DWQ-2002-002000

Utah Department of Environmental Quality
Division of Water Quality
TMDL Section

Ashley Creek TMDL

Waterbody ID Ashley-Brush Watershed: Lower Ashley Creek,
from confluence with Green River upstream
approximately 8 miles
HUC #14060002 Location Uintah County, Northeastern Utah
Pollutant of Concern Selenium (Se)
Impaired Beneficial Uses Class 3B: Protected for warm water species of
game fish and other warm water aquatic life,
including the necessary aquatic organisms in their
food chain. Current Load
Loading Capacity (TMDL)
Load Reduction
1,637 lbs/yr
361 lbs/yr
1,276 lbs/yr (78%)
Wasteload Allocation
UPDES #UT0024511 (Lagoons)
UPDES #UT0025348 (WWTP)
Load Allocation
Margin of Safety

0 lbs/yr
72 lbs/yr
271 lbs/yr
18 lbs/yr Defined Targets/Endpoints

1) Total maximum load as an annual average of
less than 361 lbs/yr
2) Load reduction of 1,276 lbs/yr
3) Water quality target of 4.75 ug/L Implementation Strategy

1) Closure of Ashley Valley Sewage Lagoons
(completed in 2001)
2) Irrigation water and riparian best
management practices This document is identified as a TMDL for waters in the Ashley Creek drainage and is
submitted under §303d of the Clean Water Act to U.S. EPA for review and approval.

Utah Department of Environmental Quality
Division of Water Quality
TMDL Section

Ashley Creek TMDL

Waterbody ID Ashley-Brush Watershed: Lower Ashley Creek, from
confluence with Green River upstream approximately
8 miles
HUC #14060002 Location Uintah County, Northeastern Utah
Pollutants of Concern Total Dissolved Solids (TDS)
Impaired Beneficial Uses Class 4: Protected for agricultural uses including
irrigation of crops and stock watering. Current Loading
Loading Capacity (TMDL)
Load Reduction
36,247 tons/yr
29,053 tons/yr
7,194 tons/yr (20%)
Wasteload Allocation
UPDES #UT0024511 (Lagoons)
UPDES #UT0025348 (WWTP)
Load Allocation
Margin of Safety

0 tons/yr
3,794 tons/yr
23,806 tons/yr
1,453 tons/yr Defined Targets/Endpoints

1) Total maximum load as an annual average of
less than 29,053 tons/yr
2) Load reduction of 7,194 tons/yr
3) Water quality target of 1,140 mg/L
Implementation Strategy

1) Closure of Ashley Valley Sewage Lagoons
(completed in 2001)
2) Irrigation water and riparian best management
practices This document is identified as a TMDL for waters in the Ashley Creek drainage and is
submitted under §303d of the Clean Water Act to U.S. EPA for review and approval.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

Table of Contents

1.0 Background..............................................................................................................1
2.0 Water Quality Standards ........................................................................................12
2.1 Utah’s Listing Methodology.................................................................................12
2.1.1 Selenium ......................................................................................................12
2.1.2 Total Dissolved Solids..................................................................................12
2.2 TMDL Endpoints.................................................................................................13
3.0 Data Inventory and Review ....................................................................................14
3.1 Flow Data ...........................................................................................................14
3.2 Water Quality Data .............................................................................................14
3.2 Water Quality Analysis........................................................................................17
3.2.1 Summary of Selenium and Total Dissolved Solids Concentrations..............17
3.2.2 Seasonal Effects on Se and TDS Concentrations.......................................20
3.2.3 Flow Versus Se and TDS Concentrations....................................................21
3.3 Critical Conditions...............................................................................................22
4.0 Source Assessment ...............................................................................................25
4.1 Assessment of Point Sources.............................................................................25
4.1.1 Ashley Valley Sewage Lagoons...................................................................25
4.2 Assessment of Non-Point Sources .....................................................................26
4.2.1 Irrigation Return Flows.................................................................................26
4.2.2 Streambank Erosion.....................................................................................27
5.0 Technical Approach................................................................................................29
5.2 Estimation of Existing Loading............................................................................31
5.2.1 Selenium ......................................................................................................31
5.2.2 Total Dissolved Solids..................................................................................32
5.3 Comparison of Existing Loading and Loading Capacity......................................34
6.0 TMDL Allocation.....................................................................................................36
6.1 Description of TMDL Allocation...........................................................................36
6.2 Selecting a Margin of Safety...........................................................................36
6.3 Allocation Summary............................................................................................36
6.4 Pollutant Loading Scenarios...............................................................................38
6.4.1 Existing Conditions.......................................................................................38
6.4.2 Loading Capacity .........................................................................................38
6.4.3 Waste Load Allocation .................................................................................40
6.4.4 Load Allocation.............................................................................................41
7. Potential Control Options .........................................................................................42
8. Future Monitoring.....................................................................................................44
9. Public Participation...................................................................................................45
10. References.............................................................................................................46

Ashley Creek Selenium and Total Dissolved Solids TMDLs

List of Figures

Figure 1-1. Location of the Ashley Creek Watershed.....................................................2
Figure 1-2. Land Use Distribution within the Ashley Creek Watershed..........................4
Figure 1-3. Geologic Formations of the Ashley Creek Watershed .................................7
Figure 1-4. Average Annual Precipitation within the Ashley Creek Watershed..............8
Figure 1-5. Vegetation Communities within the Ashley Creek Watershed .....................9
Figure 1-6. Hydrology of the Ashley Creek Watershed ................................................11
Figure 3-1. Location of Water Quality Monitoring and Flow Gauge Stations in Lower
Ashley Creek Watershed .......................................................................................16
Figure 3-2. Average monthly Se concentrations in Ashley Creek above confluence with
the Green River (Storet site 493721).....................................................................20
Figure 3-3. Average monthly TDS concentrations in Ashley Creek above confluence
with the Green River (Storet site 493721)..............................................................20
Figure 3-4. Plot of Se versus flow in Ashley Creek above confluence with the Green
River (Storet site 493721). .....................................................................................21
Figure 3-4. Plot of TDS versus flow in Ashley Creek above confluence with the Green
River (Storet site 493721). .....................................................................................21
Figure 3-5. Monthly distribution of Se concentrations at station 493721 (Ashley Creek
above confluence with the Green River)................................................................24
Figure 3-6. Monthly distribution of TDS concentrations at station 493721 (Ashley Creek
above confluence with the Green River)................................................................24
Figure 4-1. Extensive cut bank along Ashley Creek.....................................................27
Figure 4-2. Locations of Billings clay and Naples loam soils along Ashley Creek........28
Figure 5-1. Loading capacity of selenium for all observed flows in Ashley Creek........30
Figure 5-2. Loading capacity of total dissolved solids for all observed flows in Ashley
Creek.....................................................................................................................31
Figure 5-3. Existing Selenium loading by flow percentile for Ashley Creek..................32
Figure 5-4. Existing Total Dissolved Solids loading by flow percentile for Ashley Creek.
...............................................................................................................................34
Figure 5-5. Existing Se loading and loading capacity for Ashley Creek. ......................35
Figure 5-6. Existing TDS loading and loading capacity for Ashley Creek.....................35
Figure 6-1. Se and TDS loads and trends from 1998-2002...........................................40

Ashley Creek Selenium and Total Dissolved Solids TMDLs

List of Tables

Table 1-1. Ashley Creek listed waterbody characteristics.............................................1
Table 1-2. Land use distribution in the Ashley Creek watershed....................................3
Table 1-3. Vegetation Communities within the Ashley Creek watershed .......................6
Table 2-1. Applicable Utah Water Quality Criteria........................................................12
Table 2-2. 303 (d) Criteria for Assessing Beneficial Use Support ................................13
Table 3-1. Inventory of data used for the watershed source assessment ....................14
Table 3-2. USGS Flow Gages in the Ashley Creek Watershed....................................14
Table 3-3. Inventory of Available Data in Lower Ashley Creek Watershed ..................17
Table 3-4. Summary of observed Se concentrations in Ashley Creek from July 1991 to
September 2001.....................................................................................................18
Table 3-5. Summary of observed TDS concentrations in Ashley Creek from July 1991
to September 2001.................................................................................................19
Table 3-6. Se data results by flow percentile group .....................................................23
Table 3-7. TDS data results by flow percentile group...................................................23
Table 5-1. Summary of flows by percentile at USGS Gage 9271550 (Ashley Creek
below Union Canal diversion near Jensen)............................................................30
Table 5-2. Selenium loading statistics for the Ashley Creek watershed.......................32
Table 5-3. Total Dissolved Solids loading statistics for the Ashley Creek watershed...33
Table 6-1. Summary of Se TMDL for Ashley Creek watershed....................................37
Table 6-2. Summary of TDS TMDL for Ashley Creek watershed.................................37
Table 6-3. Se load capacity, existing load, and necessary reduction at various flows. 39
Table 6-4. TDS load allocation, existing load, and necessary reduction at various flows.
...............................................................................................................................39
Ashley Creek Selenium and Total Dissolved Solids TMDLs

Executive Summary

This document addresses water quality impairments within the lower Ashley
Creek watershed through the establishment of Total Maximum Daily Loads (TMDLs) for
Selenium (Se) and Total Dissolved Solids (TDS). The purpose of this TMDL is to
improve water quality and protect or restore designated beneficial uses. Lower Ashley
Creek, from the confluence with the Green River upstream approximately 8 miles, is
listed on the State’s 303D list of impaired waters and has been designated as not
meeting its warm water fishery beneficial use (3B) due to high concentrations of Se and
its agricultural beneficial use (4) due to high concentrations of TDS. The source of
impairment originates primarily from seepage from the Ashley Valley Sewage Lagoons
through an outcrop of Mancos shale, a naturally occurring geologic formation that
borders the east side of Ashley Creek. Permitted point source discharges in the
watershed include the Ashley Valley Water Reclamation Facility, a mechanical waste
water treatment plant, and five oil wells in the Ashley Oil Field.
The Ashley Creek Watershed is located in the northeast corner of the State of
Utah and encompasses 393 square miles. Elevations range from over 9,500 feet in the
Uinta Mountains to the north, down to 5,000 feet at the confluence with the Green River.
Vegetation types are characteristic of the Rocky Mountains and Colorado Plateau with
coniferous forests dominating the high elevations, Pinyon-Juniper forests at mid-
elevations and sagebrush-grass and agricultural lands in the valley bottom. Ashley
Creek is the primary drainage in the watershed flowing from the Uinta Mountains in the
north, through Ashley Valley and into the Green River forty-five miles to the southeast.
Flows in Ashley Creek vary widely due to spring snow melt, irrigation diversions, and
occasional thunderstorms. During spring runoff stream flows average 195 cubic feet
per second (cfs), 24 cfs during irrigation season and 34 cfs during the winter near
Jensen above the confluence with the Green River.
Approximately 12,000 people reside within the watershed with the majority living
in the city of Vernal. The economy of the watershed is based upon tourism, fossil fuel
production and agriculture. Recreational opportunities abound on nearby National
Forest lands, Dinosaur National Monument and Green River.
Because of the natural geologic sources of Se and TDS that underlie Ashley
Valley there will always be some Se and TDS non-point source loading into Ashley
Creek. However, several projects currently underway within the watershed will improve
the water quality and riparian habitat of Ashley Creek. A locally led watershed planning
effort, the Ashley Creek Restoration and Stabilization Committee, is addressing chronic
flooding, water quality and riparian habitat issues on Ashley Creek. The Army Corps of
Engineers is about to begin implementation on a riparian restoration project on Ashley
Creek above the Steinaker diversion, the Ashley Valley Sewer Improvement District has
recently constructed a new wastewater treatment plant, and the Uintah County Water
Conservancy District, in cooperation with the Bureau of Reclamation and Natural
Resources Conservation Service, is implementing salinity control projects on irrigated
lands throughout the watershed. It is important to recognize that data collection in
support of this TMDL is an ongoing effort and that as new data is collected this TMDL
will be revised as needed.
Ashley Creek Selenium and Total Dissolved Solids TMDLs

1

1.0 Background

EPA’s Water Quality Planning and Management Regulations (40 CFR 130) require
states to develop Total Maximum Daily Loads (TMDLs) for waters that exceed water
quality standards. This document presents TMDLs for Ashley Creek, which is listed on
Utah’s 2002 303(d) for impairments associated with excess concentrations of Selenium
(Se) and Total Dissolved Solids (TDS).

Ashley Creek, a tributary of the Green River, is located in the western portion of the
Ashley-Brush hydrologic unit (HUC 14060002) on the southeastern slope of the Uinta
Mountains in northeastern Utah (Figure 1-1). Utah’s Division of Water Quality (UDWQ)
has assessed lower Ashley Creek (from the mouth of the river to the County Road
crossing above the old Vernal Lagoons) and its tributaries and has determined that this
segment is not supporting its agricultural classification due to violations of the water
quality criterion for TDS and its warm water fisheries classification due to violations of
the criterion for Se. This stream segment also has a fish consumption advisory on it
because of elevated levels of Se found in fish tissue.

Ashley Creek has a priority ranking of low. Although Ashley Creek has been listed as a
low priority for TMDL development on the State’s 2002 303(d) list, the initiation of water
quality improvement projects directed by various federal, state and local agencies as
well as the voluntary support of local stewards to address water quality problems in the
watershed has elevated it to a higher priority. The development of this TMDL has not
disrupted the state’s scheduled completion of TMDLs for high priority waterbodies.
Table 1-1 presents the 2002 303(d) list information for Ashley Creek.

Table 1-1. Ashley Creek listed waterbody characteristics

Waterbody Name HUC Code Designated
Uses*
Pollutants of
Concern
Primary Source
of Impairment
Ashley Creek 14060002 2B, 3B, 4 Total Dissolved
Solids, Selenium
Sewage lagoon
seepage,
irrigation return
flows, natural
geologic
formations
* 2B = Recreational use and aesthetics: secondary contact recreation;
3B = Protected for warm water species of game fish and other warm water aquatic
life including the necessary aquatic organisms in their food chain
4 = Agriculture: including irrigation of crops and stock watering
Ashley Creek Selenium and Total Dissolved Solids TMDLs

2

Figure 1-1. Location of the Ashley Creek Watershed
#S
#S
#S
#S
Maeser
Vernal
Naples
40
Jensen
(/44
.-,40
A
shle
y C
reek
Steinaker ReservoirSpring Creek
Dry Fork
0102030Miles
Ownership
Forest Service
BLM
State of Utah
Private
State Parks
State Wildlife Reserve
Water
Stream Beneficial Use Support
Not Supporting
N Ashley Creek Watershed

Ashley Creek Selenium and Total Dissolved Solids TMDLs

3

The Ashley Creek watershed encompasses 251,817 acres and is encircled by the Uinta
Mountains to the north, Asphalt Ridge to the west and the Buckskin Hills to the east.
Vernal, the county seat, has the highest population within the watershed with an
estimated 7,714 residents. Other communities within the watershed include Naples with
1,300 residents and Maeser with 2,855 residents (2000 U.S. Census).

Based on 1985 land use data from U.S. Geological Survey’s (USGS) Geographic
Information Retrieval Analysis System (GIRAS), the Ashley Creek watershed is
composed primarily of rangeland and agricultural lands in the lower watershed with
forest land dominating the upper watershed. Land use distribution in the watersheds
based on the general categories is listed in Table 1-2 and shown in Figure 1-2. It
should be noted that the areal extent of these land use categories are approximate and
were derived from high altitude aerial photographs with a minimum unit size of 40 acres
for natural features. Natural land use features less than 40 acres in size were lumped
into larger, adjoining land use categories so that the total size of small isolated features
such as wetlands is underestimated.

Table 1-2. Land use distribution in the Ashley Creek watershed

Land Use Area
(acres)
% of total area
Residential/Urban 3,764 2%
Agriculture 31,013 12%
Rangeland 65,326 26%
Forest 137,182 55%
Water 712 <1%
Wetland 431 <1%
Barren 2,271 <1%
Tundra 11,118 4%

Ashley Creek Selenium and Total Dissolved Solids TMDLs

4

Figure 1-2. Land Use Distribution within the Ashley Creek Watershed
0 102030Miles
Land Use
Agriculture
Barren
Forest
Rangeland
Residential / Urban
Tundra
Water
Wetland
Streams
Perennial Stream
Intermittent Stream
Ditch or Canal
N

Ashley Creek Selenium and Total Dissolved Solids TMDLs

5

The geology of the Ashley Creek watershed has a significant influence on the
watershed’s vegetation communities, land uses, and water quality. The upper
watershed within the Uinta Mountains is predominantly metamorphic quartzite,
conglomerate and glacial deposits that have low concentrations of salts and metals
(Figure 1-3). At mid-elevations, formations that contain high concentrations of
phosphorus are exposed such as the Park City and Morrison formations. Within the
lower watershed, including Ashley Valley, formations naturally high in salt and selenium
are found including the Mancos and Chinle Shales.

Soils within Ashley Valley are formed from alluvial sediments that have been
transported into the valley from the Uinta Mountains and surrounding foothills. The
majority of irrigated soils west of Ashley Creek are mainly of medium texture and open
structure, with moderate permeability, good available moisture capacity, and relatively
low in soluble salt and alkalinity. Soils derived from Mancos shale, including Billings
clay and Naples loam, are poorly drained and high in soluble salts, selenium and
alkalinity (see Figure 4-2, p.27).

The climate of the Ashley Creek watershed is typified as temperate and arid in Ashley
valley and mesic within the Uinta Mountains with rainfall averaging between 7 inches
per year in the valley and 30 inches within the high Uintas (Figure 1-4). The majority of
precipitation is associated with frontal storms from the Pacific Northwest and falls as
snow during the cold winter months. The remaining precipitation is associated with
highly variable thunderstorms originating from the Gulf of Mexico during the summer
and early fall that can result in localized flash flooding.

The vegetation types found within the Ashley Creek watershed are the result of the
environmental factors discussed above, including agricultural and urban development.
Vegetation types within the Uinta Mountains include coniferous forests, Aspen, and
brush types. On the foothills Pinyon pine, Juniper, and sagebrush are the dominant
vegetation types. Within Ashley Valley, agricultural crops, urban development and salt
desert scrub predominate (Figure 1-5). The following Table 1-3 summarizes the total
area and relative percentage of the different vegetative communities found with the
watershed.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

6

Table 1-3. Vegetation Communities within the Ashley Creek watershed

Vegetation Community Area
(acres)
% of total area
Lodgepole Pine 53,046 21%
Spruce / Fir 40,657 16%
Agriculture 34,645 14%
Sagebrush 31,790 13%
Salt Desert Scrub 26,261 10%
Sagebrush / Perennial Grass 22,902 9%
Pinyon Pine / Juniper 15,766 6%
Alpine 6,627 3%
Mountain Fir 6,039 2%
Ponderosa Pine 2,959 1%
Aspen 2,694 1%
Barren 1,974 1%
Urban 1,646 1%
Pinyon Pine 1,248 <1%
Juniper 1,034 <1%
Dry Meadow 887 <1%
Ponderosa Pine / Mt. Shrub 432 <1%
Spruce Fir / Mt. Shrub 394 <1%
Mountain Shrub 297 <1%
Lowland Riparian 297 <1%
Wet Meadow 187 <1%
Wetland 30 <1%
Ashley Creek Selenium and Total Dissolved Solids TMDLs

7

Figure 1-3. Geologic Formations of the Ashley Creek Watershed
0 102030Miles
Geologic Formations
Mancos Shale
Chinle Shale
Surficial deposits-alluvium
Surficial deposits-older alluvium
Surfiical deposits-glacial deposits
Surficial deposits-landslides
Bishop Conglomerate
Duchesne River Fm, Uinta Fm
Green River Fm
Mesaverde Group (coal)
Dakota and Cedar Mountain Fms
Morrison Fm
Curtis FmNugget Sandstone
Moenkopi FmPark City Fm
Weber Sandstone
Morgan FormationHumbug Fm
Madison LimestoneUinta Mountain Group
Water
Streams
Perennial Stream
Intermittent Stream
Ditch or Canal
Lakes
N

Ashley Creek Selenium and Total Dissolved Solids TMDLs

8

Figure 1-4. Average Annual Precipitation within the Ashley Creek Watershed
A
shley
C
reek
Spring Creek
Dry Fork
Steinaker
Reservoir
0102030Miles
Precipitation (in.)
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
Major Streams
Lakes
N

Ashley Creek Selenium and Total Dissolved Solids TMDLs

9

Figure 1-5. Vegetation Communities within the Ashley Creek Watershed
0 102030Miles
Gap vegetation
WaterSpruce/FirPonderosa Pine
Lodgepole
Mountain Fir
Juniper
Pinyon
Pinyon/Juniper
AspenMountain ShrubSagebrush
Sagebrush/Perennial Grass
Alpine
Dry Meadow
Wet MeadowBarren
Ponderosa Pine/Mountain Shrub
Spruce Fir/Mountain Shrub
Lowland Riparian
Agriculture
UrbanSalt Desert Scrub
Desert Grassland
Wetland
Major Streams
Lakes
N

Ashley Creek Selenium and Total Dissolved Solids TMDLs

10

The hydrology of Ashley Creek has been significantly altered as the result of water
developments designed to transport and store stream flows for agricultural and
municipal use. The average annual streamflow of Ashley Creek is 119,400 acre-feet
(ac/ft) while the presently developed water supply totals 88,840 ac-ft (Utah Division of
Water Resources, 1999). There are several diversions on Ashley Creek and its
tributaries including a trans-basin diversion near the headwaters of Dry Fork into the
Mosby Canal, the Ashley Upper and Highline Canals and the Fort Thornburgh diversion
(Figure 1-6).

During low flows, Ashley Creek is completely diverted into Steinaker Reservoir via the
Fort Thornburgh Diversion. From this point downstream approximately 6.5 miles the
channel is generally dry during the summer until groundwater base flows, irrigation
return flows and the outfall of the new Ashley Valley Water Reclamation Facility
(AVWRF) re-enter the channel. The AVWRF was constructed in 2001 to replace
sewage lagoons located on a bench of Mancos Shale above Ashley Creek that were
contributing high TDS and Se loads into Ashley Creek.

Approximately 6 miles downstream of the AVWRF’s outfall is the Ashley Oil Field,
where five permitted discharge facilities are located. All of the facilities are oil wells with
wastewater treatment systems that consist of oil/water separator tanks and skimmer
ponds that discharge into the Union Canal. The discharge permits for these facilities
stipulate the daily maximum concentration of TDS is not to exceed 2,200 mg/L.
Analysis from two oil wells indicated TDS concentrations between 1,330-1,900 mg/L
with no selenium detected (Stephens et al., 1992, p.77). The Union Canal is used to
irrigate approximately 400 acres of alfalfa and grain during irrigation season and for
stock watering during the winter. The canal ends below the confluence of Ashley Creek
with the Green River. There may be the potential for Union Canal water to reach Ashley
Creek during the irrigation season via surface runoff, although salinity control efforts are
currently underway that will line the canal and install improved irrigation methods in this
area that will greatly reduce the potential for return flows to reach Ashley Creek.

Selenium and TDS loading into Ashley Creek has been significantly reduced recently
with the closing of the Ashley Valley Sewage Lagoons (Naftz, pers. comm.) and
irrigation improvements associated with the Salinity Control Program. The remaining
point source of Se and TDS loading into Ashley Creek includes the seepage from the
sewage lagoons. Non-point sources include naturally occurring shallow groundwater,
streambank erosion and irrigation return flows. These sources of Se and TDS are
described in greater detail in Section 3.0.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

11

Figure 1-6. Hydrology of the Ashley Creek Watershed
']']']']']
#0
#0
#0
#0
#0
Steinaker Res.
A
s
h
le
y C
ree
k
D
r
y

F
o
r
k
#·#
Ashley Valley WWTP
#
Old Vernal Lagoons
$Z
$Z
#
Ft. Thornburgh
Diversion
$Z#
Mosby Canal
Diversion
#Ashley Upper and
Highline Canal Diversions
#
Ashley Oil Field
#
Union Canal
9267500
9266500
9271400
9271450
9271550
0102030Miles
Irrigated lands
Lakes
Streams
Perennial Stream
Intermittent Stream
Ditch or Canal
Old Lagoons']Ashley Oil Field Sites#0 USGS Gauges
Ashley Creek Watershed
N

Ashley Creek Selenium and Total Dissolved Solids TMDLs

12

2.0 Water Quality Standards

Utah’s Standards of Water Quality for Waters of the State (Utah Division of Water
Quality, 2000) present the applicable water quality criteria for the state of Utah. Table
2-1 presents Utah’s water quality criteria for the designated uses of Ashley Creek.

Table 2-1. Applicable Utah Water Quality Criteria

Parameter Secondary Contact
Recreation (2B)
Warm Water Aquatic Life (3B) Agriculture (4)
Selenium
(Dissolved)
- 4 day average (chronic): 5 ug/L
1 hour average (acute): 20 ug/L
Maximum: 50 ug/L
Total Dissolved
Solids
- - 1,200 mg/L

2.1 Utah’s Listing Methodology

2.1.1 Selenium

To evaluate attainment of water quality standards Utah uses the acute Selenium
criterion of 20 micrograms per liter (ug/L) which is based upon a 1 hour average of
samples. In the case of the UDWQ’s sampling methodology this typically entails a
single grab sample. However, the goal for this TMDL is based upon the chronic
Selenium criterion of 5 ug/L, based upon a 4 day average of samples, which is more
applicable to loading calculations based upon annual average loads. The 303(d) listing
criteria evaluates beneficial use support based on the number of violations of the water
quality criterion for toxic parameters as listed in Table 2-2. A minimum of four samples
collected at least once each season is required for assessment.

2.1.2 Total Dissolved Solids

Utah uses the Total Dissolved Solids criterion of 1,200 milligrams per liter (mg/L) to
evaluate attainment of water quality standards. The 303(d) listing criteria evaluates
beneficial use support based on the number of violations of the water quality criterion for
conventional parameters as listed in Table 2-2. A minimum of ten samples collected
throughout the year (as in an intensive monitoring cycle) is required for assessment.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

13

Table 2-2. 303 (d) Criteria for Assessing Beneficial Use Support

Degree of
Use Support
Toxic Parameters*
(Se)
Conventional Parameter**
(TDS)
Full For any one pollutant, no more than
one violation of criterion.
Criterion exceeded in less than two
samples and in less than 10% of the
samples if there were two or more
exceedances.
Partial For any one pollutant, two or more
violations of the criterion, but violations
occurred in less than or equal to 10%
of the samples.
Criterion was exceeded two times, and
criterion was exceeded in more than
10% but not more than 25% of the
samples.
Non-support For any one pollutant, two or more
violations of the criterion, and
violations occurred in more than 10%
of the samples.
Criterion was exceeded two times, and
criterion was exceeded in more than
25% of the samples.
* Based on at least quarterly sampling
** Based on at least 10 samples during an intensive monitoring cycle

2.2 TMDL Endpoints

TMDL endpoints represent water quality targets used in quantifying TMDLs and their
individual components. Different TMDL endpoints are necessary for each impairment
type (i.e., Se and TDS). Utah’s chronic numeric water quality criteria for Se and TDS
were used to identify endpoints for TMDL development. Based on water quality
observations in lower Ashley Creek (see Section 3.3), the entire time period and all
flows were assumed to be the critical conditions in the watershed for both Se and TDS.
The TMDL endpoints applied were the chronic Warm Water Aquatic Life criteria for Se
of 5 ug/L, and the Agriculture criteria for TDS of 1,200 mg/L, established in Utah’s water
quality standards (Utah Division of Water Quality, 2000).

Ashley Creek Selenium and Total Dissolved Solids TMDLs

14

3.0 Data Inventory and Review

The data used in the development of Se and TDS TMDLs for Ashley Creek include
physiographic data that describes the physical conditions of the watershed and
environmental monitoring data that can be used to identify potential pollutant sources,
their location, and their loading contribution. Table 3-1 presents the various data types
and data sources reviewed in the watershed.

Table 3-1. Inventory of data used for the watershed source assessment

Data Category Description Data Source(s)
Land Use Utah Division of Water Resources
Stream Reach Coverage Utah Division of Water Resources, USGS 7.5” Quads
Utah Division of Water Resources Stream Characteristics Utah Division of Water Quality
Soils Natural Resources Conservation Service, USGS
Watershed
Physiographic
Data
Geology Utah Geological Survey
303(d) Listed Waters Utah Division of Water Quality
Water Quality Data Utah Division of Water Quality
UPDES Facilities Utah Division of Water Quality
Environmental
Monitoring Data
Streamflow Data USGS, Utah Division of Water Quality

3.1 Flow Data

Flow records available for USGS flow gages in Ashley Creek watershed are listed in
Table 3-1 with their gage names, station IDs, and periods of record. Only those gages
with records of daily flows available for dates since 1970 are shown in Figure 3-1.

Table 3-2. USGS Flow Gages in the Ashley Creek Watershed

Station ID Gage name
Start
date
End date
9266500 ASHLEY CREEK NEAR VERNAL, UT 10/1/14 9/30/01 9271550 ASHLEY CREEK BL UNION CANAL DIV NR JENSEN, UT 7/9/91 9/30/01
9271500 ASHLEY CREEK NEAR JENSEN, UTAH 10/1/46 10/1/83
9271450 ASHLEY CREEK BL SADLIER DRAW, NEAR NAPLES,
UT 11/18/99 9/30/01
9271400 ASHLEY CREEK NEAR NAPLES, UT 11/23/99 9/30/01

3.2 Water Quality Data

The Utah Division of Water Quality (UDWQ) maintains a water quality database for 36
sites within the Ashley Creek watershed, 7 of these are currently being monitored and
are located within the lower watershed as defined by the purposes of this study (Table
Ashley Creek Selenium and Total Dissolved Solids TMDLs

15

3-3). The USGS is currently conducting a detailed study of selenium loading in Ashley
Creek in cooperation with the US Bureau of Reclamation (BOR). The objectives of the
USGS/BOR study are to: (1) Quantify the change in salinity and selenium loading to
Ashley Creek that results from closure of the sewage lagoons. (2) Determine mobility of
salinity and selenium under current hydrologic conditions. (3) Determine mobility of
salinity and selenium under conditions present after closure of the sewage lagoons. (4)
Examine the role of evaporation processes on selenium pathways. The study is
anticipated to be completed in September of 2004. A summary of the data available at
the stations within the lower watershed is provided in Table 3-3, and station locations
are shown in Figure 3-1.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

16

Figure 3-1. Location of Water Quality Monitoring and Flow Gauge Stations in
Lower Ashley Creek Watershed

#0
#0
#0
#0
#0
#S
#S
#S
#S
#S
#S
#S 9266500
9271550
9271400
9271450
9271500
493721
493742
493743
493744
493748
493771
493772
Dry Fork
A
sh
le
y

C
r
e
e
k
0 5 10 15 Miles
Ashley Creek Watershed
Lakes
Streams
Perennial Stream
Ditch or Canal
Intermittent Stream
Beneficial Use Support
Fully Supporting
Not Supporting
Ashley Valley Sewage Lagoons#0 USGS Gauges and Monitoring Sites
#S UDWQ Monitoring Stations
N

Ashley Creek Selenium and Total Dissolved Solids TMDLs

17

Table 3-3. Inventory of Available Data in Lower Ashley Creek Watershed
Station Location Type Start date End date Count Data from Utah DWQ
493772 Ashley Ck AB Cnfl / Dry Fork Ambient 6/2/87 5/23/01 43
493771 Dry Fork AB Cnfl / Ashley Ck Ambient 3/30/95 6/6/01 28
493748 Ashley Ck NE of Vernal at Diamond Mt Rd Xing Ambient 1/17/80 6/6/01 36
493744 Ashley Ck AB Vernal Lagoons at County Rd Xing Ambient 1/17/80 11/12/97 36
493743 Ashley Ck N of US 40 at County Rd Xing Ambient 7/28/76 6/6/01 27
493742 Ashley Ck at US 40 Xing Ambient 2/4/75 4/18/96 84
493721 Ashley Ck AB Cnfl / Green R Ambient 1/17/80 5/1/02 237 Data from US Geological Survey
9266500 ASHLEY CREEK NEAR VERNAL, UT Ambient 8/12/55 7/17/91 211 9271400 ASHLEY CREEK NEAR NAPLES, UT Ambient 1/19/00 9/13/01 20
9271450
ASHLEY CREEK BL SADLIER DRAW, NEAR
NAPLES, UT
Ambient 1/19/00 9/13/01 20
9271500 ASHLEY CREEK NEAR JENSEN, UTAH Ambient 3/19/47 10/23/91 207
9271550
ASHLEY CREEK BL UNION CANAL DIV NR
JENSEN, UT
Ambient 4/6/88 9/26/01 73

3.2 Water Quality Analysis

This section provides a summary of the Se and TDS data throughout the watershed
collected from July 1991 to September 2001, as well as discussions on the evaluation of
any identifiable spatial or temporal patterns in Se and TDS data. The data set was
limited to this 10 year period because it was felt it would be more representative of
current conditions than if older data were incorporated and is more consistent in terms
of monitoring frequency and methodology. It is important to recognize that data
collection in support of this TMDL is an ongoing effort and that as new data is collected
this TMDL will be revised accordingly.

3.2.1 Summary of Selenium and Total Dissolved Solids Concentrations

Table 3-4 is a summary of the Se data, including the number of samples collected at the
site (Count), a summary of exceedances of the chronic 5 ug/L water quality standard,
and the average concentration (Mean).

Ashley Creek Selenium and Total Dissolved Solids TMDLs

18

Table 3-4. Summary of observed Se concentrations in Ashley Creek from July
1991 to September 2001

Station Location Count
#
Exceeding
(5 ug/L)
%
Exceeding
(5 ug/L)
Mean
(ug/L)*
Data from UDWQ
493772 Ashley Ck AB Cnfl / Dry Fork 11 0 0 0.5
493771 Dry Fork AB Cnfl / Ashley Ck 9 0 0 0.5
493748 Ashley Ck NE of Vernal at Diamond
Mt Rd Xing 4 0 0 0.5
493744 Ashley Ck AB Vernal Lagoons at
County Rd Xing 43 2 5 2.3
493743 Ashley Ck N of US 40 at County Rd
Xing 44 39 89 36.3
493742 Ashley Ck at US 40 Xing 6 6 100 48.3
493721 Ashley Ck AB Cnfl / Green R 49 49 100 39.7
Data from USGS
9266500 ASHLEY CREEK NEAR VERNAL, UT 1 0 0 0.5
9271400 ASHLEY CREEK NEAR NAPLES, UT 20 0 0 2.6
9271450 ASHLEY CREEK BL SADLIER
DRAW, NEAR NAPLES, UT 20 18 90 46.8
9271500 ASHLEY CREEK NEAR JENSEN,
UTAH 12 12 100 63.8
9271550 ASHLEY CREEK BL UNION CANAL
DIV NR JENSEN, UT 69 62 90 41.6
* For results with non-detect values ½ the reporting limit (1 ug/L) was used for mean of sample
calculation

Stations where water quality standards for Se are exceeded are all located downstream
from the old Ashley Valley Sewage Lagoons (see stations 493743 and 9271450 on
Figure 3-1). The influence of lagoon seepage on Se loading to Ashley Creek has been
well documented by USGS studies (Stolp, 1999). Potentiometric groundwater studies
have shown that ground-water flows away from the lagoons and toward Ashley Creek
and Sadlier Draw. The sewage lagoons cover 76 acres and were built directly on
Mancos Shale derived soils and on the Mancos Shale itself. This Cretaceous formation
contains relatively high concentrations of Se. Three distinct areas of seepage from the
lagoons to Ashley Creek have been identified in addition to the potential of more
dispersed areas. In joint studies between the BOR and USGS they found that the “…
selenium load in Ashley Creek that can be directly attributed to seepage from the
sewage lagoons may be a high as 1 kilogram per day.” (Stolp 1999) and in another
study stated that, “These seeps flow about 2.5 cubic feet per second (cfs) and
contribute approximately 9,000 tons of salt and 2,000 pounds of selenium per year to
Ashley Creek.” (USDI, 1997).
Ashley Creek Selenium and Total Dissolved Solids TMDLs

19

Table 3-5 is a summary of the TDS data collected in the watershed, including the
number of samples collected at the site (Count), a summary of exceedances of the
1,200 mg/L water quality standard, and the average concentration (Mean).

Table 3-5. Summary of observed TDS concentrations in Ashley Creek from July
1991 to September 2001

Station Location Count
#
Exceeding
(1,200 mg/L)
%
Exceeding
(1,200 mg/L)
Mean
(mg/L)
Data from UDWQ
493772 Ashley Ck AB Cnfl / Dry Fork 43 0 0 90
493771 Dry Fork AB Cnfl / Ashley Ck 28 0 0 399
493748 Ashley Ck NE of Vernal at Diamond
Mt Rd Xing 36 1 3 408
493744 Ashley Ck AB Vernal Lagoons at
County Rd Xing 36 7 19 1018
493743 Ashley Ck N of US 40 at County Rd
Xing 24 18 75 1444
493742 Ashley Ck at US 40 Xing 72 53 74 1604
493721 Ashley Ck AB Cnfl / Green R 237 185 78 1735
Data from USGS
9266500 ASHLEY CREEK NEAR VERNAL, UT 1 0 0 93
9271400 ASHLEY CREEK NEAR NAPLES, UT 20 3 15 992
9271450 ASHLEY CREEK BL SADLIER
DRAW, NEAR NAPLES, UT 20 16 80 1572
9271500 ASHLEY CREEK NEAR JENSEN,
UTAH 1 1 100 1890
9271550 ASHLEY CREEK BL UNION CANAL
DIV NR JENSEN, UT 2 2 100 2055

Stations where water quality standards for TDS are consistently exceeded are located
downstream from the old Ashley Valley Sewage Lagoons (see stations 493743 and
9271450 on Figure 3-1). As with Se, the influence of lagoon seepage on TDS loading
to Ashley Creek has been well documented by BOR and USGS studies (Stolp 1999).
Mancos shale, on which the lagoons are constructed, contains as much as 2 percent
soluble salts by weight. While inflow to the sewage lagoons has a dissolved-solids
concentration of about 600 mg/L, as the wastewater seeps down and flows through and
across the Mancos Shale, salinity and selenium concentrations increase dramatically.
TDS concentrations from the seeps have been measured as high as 25,000 milligrams
per liter.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

20

3.2.2 Seasonal Effects on Se and TDS Concentrations

In Figures 3-2 and 3-3, the average monthly Se and TDS values are plotted to show the
monthly and seasonal patterns for Ashley Creek at its confluence with the Green River
(Storet 493721). Because of the relationship between instream concentrations and
diluting flows, average monthly flows from the nearby USGS gauge (9271550, Ashley
Creek below Union Canal diversion) are also included.

Figure 3-2. Average monthly Se concentrations in Ashley Creek above
confluence with the Green River (Storet site 493721).
0
10
20
30
40
50
60
70
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
Se

(
u
g
/
L
)
0
50
100
150
200
250
300
350
Fl
o
w

(
c
f
s
)
Avg Flow Avg Se

Figure 3-3. Average monthly TDS concentrations in Ashley Creek above
confluence with the Green River (Storet site 493721).
0
400
800
1200
1600
2000
2400
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
TD
S

(
m
g
/
L
)
0
50
100
150
200
250
300
350
Fl
o
w

(
c
f
s
)
Avg Flow Avg TDS

Ashley Creek Selenium and Total Dissolved Solids TMDLs

21

The relationship between flows and Se and TDS concentrations is readily apparent from
the figures above. During spring runoff (May - June) concentrations are relatively low,
gradually rising throughout the rest of the year. This pattern suggests that Se and TDS
loading occurs throughout the year and that concentrations are primarily a function of
the availability of diluting stream flows, or hydrology, rather than seasonality.

3.2.3 Flow Versus Se and TDS Concentrations

To determine how much influence stream flow has on Se and TDS concentrations the
two variables were plotted against each other and regression lines were calculated as
shown in Figures 3-4 and 3-5. The coefficient of determination (R2) indicates the
strength of the correlation between the two variables (Se or TDS versus flow).

Figure 3-4. Plot of Se versus flow in Ashley Creek above confluence with the
Green River (Storet site 493721).
R2 = 0.5
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200 250
Flow (cfs)
Se

(
u
g
/
L
)

Figure 3-4. Plot of TDS versus flow in Ashley Creek above confluence with the
Green River (Storet site 493721).
R2 = 0.7
0
500
1000
1500
2000
2500
3000
3500
0 50 100 150 200 250
Flow (cfs)
TD
S

(
m
g
/
L
)

Ashley Creek Selenium and Total Dissolved Solids TMDLs

22

The plot of TDS versus flow shows a more identifiable trend (R2 = 0.7) than the Se data
show (R2 = 0.5). However, both plots show the general trend of decreasing Se and
TDS concentrations with increased flow. Se and TDS concentrations are likely the
highest during baseflow conditions when groundwater with elevated concentrations
provides the majority of the streamflow.

3.3 Critical Conditions

The critical condition represents the time of year or hydrologic event under which water
quality standards are exceeded. Analyzing the TMDL in consideration of the critical
condition ensures that water quality standards are met under all conditions. When the
source of a pollutant is fairly constant in its frequency and magnitude, low flow (i.e., the
period of minimum dilution) is typically the critical condition for the receiving water.
When pollutant sources are driven by precipitation (e.g., runoff from urban or
agricultural land), they affect a receiving water during or just following a storm event,
making high flows the critical condition.

Flow patterns and Se and TDS concentrations were reviewed to evaluate the critical
conditions for Ashley Creek. Flows in Ashley Creek were sorted by magnitude and
divided into percentiles that were matched with the associated Se and TDS data
including minimum, average, and maximum Se and TDS concentrations for each flow
percentile. This evaluation indicated that elevated Se concentrations and violations of
water quality standards occurred in all flow percentiles while elevated TDS
concentrations and violations of water quality standards occurred up to the 90th flow
percentile as shown in Tables 3-6 and 3-7.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

23

Table 3-6. Se data results by flow percentile group
Flow Se
Percentile
Avg.
Flow
(cfs)
Maximum
(ug/L)
Mean
(ug/L)
Minimum
(ug/L)
Standard
Deviation
Mean +
Deviation
Mean -
Deviation
# of
Obs.
0-10% 1 47 26 13 15 41 11 4 10-20% 3 73 43 29 20 63 23 4 20-30% 7 101 52 30 25 77 27 7 30-40% 12 60 57 53 5 62 47 2 40-50% 17 80 50 22 23 73 27 5 50-60% 20 63 51 25 15 66 36 5 60-70% 23 70 45 16 27 72 18 3 70-80% 34 50 39 30 9 48 30 7 80-90% 81 33 18 6 12 30 6 5 90-100% 167 14 9 6 4 13 5 3

Table 3-7. TDS data results by flow percentile group
Flow TDS
Percentile
Flow
(cfs)
Maximum
(mg/L)
Mean
(mg/L)
Minimum
(mg/L)
Standard
Deviation
Mean +
Deviation
Mean -
Deviation
# of
Obs. 0-10% 1 3104 1835 1038 794 2629 1041 7 10-20% 3 2484 2020 1632 332 2352 1688 8 20-30% 7 2464 1977 1408 315 2292 1662 10 30-40% 12 2158 1958 1852 136 2094 1822 4 40-50% 17 2080 1732 1258 297 2029 1435 9 50-60% 20 2188 1853 1630 173 2026 1680 8 60-70% 24 2118 1880 1318 325 2205 1555 5 70-80% 32 2118 1564 852 341 1905 1223 13 80-90% 72 1402 955 376 400 1355 555 9 90-100% 562 598 308 140 153 461 155 8

Ashley Creek Selenium and Total Dissolved Solids TMDLs

24

Figures 3-5 and 3-6 present the Se and TDS data at station 493721 (Ashley Creek
above confluence with Green River) and the water quality criterion of 5 ug/L and 1,200
mg/L respectively.

Figure 3-5. Monthly distribution of Se concentrations at station 493721 (Ashley
Creek above confluence with the Green River).
0
20
40
60
80
100
120
012345678910111213
Month
Se

(
u
g
/
L
)

Figure 3-6. Monthly distribution of TDS concentrations at station 493721 (Ashley
Creek above confluence with the Green River).
0
400
800
1200
1600
2000
2400
2800
3200
3600
012345678910111213
Month
TD
S

(
m
g
/
L
)

As discussed in Section 3.2 and shown in Figure 3-5, violations of the Se water quality
standard occurs during all months of the year. Water quality standards for TDS (Figure
3-6) were also violated throughout the year. Since water quality standards are
exceeded in all flows and throughout the year the critical condition is and the TMDLs will
be based on an analysis of all flow conditions and will not isolate specific time periods.
Ashley Creek Selenium and Total Dissolved Solids TMDLs

25

4.0 Source Assessment

The evaluation of Se and TDS sources in the Ashley Creek watershed included point
and non-point sources. The source assessment is an important part of defining the
TMDL for any pollutant. The data and the sources have to be understood to be able to
distinguish between point and non-point source impacts. Typically, the point source
impacts can be quantified through permit limits and/or direct measurements at a certain
location. A source assessment was performed on the Ashley Creek watershed to
determine the predominant sources of Se and TDS loading to the system. Se and TDS
are related, so it is assumed that the sources contributing to high Se concentrations in
Ashley Creek are the same sources that contribute to increased TDS concentrations.
Datasets and references used in assessing the pollutant sources in the watershed
include National Irrigation Water Quality Program Studies conducted by the BOR and
the USGS, USDA Salinity Control Program Reports, the Uintah Basin Area-wide Water
Quality Management Plan (Section 208), stream networks and characteristics,
watershed boundaries, Utah Pollutant Discharge Elimination System (UPDES)
permitted locations, and soil types and characteristics.

4.1 Assessment of Point Sources

Six permitted discharge facilities are located within the Ashley Creek watershed. The
Ashley Valley Sewage Lagoons (UPDES Permit #UT0024511) have recently been
replaced by the Ashley Valley Water Reclamation Facility (AVWRF), a mechanical
wastewater treatment plant (UPDES Permit #UT0025348). Construction of the AVWRF
began in May 1999 and was completed in May of 2001. Since this TMDL analysis is
based on data collected prior to the completion of the AVWRF, Se and TDS loading
from the lagoons will be included in the analysis. However, since the lagoons are no
longer in operation they will not be allocated any loading.

The other five facilities are oil wells all located approximately seven miles southeast of
Vernal within the Ashley Oil Field. All five facilities’ wastewater treatment systems
consist of oil/water separator tanks which discharge to skimmer ponds and thence to
the Union Irrigation Canal which ends near the Green River downstream of Ashley
Creek (see Figure 1-6). The permits specify that discharge from these facilities may
reach Ashley Creek, presumably from irrigation return flows, however a salinity control
project currently underway will line the canal and install improved irrigation systems that
will greatly reduce the potential for return flows to reach Ashley Creek. Therefore the oil
wells are not considered a significant source of Se or TDS loading into Ashley Creek.

4.1.1 Ashley Valley Sewage Lagoons

The Ashley Valley Sewage Lagoons were constructed in 1981 to replace a trickling filter
plant that discharged directly to Ashley Creek. The facility consists of 76 acres of
treatment lagoons, a winter storage reservoir, and pumping facilities. After the facility
began operating, the lagoons began to leak at a high rate. Several seeps surfaced
Ashley Creek Selenium and Total Dissolved Solids TMDLs

26

along the bluff between Ashley Creek and the lagoons. In 1987, seepage from the
lagoons was estimated at 3.3 cfs (Stephens et al., 1992, p. 73). As the water flowed
through the soil, it dissolved and carried salts and selenium. High concentrations of
selenium in Ashley Creek has had serious adverse environmental impacts on local fish
and waterfowl populations. Because of the elevated levels of selenium, the Utah
Division of Water Quality and the Tri-County Health Department issued a health
advisory in 1991 cautioning people about consumption of fish or waterfowl from the
area.
4.2 Assessment of Non-Point Sources

Non-point sources represent contributions from diffuse, non-permitted sources. The
predominant land use in the Ashley Creek watershed is irrigated agriculture and grazing
lands. Possible non-point sources of Se and TDS to Ashley Creek include irrigation
return flows and streambank erosion which are discussed in more detail in the following
sections.

4.2.1 Irrigation Return Flows

Significant natural sources of Se and TDS exist in the watershed. The geology of the
lower portion of the basin is dominated by the highly saline Mancos shale formation.
Whenever water comes into contact with this formation, or the saline soils formed from
it, selenium and other salts are dissolved and transported to Ashley Creek via surface
runoff or shallow groundwater. There have been numerous studies within the Ashley
Creek watershed on the causes and sources of salt and selenium loading and all
generally agree that, other than seepage from the sewage lagoons, the most significant
human caused sources include irrigation return flows and field drains. However, due to
implementation of salinity control efforts in the last 20 years a significant amount of the
return flows have been greatly reduced or eliminated. Salinity control practices typically
entail converting from flood irrigation to sprinklers or gated pipe and lining or piping
ditches to reduce deep percolation.

According to data collected from ephemeral drainages to the west of Ashley Creek that
constitute most of the potential return flows, some Se and TDS loading occurs. Of
particular concern is the Mantle Gulch drainage that crosses US-40 just west of Ashley
Creek. In July of 1988 a Se concentration of 540 mg/L was measured where Mantle
Gulch flows into Ashley Creek, although no accompanying flow was given (Stephens et
al., 1992). However, when considered in light of the Se and TDS loading from the
sewer lagoons, the loads originating from irrigation return flows pale in comparison.
Which is not to say that potential Se and TDS loading attributable to irrigation return
flows can be dismissed as insignificant, rather at the present time there is insufficient
information to conclusively state what the allocation should be. As the sewer lagoons
dry up and loading decreases the proportionate contribution from irrigation return flows
may increase, thereby justifying the implementation of additional best management
practices in this area.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

27

As noted in a recent study conducted by the USGS in western Colorado (Butler, 2001),
TDS concentrations in return flows tend to remain relatively constant regardless of the
amount of flow. In other words, salinity control efforts have been effective in reducing
Se and TDS loading through reduction of return flows but not in reducing
concentrations, which requires dilution with high quality water.

4.2.2 Streambank Erosion

Se and TDS loading attributable to streambank erosion is highly variable from year to
year, depending primarily on the magnitude and duration of peak flows and the
streambank’s soil type.

Figure 4-1. Extensive cut bank along Ashley Creek

Soils in the area of lower Ashley Creek are derived from alluvial material and from
Mancos Shale. The two soil types associated with high Se and TDS concentrations
include the Billings clay and Naples loam (Figure 4-2). Se concentrations of 2,000 to
8,000 ppb have been reported for six samples of Billings clay loam from Western
Colorado, and an alkaline crust resulting from seepage through Billings clay loam
contained a Se concentration of 52,000 ppb (Williams and Byers, 1935).

Ashley Creek Selenium and Total Dissolved Solids TMDLs

28

Figure 4-2. Locations of Billings clay and Naples loam soils along Ashley Creek
From “Detailed Study of Selenium and Selected Elements in Water, Bottom Sediment, and Biota
Associated with Irrigation Drainage in the Middle Green River Basin, Utah, 1988-90.” USGS Water
Resources Investigations Report 92-4084.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

29

5.0 Technical Approach

Establishing a relationship between the in-stream water quality targets and source
loading is a critical component of TMDL development. Identifying the cause-and-effect
relationship between pollutant loads and the water quality response is necessary to
evaluate the loading capacity of the receiving waterbodies. The loading capacity is the
amount of pollutant that can be assimilated by the waterbody while still attaining and
maintaining water quality standards. In other words, the load capacity, or maximum
allowable load, is calculated by multiplying existing flows by the water quality standard.
This section discusses the estimation of the loading capacity and existing Se and TDS
loadings in the Ashley Creek watershed.

Together with historical flow records, the water quality target for Se and TDS was used
to establish loading capacities for all flows expected to occur in Ashley Creek in a
typical year. Existing loads also were estimated for comparison to loading capacities
and evaluation of necessary load reductions. Existing loads were calculated based on
available monitoring data for Se, TDS and flow. The following sections discuss the
approaches used to estimate loading capacity and existing Se and TDS loadings for
Ashley Creek.

A percent ranking model based on flow was used to establish associated Se and TDS
loads. The available flow record at the USGS Gage 9271550 (Ashley Creek below
Union Canal diversion near Jensen) was used to develop the flow duration curves and
the loading analyses for the TMDLs. The observed daily USGS flows (7/91 to 9/01)
were ranked in order of magnitude and each flow was assigned a percent that reflects
the chance of a flow less than or equal to it (Table 5-1). To evaluate the allowable Se
and TDS loadings for the watershed, each flow was then multiplied by the 5 ug/L or
1,200 mg/L criterion respectively to calculate a corresponding maximum loading limit for
each flow. The individual lines were plotted to present a loading capacity line by flow
percentile, as shown in Figures 5-1 and 5-2.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

30

Table 5-1. Summary of flows by percentile at USGS Gage 9271550 (Ashley Creek
below Union Canal diversion near Jensen).

Percentile Average Flow (cfs)

Selenium
Total Dissolved
Solids 0%-10% 1.1 1.1

10%-20%

3.2 3.3

20%-30%

7.1 7.5
30%-40% 12.5 12.3
40%-50% 17.4 16.9

50%-60%

20 19.9

60%-70%

23 24.4
70%-80% 33.6 32.4
80%-90% 81.4 72.3
90%-100% 167.3 561.6

Figure 5-1. Loading capacity of selenium for all observed flows in Ashley Creek.
0.0
0.0
0.0
0.1
1.0
10.0
100.0
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Flow Percentile
Se

L
o
a
d

(
l
b
/
d
a
y
)
0.01
0.1
1
10
100
1000
10000
Fl
o
w

(
c
f
s
)
Se Load Capacity Flow

Ashley Creek Selenium and Total Dissolved Solids TMDLs

31

Figure 5-2. Loading capacity of total dissolved solids for all observed flows in
Ashley Creek.
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Flow Percentile
TD
S

L
o
a
d

(
l
b
/
d
a
y
)
0.01
0.1
1
10
100
1000
10000
Fl
o
w

(
c
f
s
)
TDS Load Capacity Flow
5.2 Estimation of Existing Loading

Existing loadings of Se and TDS for the Ashley Creek watershed were calculated using
monitoring data for station 493721 (Ashley Creek above confluence with Green River)
and flow measurements from USGS gage 9271550 recorded on the same day.
Although the USGS has collected Se samples during the same time period it was
decided that it would be more appropriate for this analysis to use the UDWQ data set
since these data were used for assessing and listing Ashley Creek and UDWQ will
continue to monitor it indefinitely, allowing more accurate tracking of changes in water
quality over the long term. The average Se concentration of the two data sets are not
significantly different, the average over the 10 year analysis period of USGS data is
40.3 ug/L while the UDWQ Se average is 39.7 ug/L. This section presents the methods
and results of the analysis of existing Se and TDS loadings in the watershed.

5.2.1 Selenium

Existing Se loadings for Ashley Creek were calculated using Daily Se loads for Ashley
Creek were calculated by multiplying the flow by the associated Se concentration
collected on the same day (Figure 5-1). The calculated existing loads were then
grouped based on the 10 flow percentile groupings from Table 5-1. Table 5-2
summarizes the maximum, minimum, average, and standard deviation of existing Se
loads for each of the 10 percentile flow groups for Ashley Creek.

Average Se loads for each flow percentile group were used to establish a line
representing existing loading for all flows for Ashley Creek. Figure 5-3 presents all
individual existing Se loadings for Ashley Creek and the representative loading line
arranged by flow percentile.
Ashley Creek Selenium and Total Dissolved Solids TMDLs

32

Table 5-2. Selenium loading statistics for the Ashley Creek watershed.
Existing load summary Flow
Percentile
Avg. Flow
(cfs)
No. of
Loads1
Violations Max
(lbs/day)
Mean+Std
(lbs/day)
Mean
(lbs/day)
Mean-Std
(lbs/day)
Min
(lbs/day)
Load Capacity2
(lbs/day)
0%-10% 1.1 4 4 0.30 0.31 0.17 0.03 0.02 0.03
10%-20% 3.2 4 4 1.57 1.31 0.78 0.25 0.47 0.09
20%-30% 7.1 7 7 3.54 2.78 1.94 1.10 1.06 0.19
30%-40% 12.5 2 2 4.53 4.82 3.84 2.86 3.14 0.34
40%-50% 17.4 5 5 7.34 6.74 4.65 2.56 2.14 0.47
50%-60% 20 5 5 6.47 7.00 5.46 3.92 2.83 0.54
60%-70% 23 3 2 8.31 8.78 5.54 2.30 1.98 0.62
70%-80% 33.6 7 7 9.44 8.74 7.07 5.40 5.18 0.91
80%-90% 81.4 5 2 14.89 12.31 7.58 2.85 2.49 2.20
90%-100% 167.3 3 0 9.44 9.34 7.83 6.32 6.43 4.51 1Number of loads calculated using flows within the specified percentile range. This number reflects the number of
available paired Se and flow measurements available within the specific flow range. 2Based on water quality criterion of 5 ug/L.

Figure 5-3. Existing Selenium loading by flow percentile for Ashley Creek
0
2
4
6
8
10
12
14
16
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Flow Percentile
Se

L
o
a
d

(
l
b
s
/
d
a
y
)
Existing Se Loads Average Se Load

5.2.2 Total Dissolved Solids

As for Se, existing TDS loadings for Ashley Creek were calculated using monitoring
data for station 493721 (Ashley Creek above confluence with Green River) and flow
measurements from USGS gage 9271550. Daily TDS loads for Ashley Creek were
calculated by multiplying the flow by the associated TDS concentration (Figure 5-2).
Ashley Creek Selenium and Total Dissolved Solids TMDLs

33

The calculated existing loads were then grouped based on the 10 flow percentile
groupings from Table 5-1. Table 5-3 summarizes the maximum, minimum, average,
and standard deviation of existing TDS loads for each of the 10 percentile flow groups
for Ashley Creek.

Average TDS loads for each flow percentile group were used to establish a line
representing existing loading for all flows for Ashley Creek. Figure 5-3 presents all
individual existing Se loadings for Ashley Creek and the representative loading line
arranged by flow percentile.

Table 5-3. Total Dissolved Solids loading statistics for the Ashley Creek
watershed.
Existing load summary Flow
Percentile
Flow
(cfs)
No. of
Loads1
Violations Max
(lbs/day)
Mean+Std
(lbs/day)
Mean
(lbs/day)
Mean-Std
(lbs/day)
Min
(lbs/day)
Load
Capacity2 (lbs/day)
0%-10% 1.1 7 4 27,207 19,116 10,916 2,716 1,962 8,608
10%-20% 3.3 8 8 49,709 45,693 36,472 27,251 24,647 24,143
20%-30% 7.5 10 10 113,377 105,330 80,888 56,446 43,281 50,033
30%-40% 12.3 4 4 162,957 153,264 129,866 106,468 113,205 82,136
40%-50% 16.9 9 9 201,943 182,924 157,086 131,248 122,137 107,509
50%-60% 19.9 8 8 224,230 212,354 198,165 183,976 184,629 128,544
60%-70% 24.4 5 5 308,448 305,980 249,136 192,292 163,507 156,829
70%-80% 32.4 13 12 354,145 320,176 269,804 219,432 179,224 209,904
80%-90% 72.3 9 3 480,348 441,586 342,673 243,760 172,385 418,643
90%-100% 561.6 8 0 2,426,120 1,455,152 750,491 45,830 254,856 3,352,059 1Number of loads calculated using flows within the specified percentile range. This number reflects the number of
available paired TDS and flow measurements available within the specific flow range. 2Based on water quality criterion of 1,200 mg/L.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

34

Figure 5-4. Existing Total Dissolved Solids loading by flow percentile for Ashley
Creek.
0
100,000
200,000
300,000
400,000
500,000
600,000
700,000
800,000
900,000
1,000,000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Flow Percentile
TD
S

L
o
a
d

(
l
b
s
/
d
a
y
)
Existing TDS Loads Average TDS Load

5.3 Comparison of Existing Loading and Loading Capacity

To evaluate the load reductions and controls necessary to maintain water quality
standards in Ashley Creek, the existing Se and TDS loadings in each flow percentile
group were compared to their respective loading capacities. Figures 5-5 and 5-6
present the estimated loading capacity curve and existing loadings based on monitoring
data, arranged by flow percentile, for Se and TDS, respectively. Table 5-2 presents the
existing loadings for the Se, grouped into the 10 percentile ranges, and the discrete
loading capacity based on the 5 ug/L target and average flow for the percentile grouping
(e.g., 1.1 ft3/s multiplied by the 5 ug/L criterion, multiplied by conversion factors equals
0.03 lb/d). Table 5-3 presents the existing loadings and loading capacity for TDS for
each flow range. For Se, all flow percentile groups have a maximum load above the
load capacity limit, whereas for TDS all but the highest two flow percentile groups
exceed the load capacity, indicating the need for reductions of Se and TDS loads at
most flows for Ashley Creek.

By plotting the loading capacities and individual existing loads by flow percentile, the
specific dates of flows and loads are removed and the curve can be applied to different
time periods. The curve illustrates an average year, with all flows and associated
loadings expected to occur during a typical year. Therefore, Figure 5-5 presents the
estimated annual existing loading and loading capacity of Se, and Figure 5-6 presents
the estimated annual existing loading and loading capacity of TDS.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

35

Figure 5-5. Existing Se loading and loading capacity for Ashley Creek.
0
2
4
6
8
10
12
14
16
18
20
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Flow Percentile
Se

L
o
a
d

(
l
b
s
/
d
a
y
)
Se Load Capacity Existing Se Load

Figure 5-6. Existing TDS loading and loading capacity for Ashley Creek.
0
50,000
100,000
150,000
200,000
250,000
300,000
350,000
400,000
450,000
500,000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Flow Percentile
TD
S

L
o
a
d

(
l
b
s
/
d
a
y
)
TDS Load Capacity Existing TDS Loading

Ashley Creek Selenium and Total Dissolved Solids TMDLs

36

6.0 TMDL Allocation
6.1 Description of TMDL Allocation
A TMDL is composed of the sum of individual waste load allocations (WLAs) for point
sources and load allocations (LAs) for non-point sources and natural background levels.
In addition, the TMDL must include a margin of safety (MOS), either implicitly or
explicitly, that accounts for the uncertainty in the relationship between pollutant loads
and the quality of the receiving waterbody. Conceptually, this definition is denoted by
the equation
TMDL = Σ WLAs + Σ LAs + MOS

The TMDL is the total amount of a pollutant that can be assimilated by the receiving
water while still achieving water quality standards. The TMDLs for Se and TDS for
Ashley Creek are expressed on a mass loading basis.

6.2 Selecting a Margin of Safety
The MOS is a required part of the TMDL development process. There are two basic
methods for incorporating the MOS (USEPA, 1991). Implicit methods incorporate the
MOS using conservative model assumptions to develop allocations. Explicit methods
specify a portion of the total TMDL as the MOS, allocating the remainder to sources.

For the Ashley Creek TMDLs, the MOS was included explicitly by allocating 5 percent of
the loading capacity to the MOS.
6.3 Allocation Summary
The TMDLs for Ashley Creek have been developed for the range of flows measured
from July 1991 to September 2001 at the USGS gage 9271550 (Ashley Creek below
Union Canal diversion near Jensen). Using the observed flows and the targets of 5
ug/L Se and 1,200 mg/L TDS, average annual loading capacities were calculated.
Average existing Se and TDS loadings were developed within each flow percentile
group using the USGS flow data and the corresponding observed water quality data at
UDWQ’s Ashley Creek above confluence with Green River (493721) monitoring site.
These flow percentile group loadings were used to calculate the annual existing load
(average daily loading multiplied by 365). These existing loads were compared to the
loading capacity, and necessary reductions were calculated. Tables 6-1 and 6-2
provide summaries of the Se and TDS TMDLs for Ashley Creek, respectively.

Section 6.4 provides more detail on the calculation of the loading capacity and
determination of the allocations.
Ashley Creek Selenium and Total Dissolved Solids TMDLs

37

Table 6-1. Summary of Se TMDL for Ashley Creek watershed.
Source
Annual existing
Se load
Estimated percent
reduction
Annual allocated Se
load
Non-point Sources:
Ashley Creek watershed 271 lbs/yr 0 % 271 lbs/yr

Point Sources:
Ashley Valley Water
Reclamation Facility
(UPDES #UT0025348)
10 lbs/yr 0 % 72 lbs
Ashley Valley Sewer
Lagoons (UPDES
#UT0024511)
1,356 lbs/yr 100 % 0 lbs/yr
Total Existing Load 1,637 lbs/yr Load Allocation 271 lbs/yr
Wasteload Allocation 72 lbs/yr
Total Annual Load Reduction = 78%
Margin of Safety1
18 lbs/yr
TMDL = Loading Capacity =

361 lbs/yr
1 Margin of safety. The MOS was included in the analysis explicitly by allocating 5 percent of the loading
capacity to the MOS.

Table 6-2. Summary of TDS TMDL for Ashley Creek watershed.

Source
Annual existing
TDS load
Estimated percent
reduction
Annual allocated TDS
load
Non-point Sources:
Ashley Creek watershed 25,580 tons/yr 7 % 23,806 tons/yr
Point Sources:
Ashley Valley Water
Reclamation Facility
(UPDES #UT0025348)
1,667 tons/yr 0 % 3,794 tons/yr
Ashley Valley Sewer
Lagoons (UPDES
#UT0024511)
9,000 tons/yr 100 % 0 tons/yr
Total Existing Load 36,247 tons/yr Load Allocation 23,806 tons/yr
Wasteload Allocation 3,794 tons/yr
Total Annual Load Reduction = 20% Margin of Safety1 1,453 tons/yr
TMDL = Loading Capacity =

29,053 tons/yr
1 Margin of safety. The MOS was included in the analysis explicitly by allocating 5 percent of the loading
capacity to the MOS.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

38

6.4 Pollutant Loading Scenarios

The TMDL process is designed to establish the total loading a stream can assimilate
without causing violation of the water quality standards. Because of the complex
hydrology, the interconnectedness of the sources, and the location and temporal record
of the monitoring data, the TMDLs do not distinguish between the contribution of Se and
TDS from the various tributaries. Therefore, the TMDL analyses focus on and establish
the TMDLs for the entire watershed of Ashley Creek based on flow. The TMDLs are
calculated on an annual basis to account for complex and varying hydrology and critical
conditions in the watersheds and consistent violations of Se and TDS water quality
standards.

6.4.1 Existing Conditions

The existing conditions represent Se and TDS loadings in the Ashley Creek watershed
calculated from existing monitoring data. As discussed in Section 5.3, existing loads
were calculated for days that had recorded Se and TDS concentrations. These
individual daily loadings were calculated by multiplying the observed Se and TDS
concentrations by the flow for that day and were used to establish the existing loading
for each parameter (Figures 5-3 and 5-4). These lines represent the estimated existing
Se and TDS loads within each flow percentile group occurring over the 10-year analysis
period. The area under the existing loading curve represents the total loading over the
analysis period. As summarized in Tables 6-1 and 6-2, the average annual existing Se
and TDS loading in the Ashley Creek watershed is 1,637 lbs/yr and 36,247 tons/yr
respectively.

6.4.2 Loading Capacity

As discussed in Section 5.1, USGS observed flows were used with the 5 ug/L target to
establish the Se loading capacity over the range of observed flows (as shown in Figure
5-5). This results in an average annual loading capacity of 361 lbs/yr of Se for Ashley
Creek (Table 6-1). Because 5 percent of the TMDL loading capacity is reserved for the
margin of safety the allocatable portion of the loading capacity for Se is 343 lbs/yr.

Likewise, USGS observed flows were used with the 1,200 mg/L target to establish the
loading capacity for TDS (Figure 5-6). The analysis resulted in an average annual
loading capacity of 29,053 tons of TDS for Ashley Creek, as summarized in Table 6-2.
After the 5 percent reservation for the margin of safety, the allocatable portion of the
loading capacity for TDS is 27,600 tons/yr.

To illustrate the range of reductions needed at various flow and loading conditions for
the TMDLs, Tables 6-3 and 6-4 present the loading capacity and average existing loads
used to develop the existing loading (and, therefore, the annual existing loads) in Ashley
Creek.
Ashley Creek Selenium and Total Dissolved Solids TMDLs

39

Table 6-3. Se load capacity, existing load, and necessary reduction at various
flows.
Flow range
Avg. Flow
(cfs)
Existing load
(lbs/day)
Loading
Capacity
(lbs/day)
Percent
reduction
0%-10% 1.1 0.17 0.03 82%
10%-20% 3.2 0.78 0.09 88%
20%-30% 7.1 1.94 0.19 90%
30%-40% 12.5 3.84 0.34 91%
40%-50% 17.4 4.65 0.47 90%
50%-60% 20.0 5.46 0.54 90%
60%-70% 23.0 5.54 0.62 89%
70%-80% 33.6 7.07 0.91 87%
80%-90% 81.4 7.58 2.20 71%
90%-100% 167.3 7.83 4.51 42%
Average Annual Load* 1,637 lbs/yr 361 lbs/yr 78%
*Calculated by multiplying the average of existing loads and load capacities by 365.

Table 6-4. TDS load allocation, existing load, and necessary reduction at various
flows.
Flow range
Avg. Flow
(cfs)
Existing load
(lbs/day)
Loading
Capacity
(lbs/day)
Percent
reduction
0%-10% 1.1 10,916 6,842 37% 10%-20% 3.3 36,472 21,602 41% 20%-30% 7.5 80,888 48,479 40% 30%-40% 12.3 129,866 79,289 39% 40%-50% 16.9 157,086 109,314 30% 50%-60% 19.9 198,165 128,642 35% 60%-70% 24.4 249,136 157,930 37% 70%-80% 32.4 269,804 203,979 24% 80%-90% 72.3 342,673 324,769 5% 90%-100% 561.6 511,116 511,116 0%
Average Annual Load (tons)* 36,247 29,053 20%
*Calculated by multiplying the average of existing loads and load capacities by 365.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

40

6.4.3 Waste Load Allocation

Practically all of the Se and TDS load reductions needed to meet the TMDL have been
allocated to the Ashley Valley Sewage Lagoons. According to studies conducted by the
Bureau of Reclamation and USGS the loading attributable to the sewage lagoons totals
approximately 2,000 pounds of selenium and 9,000 tons of salt per year to Ashley
Creek (USDI, 1997). With the closure of the lagoons the seeps have begun to dry up
and the latest Se and TDS data from Ashley Creek (493721) shows that loads of Se
and TDS are decreasing (Figure 6-1). The USGS is continuing to monitor the seeps in
a study scheduled to be completed in September of 2003 (Naftz, pers. comm.).
Preliminary data shows that the seeps are decreasing in flow and Se concentrations are
declining, but there is no identifiable trend in TDS concentrations which is similar to the
USGS’s findings in Colorado (Butler, 2001).

Figure 6-1. Se and TDS loads and trends from 1998-2002.
0
50
100
150
200
250
300
350
1998 1999 2000 2001 2002
Year
TD
S

l
o
a
d

(
t
o
n
s
)
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
Se

l
o
a
d

(
p
o
u
n
d
s
)
Total Dissolved Solids Selenium TDS Trend Line Selenium Trend Line
Load allocations for Se and TDS have also been established for the AVWRF, based
upon the chronic water quality standard concentration of 5 ug/L for selenium and no
more than a 400 mg/L increase in TDS concentration from the culinary source, along
with the plant’s designed capacity outflow of 4.7 million gallons per day. Since the
current permit limit for TDS is relative to the concentration at the culinary source, the
maximum concentration recorded of 130 mg/L plus the permitted 400 mg/L increase
was used in calculating the load allocation to the AVWRF. However, this will not
supercede the current permit limit of no more than a 400 mg/L increase from the
culinary source. It is recognized that there is the potential for the culinary source to be
higher than 130 mg/L, which would not exceed the TMDL unless the annual average
concentration also exceeded 130 mg/L in which case the load allocation established by
Ashley Creek Selenium and Total Dissolved Solids TMDLs

41

this TMDL would be revised accordingly to account for the natural change in water
chemistry.

It should also be noted that current concentrations of Se from the AVWRF’s discharge
fall well below its established permit limit. The AVWRF’s UPDES permit also states that
if Se is not detected, or is found in amounts substantially below the permit limits then
the AVWRF’s monitoring requirement for Se could be eliminated after one year (12
samples). Given the low concentrations of Se currently being discharged by the
AVWRF it is proposed that this provision be honored with the State continuing to
periodically monitor Se as part of its regular monitoring program. If, at the time of the
AVWRF’s permit renewal date of January 31, 2006, Se concentrations have not
exceeded the permit limit it is proposed that the AVWRF may petition to have Se
removed from its UPDES permit.

6.4.4 Load Allocation

Because of the natural geologic sources of Se and TDS that underlie Ashley Valley
there will always be some Se and TDS non-point source loading into Ashley Creek.
However, due to the past and on-going implementation of salinity control projects within
the watershed, the proportion of loading directly attributable to human causes (e.g.
irrigation return flows) has and will likely continue to decline. Improving riparian habitat
and reducing streambank erosion will also reduce Se and TDS loading, thereby
improving aquatic habitat as well. Although only a small non-point source load
reduction is needed it is anticipated that efforts to reduce salt loading and improve
aquatic habitat will continue with the provision of cost-share funding and other grants
directed at improving water quality.
Ashley Creek Selenium and Total Dissolved Solids TMDLs

42

7. Potential Control Options

Although practically all of the required Se and TDS load reductions have been allocated
to the Ashley Valley Sewage Lagoons it is important to recognize that other sources
associated with natural background (streambank erosion) and non-point sources
(irrigation return flows) can and should be controlled to the maximum extent practicable
on a voluntary basis through the implementation of best management practices.

Potential options for reducing non-point source Se and TDS loads in the Ashley Creek
watershed include:

− Increase irrigation efficiency by providing sprinkler irrigation, properly scheduling
irrigation turns, reducing flood length and leveling land.
− Line canals and ditches with open concrete lining or replace them with pipe.
Seepage losses in canals and ditches can result in mineral pickup and return flow to
streams through springs and drains.
− Construct weirs at turnouts to ensure that proper amounts of water are applied.
− Maintain grassed waterways and construct check dams on return flows.
− Maintain uncultivated buffer strips along streams and channels.
− Re-establish and preserve existing flood plains along Ashley Creek through planning
and zoning

In addition to reducing deep percolation of irrigation water it is anticipated that
controlling soil erosion from streambanks and uplands will also reduce Se and TDS
loading since soils in the lower watershed are slightly to highly saline. Potential control
options for reducing soil and streambank erosion include:

− Promoting proper grazing management on uplands and riparian areas to maintain
sufficient plant cover to protect the soil.
− Improve condition of riparian areas through plantings, temporary grazing exclusion
and development of alternate watering sites.
− Stabilize streambanks through planting deep rooted species of woody plants,
placement of rock barbs and revetment to deflect flow away from erosive banks and
sloping vertical streambanks to reconnect the stream channel to its floodplain and
allow native vegetation to re-establish.

The Ashley Creek Stabilization/Restoration Report, completed in May of 2000 for Uintah
County and the Uintah County Water Conservancy District, summarized the results of
several studies designed to address problems along Ashley Creek including chronic
flooding during spring run-off, lack of riparian habitat and fisheries, water quality and low
flow summer conditions. After extensive discussion and consultation with local officials
and the general public the authors of the report recommended the construction of an
off-stream reservoir within the intermittent Spring Creek drainage to store peak flood
stage flows in the spring for gradual release over the summer to provide instream flows
necessary for the establishment of riparian habitat and a fishery (Franson-Noble, 2000).
Work on this project has begun with a detailed site investigation of the proposed dam-
Ashley Creek Selenium and Total Dissolved Solids TMDLs

43

site location currently underway. It is anticipated that this project would ultimately
benefit water quality downstream, particularly during the summer, by diluting
groundwater base-flows high in TDS that enter the channel in lower reaches.

These TMDLs are based on a representative flow regime that is determined using
historical flow records. Therefore the allocated loadings and associated load reductions
are calculated to meet water quality standards assuming the flow conditions remain
similar to those established in the TMDL. However, it is possible with salinity control
efforts focusing on decreasing TDS loads that instream TDS concentrations may
increase. This could be the result of less dilution water available from flood irrigation
return flows or higher TDS concentrations of groundwater baseflow. To offset this, the
control options for the Ashley Creek watershed should focus on minimizing deep
percolation of irrigation water through improving the efficiency of irrigation practices and
conveyances on saline soils.

To address the possibility that implementation may lead to increased instream TDS
concentrations and non-attainment of water quality standards this TMDL will utilize an
approach that provides for the implementation of load reduction strategies while
continuing to collect additional data. If when the load reductions identified in this TMDL
are attained or a reasonable effort towards implementation has occurred, and water
quality standards are still violated, the TMDL will be revised accordingly based upon the
additional data collected. Regardless of the short-term effect on instream flows and
concentrations, the available and recommended control efforts should improve irrigation
efficiencies and water quality will ultimately benefit.

The reasonable assurance that these implementation activities will occur and attempt to
meet the load reduction goals is that implementation is currently ongoing under the
cooperative efforts of local agricultural producers and the USDI/USDA Salinity Control
Program. In fact, approximately 17,000 acres of irrigated land within the watershed
have already been treated. There is a great deal of local interest among watershed
stakeholders to participate in the salinity control program. The availability of cost-share
funding is the primary limitation on implementation. It is anticipated that with the
establishment of this TMDL for the Ashley Creek watershed some of the funding
shortfalls will be alleviated with 319 funding along with the priority status of other
sources of funding associated with approved TMDL watersheds.

Ashley Creek Selenium and Total Dissolved Solids TMDLs

44

8. Future Monitoring

Continued water quality monitoring is essential to evaluate the effects of best
management practices as well as progress towards meeting water quality goals and
beneficial use support. In addition to the regular and intensive monitoring already
conducted by the state and USGS, additional monitoring of shallow wells is
recommended to determine the influence of groundwater and deep aquifers on surface
water Se and TDS concentrations. Periodic monitoring will also be needed on the
ephemeral drainages to the west of Ashley Creek, particularly Mantle Gulch, to
determine the influence of irrigation return flows versus natural precipitation on Se and
TDS loading.

In addition to regular water quality monitoring, upland and riparian areas should be
monitored periodically. The purpose for monitoring these areas is to identify where
significant sources of sediment and salt originate from. This monitoring will be
conducted through the cooperative efforts of the Uintah County Soil Conservation
District and the Ashley Creek Stabilization and Restoration Committee.

It is important to recognize that data collection in support of this TMDL is an ongoing
effort and that as new data is collected this TMDL will be revised as appropriate.
Ashley Creek Selenium and Total Dissolved Solids TMDLs

45

9. Public Participation

Public participation for this TMDL was accomplished through a series of public meetings
with the local Uintah County Soil Conservation District and the Ashley Creek
Stabilization and Restoration Committee (ACSRC). The ACSRC is comprised of
individuals representing key interests within the watershed.
A public hearing on the TMDLs was held on March 5, 2003 with notification of the
hearing published in the local newspaper on February 24, 2003 (Vernal Express). The
comment period was opened on February 24 and closed on March 18, 2002. Formal
comments and responses are included in Appendix B.
In addition, the TMDL and dates for public comment were posted on the Division of
Water Quality’s website at (www.deq.state.ut.us/EQWQ/TMDL/TMDL_WEB.HTM).

Ashley Creek Selenium and Total Dissolved Solids TMDLs

46

10. References

Bureau of Reclamation. 1957. Vernal Unit, Central Utah Project, Definite Plan Report,
May 1957. Appendix A, Project Lands, Land Drainage.

Butler, D.L. 2001. Effects of Piping Irrigation Laterals on Selenium and Salt Loads,
Montrose Arroyo Basin, Western Colorado. USGS/WRIR 01-4204.

Evaluation of Salinity and Selenium Loading to Lower Ashley Creek, Uintah County,
Utah. UT-00-259. U.S. Bureau of Reclamation and U.S. Geological Survey. B.J. Stolp.
http://wwwdutslc.wr.usgs.gov/publications/Activities00/Text.html#UT_259

Franson-Noble. 2000. Ashley Creek Stabilization/Restoration Report, Spring Creek
Dam and Reservoir.

Naftz, D. 2002. Personal communication (regarding Selenium and Total Dissolved
Solids loading from Ashley Valley Sewer Lagoons).

Stephens, D.W., B.W. Waddell, L.A. Peltz, J.B. Miller. 1992. Detailed Study of Selenium
and Selected Elements in Water, Bottom Sediment, and Biota Associated with Irrigation
Drainage in the Middle Green River Basin, Utah, 1988-90. USGS/WRIR 92-4084.

U.S. Dept of Interior, National Irrigation Water Quality Program. 1997. Middle Green
River Basin Study, Stewart Lake Waterfowl Management Area, Final Environmental
Assessment. PRO-EA-97-001.

Utah Division of Water Resources. 1999. Utah State Water Plan, Uintah Basin. Salt
Lake City, Utah.

Utah Division of Water Quality. 2001. Standards of Quality for Waters of the State, Utah
Administrative Code R317-2. Salt Lake City, Utah.

Williams, K.T. and H.G. Byers. 1935. Occurrence of selenium in the Colorado River and
some of its tributaries: Industrial and Engineering Chemistry. 7:6:431-432.