DWQ-2008-020003

MILL HOLLOW RESERVOIR

LIMNOLOGICAL ASSESSMENT OF WATER QUALITY

March 2008

1
Mill Hollow Reservoir Report
March 6, 2008

Mill Hollow Reservoir is listed by the State of Utah as an impaired water body because of
high total phosphorus concentrations and pH values that exceed State water quality
standards (Utah, State of. 2006). In partnership with Utah Division of Water Quality
(UDWQ), the Uinta National Forest (UNF) collected data from Mill Hollow Reservoir
from March 2006 to August 2006 to provide recent detailed water quality information to
support a Total Maximum Daily Load (TMDL) analysis. This report contains
information listed below.
• Sections 1.0 and 2.0: Description of the water body and associated watershed, the
nature of the impairment and water quality standards for the parameters of
concern for the water body.
• Section 3.0: Discussion of whether the impairments are naturally occurring and,
if not, what water quality targets and endpoints should be recommended.
• Section 4.0: Discussion of which land management activities are contributing to
the impairment, what practices may be recommended to reduce sources of
impairment, and an estimate of the acceptable load or the degree to which the
current pollutants (loads) need to be decreased to attain the defined endpoints.
• Section 5.0: Identification of significant pollutant sources through use of existing
information (maps, reports, inventories, and analyses) and new data.
• Section 6.0: Description of water quality data in relationship to abiotic and
biological processes.
• Section 7.0: An evaluation of all sources contributing to impairment and a
determination of beneficial use support.
• Section 8.0: The rationale for addressing all sources and causes that are
significant for the attainment of water quality targets.
2

Figure 1. Mill Hollow Reservoir Location Map
3
1.0 Introduction

Mill Hollow Reservoir is an artificial stabilized lake maintained by the Utah Division of
Wildlife Resources to provide angling opportunities for Utahns. The reservoir was
created in 1962 by the construction of an earth-filled dam and has a surface area of 15
acres, volume of 315 acre-feet, and a mean depth of 5.4 meters (18 feet). It is located at
an elevation of 8,843 feet above mean sea level (Judd 1997). In September 2006, the
reservoir was drained to allow work to be done on the dam and is expected to be empty
until work is completed, possibly in the fall of 2009.

Surface inflows to the reservoir consist of Mill Hollow Creek to the south, a small
spring/wetland area to the east, and two small seeps on the north side of the reservoir.
Water leaves the reservoir via Mill Hollow Creek which is about 6 feet wide above the
reservoir and 10 feet wide below the reservoir. From data collected in 2006 and 2007,
estimates of water flowing into Mill Hollow Reservoir range from 1.1 cubic feet per
second (cfs) during low flows to 9.1 cfs during high flows. The average stream gradient
on Mill Hollow Creek is 6.9 percent (Judd 1997).

The watershed above the lake is rather small, about 168 acres in size. The watershed
receives 30 inches of precipitation annually with a frost-free season of 20-40 days. Most
of the precipitation occurs in the form of snow that falls during the winter. The soil is of
limestone origin and has good permeability and moderately slow erosion and runoff. The
vegetation communities in the watershed are aspen, spruce-fir, and alpine meadows. The
watershed is made up of high mountains with many rock outcroppings characteristic of
the Wasatch Plateau (Judd 1997).

Many land uses occur within the watershed draining into the reservoir with grazing and
recreation being the dominant uses. The shoreline around the reservoir is owned and
managed by the Forest Service with unlimited public access. A 46-unit campground is
located above Mill Hollow Reservoir and has a gravel road through it with restrooms
south of the reservoir and drinking water from a spring nearby. Several dispersed
campsites are located in the drainage above the reservoir. Other land uses include ATV
riding and snowmobiling, camping, hunting and fishing (Judd 1997). Old timber harvest
units are located in the watershed above the reservoir that have since re-vegetated with no
evidence of accelerated erosion.

The ground cover in the watershed above the reservoir is in good condition. The main
road through the canyon near the reservoir is gravel surfaced and well maintained. The
shore surrounding the reservoir has dense vegetation in most areas. The riparian area
along Mill Hollow Creek is in good condition and shows no sign of bank erosion. The
dispersed campsites above the reservoir have user created two-track roads which lead to
Mill Hollow Creek.

4
2.0. Water Quality Standards

This section discusses the associated impairment and parameters of concern with respect
to state water quality standards, antidegredation policies, and designated beneficial uses
for Mill Hollow Reservoir.

The State of Utah has designated the waters within the lake as Antidegradation Segments
indicating that the existing water quality is better than the established standards for the
designated beneficial uses. Water quality is required by state regulation to be maintained
at this level. The beneficial uses of streams within the Forest, as designated by the Utah
Department of Environmental Quality, Division of Water Quality, are Class 2B –
protected for recreation; Class 3A – protected for cold water species of game fish and
other cold water aquatic species; and Class 4 – protected for agricultural uses (Utah, State
of 2005).

Mill Hollow Reservoir is listed as impaired for total phosphorus and pH for Cold Water
Species of Game Fish (Beneficial Use Class 3A). The methodology for listing for these
parameters is described below.

Listing Methodology for Total Phosphorus – Total phosphorus does not directly affect
aquatic life, but as a nutrient it can stimulate growth of aquatic algae and emergent plants.
Nuisance blooms of algae and other aquatic plants can have an effect on the amount of
dissolved oxygen and habitat that fish and macroinvertebrates occupy. The assessment
methodology employed by Utah for total phosphorus in lakes and reservoirs states that an
assessment unit needs further study if more than 10% of the phosphorus concentration
measurements at given depths (21,23, 27, 29) exceed 0.025mg/l. If further study is
Figure 2. Total Phosphorus as P - Mill Hollow
Reservoir
0.000
0.050
0.100
0.150
0.200
0.250
0.300
Aug-80
Jun-81
Jun-81
Jun-89
Aug-89
Mar-90
Jun-91
Aug-91
Jun-93
Sep-93
Aug-95
Aug-95
Sep-95
Jul-97
Sep-97
Jun-99
Aug-99
Jul-01
Sep-01
Jul-03
Sep-03
Jul-05
Sep-05
Mar-06
Apr-06
May-06
Jun-06
Aug-06
Aug-06
Date
T
P

(
m
g
/
21: Surface
23: 1 meter
above
thermocline27: 1 meter
below
thermocline29: 1 meter
from bottom
5
needed, other factors such as fish kills, low dissolved oxygen, amount of blue-green
algae, and the Trophic State Index (TSI) are considered in the evaluation (Utah, State of.
2007).

Listing Methodology for pH – The listing methodology employed by Utah for pH to
assess Class 3A (aquatic life) beneficial use involves looking at pH profile data collected
at the surface and at one meter intervals against the pH standard of greater than 6.5 and
less than 9.0. For a given monitoring cycle, the beneficial use is supported if the number
of violations are less than or equal to 10 percent (≤10%) of the measurements.

An assessment of the water quality conditions in Mill Hollow Reservoir in 1997 (Judd
1997) is described below.

The water quality of Mill Hollow Reservoir is good. It is considered to be soft with a recent hardness
concentration value of approximately 68 mg/L (CaCO3). The only parameters that have exceeded
State water quality standards for defined beneficial uses are phosphorus and dissolved oxygen. On
occasion pH values will exceed the criteria of 9.0 when a heavy algal bloom is in progress. The
average concentration of total phosphorus in the water column in 1981 and 1991 was 135 and 43
ug/L which exceeds the recommended pollution indicator for phosphorus of 25 ug/L. The
phosphorus concentration in the hypolimnion in September, 1991 reached a level of 118 ug/L. This
increased concentration occurred when the reservoir was stratified, and low dissolved oxygen was
present near the bottom. Dissolved oxygen concentrations in late summer substantiate the fact that
water quality impairments do exist. Concentrations dropped dramatically below the thermocline to
approximately 1.4 mg/L during the summer. A review of a reservoir profile obtained on March 28,
1990 indicates that anoxic conditions are prevalent throughout the winter. Concentrations of
dissolved oxygen were 1.8 mg/L at 1 meter, 1.2 mg/L at 2 meters and virtually 0.0 down to a depth
of 9 meters. These conditions are deleterious to the fishery rendering approximately the entire
reservoir unsuitable for a fishery. It is apparent that the only carryover of fish would be in the inlet
area where oxygen supplies are sufficient to maintain a limited fishery. Current data suggest that the
Figure 3. Mill Hollow Reservoir: pH vs. Depth
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0
pH
De
p
t
h

(
m
)
3/23/2006
4/13/2006
5/31/2006
6/29/2006
8/2/2006
8/30/2006
7/21/2001
9/5/2001
9/16/2003
7/5/2005
9/1/2005

6
reservoir is a nitrogen limited system. TSI values indicate the reservoir is mesotrophic approaching
the eutrophic range with fairly high productivity.

The reservoir was stratified during a summer monitoring trip on August 21, 1991. The profile
indicates that a thermocline developed at a depth of 3-7 meters. This is consistent with a noticeable
decline in the concentration of dissolved oxygen in the water column.

According to DWR no fish kills have been reported in recent years, however it is evident that some
winter kills may occur. The reservoir supports populations of brook trout (Salvelinus fontinalis),
rainbow trout and albino rainbow trout (Oncorhynchus mykiss). The lake has not been treated for
rough fish competition, so populations of native fishes may still be present in the lake. According to
the Utah State Division of Wildlife Resources, Mill Hollow Reservoir is regularly stocked with
8,000 catchable rainbow trout, 4,200 catchable albino rainbow trout, and 7,500 fingerling brook
trout. DWR also reports that the water flea, Daphnia, is also present in the reservoir.

The phytoplankton community is dominated by flagellates, diatoms and blue-green algae. This
supports the water quality analysis of the reservoir with moderate productivity and generally good
water quality.

Pollution Assessment
Nonpoint pollution sources include the following: sedimentation and nutrient loading from grazing
and litter or waste from recreation. Grazing takes place throughout the watershed.
There are no point sources of pollution in the watershed.

Water quality data collected since 2001 indicate that the assessment described above is an
accurate description of water quality conditions in Mill Hollow Reservoir. Total
phosphorus exceeded the State pollution indicator value in 42 of 76 samples (55.3 % of
samples) between 1980 and 2005 and exceeded State standards in 12 of 20 samples (60%
of samples) in 2006. Since 2001, Mill Hollow Reservoir exceeded the pH State standard
of 9.0 in 17 out of 90 samples (19% of the samples). In 2006, Mill Hollow Reservoir
exceeded the pH State standard of 9.0 in 9 out of 45 samples (20% of the samples).

3.0 Water Quality Targets/Endpoints

This section discusses whether the impairments are naturally occurring and if not, what
quantifiable targets or endpoints will achieve water quality standards.

A review of potential pollution sources have identified a few sources of man-made
pollution that may be contributing to elevated total phosphorus and pH in the reservoir.
These are discussed in Sections 4.0 and 5.0. The primary recommended endpoints for
Mill Hollow Reservoir based on standards are at least 90% of mean in-lake
concentrations of total phosphorus are less than or equal to 0.025 mg/l, and for pH, at
least 90% of the pH measurements are within the pH standard range of 6.0 to 9.0.

7

4.0 Beneficial Use Assessment

This section discusses which land management activities are potentially contributing to
the impairment, what practices may be recommended to reduce sources of impairment,
and, if applicable, an estimate of the acceptable load or the degree to which the current
pollutants (loads) need to be decreased to attain the defined endpoints.

There area no point sources of pollution within the Mill Hollow Reservoir watershed. The
primary man-made non-point sources of total phosphorus are likely to be from sediment
that enters the reservoir and a few sources have been identified in the drainage above the
reservoir. Dispersed camping is occurring along the drainage bottom above the reservoir
and dirt access roads are located near the drainage bottom. These roads are causing
erosion and the close proximity of the dispersed sites and dirt roads to the drainage
bottom are likely to be causing sediment to enter the stream that flows into the reservoir.
Other potential sources do not appear to contribute to phosphorus entering the reservoir.
Human waste is contained in vault toilets that are functioning and maintained properly.
Timber harvest activities are not contributing sediment because the land surface is well-
vegetated and trees in the harvest units have grown back.

Recommended practices that may reduce total phosphorus and pH levels in the reservoir
are:
• Identify dispersed camping areas that are authorized under Forest Plan Travel
Management, but may need to be closed or need to use best management practices
to control soil erosion.
• Limit vehicle access in the drainage above the reservoir by decommissioning and
rehabilitating unauthorized dirt roads above the reservoir.

It is likely that these practices will reduce some of the total phosphorus in the reservoir
but it is not known whether that these practices will result in meeting the endpoints
(meeting State water quality standards).

5.0 Significant Sources

The main sources of man-made pollution are from soil erosion and sedimentation
including dispersed camping areas and dirt roads located in the drainage bottom in the
area above the reservoir.

6.0 Technical Analysis

This section contains a description of water quality data conditions at Mill Hollow
Reservoir and at the end, a discussion containing a summary by Bronmark and Hansson
(2005) of abiotic and biological processes that occur in lakes and ponds and a comparison
of these concepts with the water quality conditions of Mill Hollow Reservoir.
8

Trophic State – Carlson’s Trophic State Index (TSI) is used to determine the living
biological material or biomass of a lake and uses a continuum of states to indicate the
amount of biomass of the lake. The TSI for a lake can be determined using regression
equations and values for chlorophyll a, secchi depth, and total phosphorus. Carlson states
that the best parameter to use for the index is chlorophyll a and transparency should be
used only if no other parameter is available (Kent State 2005).

Based on sampling from 1989, the trophic states using chlorophyll-a are oligotrophic to
mesotrophic (TSI (Chl) of 30-45) during winter and spring seasons and are mesotrophic
to eutrophic (TSI (Chl) of 45-55) in the summer and fall seasons. The clarity of the water
reflects the chlorophyll-a pattern but indicates a slightly higher trophic level. The trophic
state using secchi depth and total phosphorus gave trophic levels that were between upper
mesotrophic and eutrophic. Total phosphorus samples were near or below the detection
limit.

Carlson presents characteristics of northern temperate lakes based on the trophic state and
says that when lakes become mesotrophic, the hypolimnia of shallow lakes is likely to
become anoxic and that may result in a loss of salmonids and when lakes are eutrophic,
the hypolimnia is anoxic and macrophyte problems are possible (Kent State 2005). Mill
Hollow Reservoir has these characteristics throughout most of the year when the lake is
mesotrophic and eutrophic.

Lake Morphology – Mill Hollow is somewhat square-shaped and is about 1,162 feet
wide, 1,426 feet long, and has a mean depth of 18 feet (5.4 meters).
Figure 4. Mill Hollow Reservoir: Trophic State Indicies
0
10
20
30
40
50
60
70
80
Aug
-80
Ju
n-89
Ju
n-91
Jun-93
Aug
-95
Jul-97
Ju
n-99
Jul-01
Jul-03
Jul-05
Mar-06
May-06
Aug
-06
Date
TS
I

V
a
l
u
Chlorophyll A
Total Phosphorous
Secchi
9

Temperature – The temperature of Mill Hollow Reservoir ranges in winter from 0ºC to
4.9 at the surface to 3.2ºC at the bottom and in summer from 12.7 to 16.8ºC at the surface
and from 4.8 to 14.0ºC near the bottom at a depth of about 7.0 meters. The temperature
profile indicates that the reservoir is weakly stratified during the summer season.

Figure 5. Mill Hollow Reservoir: Temperature vs. Depth
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0Temperature ( degrees C)
De
p
t
h

(
m
)

3/23/2006
4/13/2006
5/31/2006
6/29/2006
8/2/2006
8/30/2006
7/21/2001
9/5/2001
9/16/2003
7/5/2005
9/1/2005

Light (secchi depth, chlorophyll concentration) –The secchi depth in Mill Hollow
Reservoir in 2006 ranged from 1.5 to 2.5 meters. In April 2006, ice was 1.5 meters thick
with one foot of slush and 0.5 feet of snow covering the slush. From May to August the
reservoir was ice-free.

Catchment Area (size of catchment, type of geology) – The watershed draining into
Mill Hollow Reservoir is about 168 acres in size and is located within the Wasatch
Montane Zone of the Wasatch and Uinta Mountains Level III ecoregion. This zone is
partially glaciated with surficial Quaternary glacial till, collovium and alluvium. (Woods
et al. 2001). The reservoir is located within a phosphate deposit zone identified by the
Bureau of Mines Special Report "Availability of Federally Owned Minerals for
Exploration and Development in Western States: Utah, 1988." These deposits,
depending on their proximity to the surface, may contribute phosphorus loading into the
reservoir.

The reservoir is located in a northeast-southwest oriented valley that is about 0.7 mile
wide and the distance from the reservoir to the southwest end of the valley is 2.8 miles.
The valley is v-shaped and has steep side slopes off of the ridges that are 1,000 higher
than the valley bottom. The predominant vegetation type in the valley bottom is
sagebrush/mountain brush and on the steep sideslopes are aspen and conifer.

10
pH – The pH values for water samples collected in Mill Hollow Reservoir are between
6.6 and 12.0. As seen in Table 1, pH is more alkaline in the upper half of the water
column during summer/fall and generally returns to less alkaline conditions in the winter.
Since 2001, Mill Hollow Reservoir exceeded the pH State standard of 9.0 in 17 out of 90
samples (19% of the samples). In 2006, Mill Hollow Reservoir exceeded the pH standard
of 9.0 in 9 out of 45 samples (20% of the samples). The pH of the streams flowing into
the reservoir was measured in 2006 and values are fairly constant throughout the year and
range between 7.8 and 8.5. As shown in Table 2, the pH of the water flowing into the
reservoir is alkaline but does not exceed State standards for pH. On August 3, 2006, the
water flowing out of the reservoir is at the upper limit of the pH standards (9.0). All other
samples collected below the reservoir are below the pH standard. Mill Hollow Reservoir
is alkaline and most of the pH values are typical of most lakes of the earth. According to
Bronmark and Hansson (2005), the majority of lakes on earth have a pH between 6 and 9.

Table 1. Mill Hollow Reservoir pH field data.
Depth (m)
Date 0.5 1.0 2.0 3.0 4.0 5.0 6.0 7.0 7.4 8.0 8.3 8.6
07/21/2001 12.0 9.1 9.1 9.0 8.7 8.5 8.1 7.7 ---- ---- ---- ----
09/05/2001 8.0 8.0 8.0 8.0 8.0 7.8 7.1 6.9 6.6 ---- ---- ----
09/16/2003 8.3 8.3 8.3 8.3 8.3 8.3 8.2 8.2 ---- 8.0 8.0 ----
07/05/2005 8.3 8.4 8.4 8.6 8.5 8.2 7.9 7.8 ---- 7.7 ---- ----
09/01/2005 9.4 9.4 9.4 9.3 8.9 8.5 8.3 8.2 ---- 8.1 ---- 8.0
03/23/2006 8.1 ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
04/13/2006 8.7 8.8 8.4 8.3 8.2 8.1 8.0 7.9 ---- 7.9 ---- ----
05/31/2006 8.2 8.1 8.1 8.0 8.0 8.0 7.9 7.6 ---- 7.5 ---- ----
06/29/2006 8.3 8.3 8.4 8.5 8.6 8.2 7.5 7.3 ---- 7.1 ---- ----
08/02/2006 9.2 9.2 9.1 9.1 8.6 8.1 7.9 7.8 ---- 7.7 ---- ----
08/30/2006 9.4 9.3 9.3 9.3 9.3 8.1 7.5 7.3 ---- ---- ---- ----
Note: Values were rounded off to the nearest depth. Red highlighted values indicate pH exceeds
standard.
11

Nutrients – Table 2 contains a summary of exceedances and Table 4 contains the
dissolved and total phosphorus and nitrogen concentrations in Mill Hollow Reservoir for
samples collected since 1980. For calculating the following averages, the assumption is
made that a sample that had a non-detect was given a value 0.01 mg/L, one half the
detection limit. Since 2001, most of the total phosphorus is in the dissolved fraction. For
all sampling events, the average concentration of total phosphorus as P at the surface (21)
is 0.028 mg/l, above the thermocline (23) is 0.023 mg/l (excluding a suspiciously high
value of 0.765 mg/L collected on 7/17/1997), below the thermocline (27) is 0.03 mg/l,
and just above the bottom (29) is 0.098 mg/l. For all samples collected, nitrogen as
dissolved nitrite+nitrate was well below the standard of 4.0 mg/l and most of the samples
(68%) did not detect nitrogen.

Table 2. Summary of Total Phosphorus exceedances and concentrations.

Time Period Number of
Exceedances
Number of
Samples
Percent of
Exceedances Average Concentration (mg/l)
1980 – 2005 42 76 55.3 0.056
2006 12 20 60.0 0.020
1980 - 2006 54 96 56.3 0.048

Table 3. Mill Hollow Reservoir - Nutrients by depth level.

Dissolved Total Phosphorus
as P (mg/l) Total Phosphorus as P (mg/l)
D-NO2+NO3, N
(mg/l)
Date 21 23 27 29 21 23 25 27 29 21 23 27 29
08/07/1980 ----- ----- ----- ----- 0.060 ----- ----- ----- 0.620 0.10 ----- ----- 1.00
06/10/1981 ----- ----- ----- ----- 0.030 ----- ----- ----- ----- 0.10 ----- ----- -----
06/16/1981 ----- ----- ----- ----- ----- ----- ----- ----- 0.240 ----- ----- ----- -----
06/14/1989 ----- ----- ----- ----- 0.042 0.036 0.020 ----- 0.016 ----- ----- ----- -----
08/30/1989 ----- ----- ----- ----- ND 0.048 ND ----- ND ----- ----- ----- -----
03/28/1990 ND ----- ----- ----- ----- ----- ----- ----- 0.244 ----- ----- ----- -----
06/12/1991 0.019 0.021 0.022 0.024 0.042 0.039 ----- 0.044 0.047 0.22 0.21 0.25 0.22
08/21/1991 ND 0.040 0.025 0.083 0.018 0.011 ----- 0.025 0.118 ND 0.04 ND ND
06/30/1993 0.024 0.021 0.055 0.062 0.023 0.022 ----- 0.094 0.139 0.05 0.03 0.03 0.10
09/06/1993 ND ND ND 0.037 ND 0.034 ----- 0.011 0.070 ND ND ND ND
08/01/1995 0.010 ----- ----- ----- 0.020 ----- ----- ----- ----- ND ----- -----
08/02/1995 ----- 0.010 ND 0.030 0.030 ----- 0.030 0.060 ----- ND ND ND
09/21/1995 ----- 0.010 ----- 0.060 0.020 0.020 ----- ----- 0.080 ND ND ----- 0.03
07/17/1997 ----- ----- ----- ----- 0.077 0.765 ----- 0.060 0.083 ND ND ND ND
09/05/1997 ----- ----- ----- ----- ----- ----- ----- ----- ----- ND 0.02 -----ND
06/15/1999 0.024 0.022 0.039 0.028 0.125 0.034 ----- 0.039 0.033 ND 0.10 0.20 0.10
08/11/1999 ND ND ND 0.026 0.035 0.029 ----- 0.032 0.062 ND ND ND ND
07/17/2001 ND ND ND 0.024 0.020 0.022 ----- 0.021 0.032 ND ND ND ND
09/05/2001 ND ND ND 0.047 ND ND ----- ND 0.083 ND ND ND ND
07/09/2003 ND ND ND ND ND ND ----- ND ND ND ND ND ND
09/16/2003 ND ND ND 0.022 0.026 0.027 ----- 0.021 0.047 0.19 ND ND ND
07/06/2005 ND ND ND ND ND ND ----- ND ND ND 1.15 ND ND
09/01/2005 ND ND 0.026 0.129 ND ND ----- 0.030 0.184 ND ND ND ND
12
03/23/2006 ND ----- ----- ----- ND ----- ----- ----- ----- 1.20 ----- ----- -----
04/13/2006 ND 0.022 0.023 0.022 ND ND ----- 0.027 0.033 0.40 0.32 0.33 0.17
05/31/2006 ND ND ----- 0.020 0.028 0.025 ----- ----- 0.031 ND ND ----- 0.22
06/29/2006 ND ND ND ND 0.022 0.024 ----- ND 0.030 ND ND ND ND
08/02/2006 ND ND 0.024 0.031 ND ND ----- 0.028 0.043 0.35 ND ND ND
08/30/2006 0.024 0.023 0.023 ND 0.027 0.025 ----- 0.029 0.026 ND ND ND ND
Note: ND means Non-detect. Red highlighted values exceed pollution indicator limit (0.025 mg/l for
phosphorus and 4.0 mg/l for NO3+NO2).

Dissolved and total phosphorus and nitrogen concentrations in water samples collected in
Mill Hollow Creek flowing into and out the reservoir are shown in Tables 4 and 5. For all
samples collected, total phosphorus as P did not exceed the indicator value of 0.05 mg/l
above the reservoir and exceeded it below the reservoir two times, once in April and once
in August. Only a few samples collected at these sites were below the detection limit for
dissolved and total phosphorus and these were all from samples collected below the
reservoir. For all samples collected, nitrogen as dissolved nitrite+nitrate was well below
the standard of 4.0 mg/l although nitrogen was detected in the majority of samples (64%).

Table 4. Mill Hollow Creek field data above reservoir.
Date Time
Water
Temp
(C) pH
DO
(mg/l)
Spec.
Cond.
( S/cm2)
Discharge
(cfs)
Units
of
flow
Total
Phosphorus
as P (mg/l)
Dissolved
Total
Phosphorus
as P (mg/l)
Dissolved
Nitrogen
NO2+NO3
03/23/06 1230 0.4 8.6 6.90 ---- ---- cfs 0.027 0.025 0.280
04/13/06 1430 2.0 8.8 10.40 205 1.20 cfs 0.029 0.024 0.264
05/31/06 1225 5.5 7.9 7.93 95 9.12 cfs 0.031 0.020 ND
06/29/06 1205 8.9 8.1 8.10 130 2.61 cfs 0.027 0.024 0.120
08/02/06 1200 7.9 8.5 6.46 184 1.51 cfs 0.026 0.027 0.130
08/30/06 1240 10.2 8.5 6.66 192 1.09 cfs 0.037 0.042 0.120
Note: ND means Non-detect. Red highlighted values exceed pollution indicator limit (0.05 mg/l for phosphorus and 4.0 mg/l
for NO3+NO2).

Table 5. Mill Hollow Creek field data below reservoir.
Date Time
Water
Temp
(C) pH
DO
(mg/l)
Spec.
Cond.
( S/cm2)
Discharge
(cfs)
Units
of
flow
Total
Phosphorus
as P (mg/l)
Dissolved
Total
Phosphorus
as P (mg/l)
Dissolved
Nitrogen
NO2+NO3
03/23/06 ---- ---- ---- ---- ---- ---- cfs ---- ---- ----
04/13/06 1400 2.1 8.1 10.50 231 ---- cfs 0.059 0.033 0.246
05/31/06 1315 5.9 8.0 8.07 119 10.90 cfs 0.028 ND ND
06/29/06 1245 15.4 8.2 6.04 130 4.24 cfs ND ND ND
08/02/06 1300 15.0 9.0 4.96 131 2.39 cfs 0.046 0.035 0.120
08/30/06 1240 12.0 8.7 4.81 144 1.33 cfs 0.180 0.152 ND
Note: ND means Non-detect. Red highlighted values exceed pollution indicator limit (0.05 mg/l for phosphorus and 4.0 mg/l
for NO3+NO2).
13

Phosphorus in the inflow stream is primarily in the dissolved fraction. It is interesting to
note that although in-lake phosphorus concentrations regularly exceed the pollution
indicator value of 0.025 mg/L none of the tributary stream data exceed the 0.05 mg/L
indicator value possibly indicating another source of phosphorus loading into the
reservoir. In reviewing the profile data it appears the additional phosphorus loading may
originate from the bottom sediments (internal loading). Given that Mill Hollow
Reservoir is located within a zone identified as rich in phosphate by the Bureau of Mines
much of this internal phosphorus load may originate from the native soils and bedrock on
which the reservoir is located.

Oxygen – From measurements collected since 2001, the dissolved oxygen profile shows
very little stratification at any time of the year. The only month where stratification
appears below the first meter in depth is in September 2005. Almost all of the profiles
show a pronounced drop in oxygen in the bottom two meters. For all the profiles, the
percent of the water column supporting the 4 mg/l dissolved oxygen criteria range from
60 to 100 percent. Before 2006, Mill Hollow Reservoir exceeded the dissolved oxygen
standard of 4 mg/l in 5 out of 46 samples (11% of the samples). In 2006, Mill Hollow
Reservoir exceeded the dissolved oxygen standard of 4 mg/l in 9 out of 45 samples (20%
of the samples). For all samples collected, Mill Hollow Reservoir exceeded the dissolved
oxygen standard of 4 mg/l in 14 out of 91 samples (15% of the samples). All of the
exceedances of the State dissolved oxygen standard occurred in the bottom 2 meters of
the reservoir.

Figure 6. Mill Hollow Reservoir: Dissolved Oxygen vs. Depth 2006
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0D.O. (mg/l)
De
p
t
h

(
m
3/23/2006
4/13/2006
5/31/2006
6/29/2006
8/2/2006
8/30/2006
7/21/2001
9/5/2001
9/16/2003
7/5/2005
9/1/2005
14
Macrophytes – Macrophytes cover about 50% of the bottom of the reservoir as indicated
in photos of the reservoir bottom when the reservoir was drained in September 2006.

Algae –For all samples collected from 2001, chlorophyll a, uncorrected for pheophytin
ranges from <0.07 to 21.0 ug/l with the largest value in August 2006 and the lowest value
in April 2006. No seasonal trends were observed in the data. No algal masses were seen
during any sample event.

A taxonomic survey of phytoplankton was conducted on Mill Hollow Reservoir from a
sample of the water column collected in August 2006. The results of this sample
compared to those in the Judd (1997) inventory is presented in Table 6.

Table 6. Diversity measurements for Mill Hollow Reservoir.
Diversity Measure Judd (1997) 2006 Sample
Shannon-Weiner Index 1.57 1.14
Species Evenness 0.68 0.71
Species Richness 0.40 1.13

Discussion – Non-point sources of pollution can contaminate lakes through runoff and
groundwater. Runoff can carry sediment and nutrients from roads, bare soil, and
agricultural wastes such as livestock manure. Nutrients and bacteria can enter a lake
through malfunctioning septic systems. When bacteria consume nutrients, dissolved
oxygen is consumed, particularly in the hypolimnetic zone. This can result in low
dissolved oxygen levels, fish kills, odors, and noxious conditions. In addition nutrients
act as a fertilizer and can stimulate excessive growth of algae and macrophytes that may
contribute to additional loss of dissolved oxygen. Excessive algae and macrophyte
growth can create supersaturated oxygen conditions from photosynthesis during the day,
which then plunges to low levels of dissolved oxygen at night from high levels of
respiration and lack of photosynthesis, creating dramatic diel fluctuations.

pH is a measure of the acidity or alkalinity of water, and determines the solubility and
biological availability of chemical constituents such as nutrients and heavy metals. When
pollution results in higher algal and plant growth, pH levels may increase. This is
because photosynthesis uses up dissolved carbon dioxide, which disassociates into
carbonic acid in water. As carbon dioxide is decreased, the alkalinity of the water
increases. These changes in pH can increase the solubility and aggravate nutrient
problems of phosphorus, making it more available for plant growth.

Researchers of the U. S. Geological Survey (Winter et al.1998) summarized ground water
and surface water processes affecting chemicals in lakes and wetlands and an excerpt
from their discussion that applies to nutrients is presented below.

“Lakes and wetlands also have distinctive biogeochemical characteristics with
respect to their interaction with ground water. The chemistry of ground water and
the direction and magnitude of exchange with surface water significantly affect
15
the input of dissolved chemicals to lakes and wetlands. In general, if lakes and
wetlands have little interaction with streams or with ground water, input of
dissolved chemicals is mostly from precipitation; therefore, the input of chemicals
is minimal. Lakes and wetlands that have a considerable amount of ground-water
inflow generally have large inputs of dissolved chemicals. In cases where the
input of dissolved nutrients such as phosphorus and nitrogen exceeds the output,
primary production by algae and wetland plants is large. When this large amount
of plant material dies, oxygen is used in the process of decomposition. In some
cases the loss of oxygen from lake water can be large enough to kill fish and other
aquatic organisms.

The magnitude of surface-water inflow and outflow also affects the retention of
nutrients in wetlands. If lakes or wetlands have no stream outflow, retention of
chemicals is high. The tendency to retain nutrients usually is less in wetlands that
are flushed substantially by throughflow of surface water. In general, as surface-
water inputs increase, wetlands vary from those that strongly retain nutrients to
those that both import and export large amounts of nutrients. Furthermore,
wetlands commonly have a significant role in altering the chemical form of
dissolved constituents. For example, wetlands that have throughflow of surface
water tend to retain the chemically oxidized forms and release the chemically
reduced forms of metals and nutrients.”

Most of the surface water flowing into the reservoir is from spring sources and from Mill
Hollow Creek. Total phosphorus concentrations in the creek primarily consisted of the
dissolved fraction. This indicates that ground water is the most likely source of this
inflow water. Although there are some man-made sources of sediment that may be
contributing to the reservoir, it is likely that most of the phosphorus entering the lake is
from natural geological sources and ground water.

The pH values within a lake may vary due to several factors. The geology and hydrology
of a catchment area will determine the regional differences in pH. Within a lake or pond,
variations in pH are strongly affected by biological processes such as photosynthesis and
respiration. The pH will increase when plants consume carbon dioxide during
photosynthesis and will decrease when plants decay and respire (Bronmark and Hansson
2005).

The pH of water in Mill Hollow Reservoir appears to reflect the ground water chemistry
of the drainage and the biochemical reactions occurring in the reservoir. Inflow water to
the reservoir is alkaline (between 8.0 and 9.0) and the pH of the lake increases during the
summer season as a result of photosynthesis that results in pH levels that exceed 9.0 for
part of the year.

7.0 Source Assessment

16
This section identifies whether load reductions are necessary, and if so, what would be an
appropriate margin of safety for limits on sources of pollution while considering the
seasonal changes of the parameters of concern.

The results of the data and land management activities in the watershed indicate that
natural processes are primarily causing exceedances of the total phosphorus pollution
indicator value and pH standards although there appears to be some man-made activities
that are contributing to the amount of total phosphorus that is entering the reservoir.

Most of the anthropogenic sources could be mitigated through development and
implementation of appropriate best management practices. Natural inputs of phosphorus
due to the phosphate geology of the watershed might also be able to be reduced through
BMPs that reduce sediment inputs. Since anthropogenic activities have not caused the
impairment, Mill Hollow Reservoir is recommended to be placed in Category 4C of the
State of Utah’s 303d List as not impaired by a pollutant.

Although Mill Hollow Reservoir is likely to not be impaired due to anthropogenic causes
several recommended projects are included to help guide land management efforts to
preserve and improve current water quality conditions

8.0 Best Management Practices

This section discusses the rationale for the means of addressing all sources and causes
that are significant in the attainment of the water quality endpoints/targets. In addition,
this section discusses whether land management activities are contributing to the
impairment and what practices may be recommended to reduce sources of impairment.

Since most of the total phosphorus is in the dissolved fraction, the main contributor of
total phosphorus is likely to be from the spring sources entering the reservoir. Soil
erosion is not likely to be a major cause of the phosphorus impairment. The spring
sources around the reservoir are in very good condition and no management practices are
recommended for these.

Recommended practices that may reduce total phosphorus and pH levels in the reservoir
are:
• Identify dispersed camping areas that are authorized under Forest Plan Travel
Management, but may need to be closed or need to use best management practices
to control soil erosion.
• Limit vehicle access in the drainage above the reservoir by decommissioning and
rehabilitating unauthorized dirt roads above the reservoir.

It is likely that these practices will reduce some of the total phosphorus in the reservoir
and may reduce the pH levels as a result of the reduced phosphorus. It is not known
whether these practices will result in meeting the endpoints (meeting State water quality
standards). The Forest Service would be responsible for implementing work on National
17
Forest lands but when implementation would occur would depend upon available funding
and Forest Service priorities.

REFERENCES

Baker, J.P., H. Olem, C.S. Creager, M.D. Marcus, and B.R. Parkhurst. 1993. Fish and
Fisheries Management in Lakes and Reservoirs. EPA 841-R-93-002. Terrene Institute
and U.S. Environmental Protection Agency, Washington, D.C.

Bronmark, Christer and Lars-Anders Hansson. 2005. The Biology of Lakes and Ponds.
Oxford University Press.

Bureau of Mines Special Report "Availability of Federally Owned Minerals for
Exploration and Development in Western States: Utah, 1988."

Judd, H.J. 1997. Utah’s Lakes and Reservoirs, An Inventory and Classification of Utah’s
Priority Lake and Reservoirs. Utah Department of Environmental Quality, Division of
Water Quality.

Kent State. 2005. Internet site http://dipin.kent.edu.tsi.htm, The Great North American
Secchi Dip-In. excerpted and modified from Carlson, R.E. and J. Simpson. 1996. A
Coordinators Guide to Volunteer Lake Monitoring Methods. North American Lake
Management Society. 96 pp.

Olem, H. and G. Flock, eds. 1990. Lake and Reservoir Restoration Guidance Manual, 2nd
edition. EPA -440/4-90-006. Prep. by N. Am. Lake Manage. Soc. for U.S. Environ. Prot.
Agency, Washington DC.

Utah, State of. 2005. Standards of Quality for the Waters of the State. R317-2, Utah
Administrative Code. Utah Department of Environmental Quality, Division of Water
Quality, Revised December 1, 2005.

Utah, State of. 2006. Utah’s 2004 303(d) List of Impaired Waters. Utah Department of
Environmental Quality, Division of Water Quality.

Utah, State of. 2007. Draft Water Quality Assessment Guidance. Utah Department of
Environmental Quality, Division of Water Quality, August 31, 2007.

Woods, A.J., Lammers, D.A., Bryce, S.A., Omernik, J.M., Denton, R.L., Domeier, M.,
and Comstock, J.A. 2001, Ecoregions of Utah (color poster with map, descriptive text,
summary tables, and photographs): Reston, Virginia, U.S. Geological Survey (map scale
1:1,175,000).