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Home My WebLink AboutDWQ-2024-007818ANALYSIS OF SEPTIC-TANK DENSITY FOR ROCKVILLE, WASHINGTON COUNTY, UTAH by Trevor H. Schlossnagle and Torri Duncan REPORT OF INVESTIGATION 288 UTAH GEOLOGICAL SURVEY UTAH DEPARTMENT OF NATURAL RESOURCES 202475 ANNIVERSARYth 19492024 Blank pages are intentional for printing purposes. REPORT OF INVESTIGATION 288 UTAH GEOLOGICAL SURVEY UTAH DEPARTMENT OF NATURAL RESOURCES 2024 ANALYSIS OF SEPTIC-TANK DENSITY FOR ROCKVILLE, WASHINGTON COUNTY, UTAH by Trevor H. Schlossnagle and Torri Duncan Cover photo: View to the northwest of Rockville and surrounding mesas. Suggested citation: Schlossnagle, T.H, Duncan, T., 2024, Analysis of septic-tank density for Rockville, Washington County, Utah: Utah Geological Survey, Report of Investigation 288, 14 p., https://doi.org/10.34191/RI-288. 75 ANNIVERSARYth 1949–2024 STATE OF UTAH Spencer J. Cox, Governor DEPARTMENT OF NATURAL RESOURCES Joel Ferry, Executive Director UTAH GEOLOGICAL SURVEY R. William Keach II, Director PUBLICATIONS contact Natural Resources Map & Bookstore 1594 W. North Temple Salt Lake City, UT 84116 telephone: 801-537-3320 toll-free: 1-888-UTAH MAP website: utahmapstore.com email: geostore@utah.gov UTAH GEOLOGICAL SURVEY contact 1594 W. North Temple, Suite 3110 Salt Lake City, UT 84116 telephone: 801-537-3300 website: geology.utah.gov The Utah Department of Natural Resources, Utah Geological Survey, makes no warranty, expressed or implied, regarding the suitability of this product for a particular use, and does not guarantee accuracy or completeness of the data. The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims by users of this product. CONTENTS ABSTRACT ..................................................................................................................................................................................1 INTRODUCTION ........................................................................................................................................................................1 Purpose and Scope .................................................................................................................................................................1 Background ............................................................................................................................................................................1 Location, Geography, and Climate ...................................................................................................................................1 Population and Land Use .................................................................................................................................................3 Geologic Setting ...............................................................................................................................................................3 Hydrogeologic Setting ......................................................................................................................................................3 SEPTIC-SYSTEM DENSITY ANALYSIS ..................................................................................................................................3 Groundwater Contamination from Septic-Tank Systems ......................................................................................................5 The Mass-Balance Approach ................................................................................................................................................5 General Methods ..............................................................................................................................................................5 Groundwater Flow Calculations .......................................................................................................................................6 Environmental Tracers .....................................................................................................................................................6 Limitations .......................................................................................................................................................................7 Region-Specific Septic-Tank Density Evaluations ................................................................................................................7 Virgin River Corridor .......................................................................................................................................................7 South Mesa .....................................................................................................................................................................11 SUMMARY AND CONCLUSIONS ..........................................................................................................................................11 ACKNOWLEDGMENTS ..........................................................................................................................................................12 REFERENCES ...........................................................................................................................................................................12 FIGURES Figure 1. Location map of Rockville, Washington County, Utah .................................................................................................2 Figure 2. Simplified geologic map of the Rockville area .............................................................................................................4 Figure 3. Nitrate concentrations and groundwater flow data ........................................................................................................8 Figure 4. Projected nitrate concentration versus septic-system density .....................................................................................10 TABLES Table 1. Aquifer properties determined from water-well log specific capacity data and aquifer test data ...................................8 Table 2. Aquifer parameters used to compute groundwater flow available for mixing ................................................................9 Table 3. Major ion and environmental tracer chemistry data .......................................................................................................9 Table 4. Nitrate data and field parameters.....................................................................................................................................9 Table 5. Results of mass balance analysis for different nitrate concentration projections in Rockville study subdomains ...........11 ABSTRACT Rockville is a small rural town in southwestern Utah that is experiencing an increase in residential development. New developments in rural areas often use septic tank soil- absorption systems for wastewater disposal, although there is potential to utilize an existing community sewer system. Because potential future septic-tank systems may overlie the principal drinking water aquifer for Rockville, city of- ficials asked the Utah Geological Survey to conduct a septic- tank density analysis. The purpose of our study is to provide tools for water-resource management and land-use planning. In this study we (1) characterize the groundwater quality of Rockville with an emphasis on nitrate, and (2) provide a mass-balance analysis for three subdomains (Virgin River corridor, South Mesa, South Mesa subdivision) based on numbers of septic-tank systems, groundwater flow available for mixing, and baseline nitrate concentrations, and thereby determine appropriate septic-system density requirements to limit water-quality degradation. There are two aquifers commonly utilized in Rockville: The Shinarump Conglomerate (primary aquifer) and the Shnab- kaib Member of the Moenkopi Formation and overlying al- luvial deposits along the Virgin River (secondary aquifer). Using 11 groundwater samples taken from water wells and a spring, we established that baseline nitrate concentrations for these two Rockville aquifers are low, at 0.1 mg/L for the primary aquifer and 0.26 mg/L for the secondary aquifer. To determine the ideal septic-tank density, we employed a mass-balance approach using existing septic systems and baseline nitrate concentrations. Nitrogen in the form of nitrate is one of the principal indicators of pollution from septic tank soil-absorption systems. For the mass-balance approach, the nitrogen mass from projected additional septic tanks is added to the current nitrogen mass and then diluted with ground- water flow available for mixing plus the water added by the septic-tank systems themselves. We used an allowable deg- radation of 1 mg/L with respect to nitrate. Groundwater flow volume available for mixing was calculated from existing hydrogeologic data. We used data from aquifer tests and spe-cific capacity data from water well logs to derive hydraulic conductivity for Rockville’s aquifers. Existing publications and potentiometric surface datasets were used to determine groundwater flow directions and hydraulic gradients. Our results using the mass balance approach indicate that the most conservative septic-tank densities for the Virgin River corridor, South Mesa, and South Mesa subdivision subdo- mains are 11 acres per system, 13 acres per system, and 8 acres per system, respectively. Due to the low baseline nitrate concentrations, higher septic-tank densities may still be con- servative enough to protect water quality. These determina-tions are based on hydrogeologic parameters used to estimate groundwater flow volume. Connecting to existing commu- nity sewer systems may be a safer alternative to septic-tank systems where feasible. INTRODUCTION Purpose and Scope Rockville is a rural town in southwestern Utah that is experi-encing an increase in residential development. Although much of the town is on community sewer systems, many residents use septic tank soil-absorption systems for wastewater disposal. Existing and future septic-tank systems overlie sediments and rocks that constitute the primary aquifers for the area. Preser-vation of groundwater quality and the potential for groundwater quality degradation are critical issues that should be considered in determining the extent and nature of future development in Rockville. Local government officials in Rockville have ex-pressed concern about the potential impact that development may have on groundwater quality, particularly development that uses septic tank soil-absorption systems for wastewater disposal. These officials have asked the Utah Geological Sur-vey to provide a basis for defensible land-use regulations to protect groundwater quality, and for determining densities for septic-tank systems as a land-use planning tool. The purpose of our study is to provide land-use planners with science-based tools for approving new development in a manner that will protect groundwater quality. To accomplish this, we used a mass balance approach to determine the po- tential impact of projected increased numbers of septic-tank systems on water quality in the main aquifers, and thereby determine appropriate septic-system density requirements to limit water-quality degradation. Background Location, Geography, and Climate Rockville occupies 8.4 square miles just outside the bound-aries of Zion National Park in southeast Washington County, by Trevor H. Schlossnagle and Torri Duncan ANALYSIS OF SEPTIC-TANK DENSITY FOR ROCKVILLE, WASHINGTON COUNTY, UTAH Utah Geological Survey2 Figure 1. Location map of Rockville, Washington County, Utah. southwest Utah (Figure 1). The main stem of the Virgin Riv- er flows east to west through the population center of Rock- ville; the confluence of the East and North Forks of the Vir- gin River is just outside the Rockville boundary to the east. Several ephemeral tributaries enter the Virgin River from the north (Coalpits Wash and Huber Wash) and south (Horse Valley Wash) within the study area (Figure 1). Upland areas in or adjacent to the study area consist of mesas and small buttes that include Rockville Bench to the north, and Wire Mesa, Smithsonian Butte, an unnamed mesa referred to lo- cally as South Mesa, and Eagle Crags to the south. Elevation in the study area ranges from about 5500 feet on the eastern flanks of Smithsonian Butte to the south to 3650 feet along the Virgin River at the western edge of Rockville. The climate of Rockville is semi-arid and characterized by warm, dry summers and moderately cold winters. Average annual precipitation for the Rockville area from PRISM 30- year normals (1991–2020) is 13 to 14 inches (PRISM Climate Group, 2021). 37 1 0 37 9 37 8 37 7 -1131 -1132 -1133 -1134 -1135 Rockville boundary Zion National Park boundary 0 0.5 1 Miles 0 10.5 Kilometers RimrockSpring Project area Utah RockvilleBench EagleCrags WireMesa SmithsonianButte SouthMesa Virgin R i v e r Coalpits Wash Huber Wash H o r s e V a l l e y W a s h 9 Figure 1. Location map of Rockville, Washington County, Utah. ± Zion National Park 3Analysis of septic-tank density for Rockville, Washington County, Utah Population and Land Use Rockville has a population of approximately 226 people (U.S. Census Bureau, 2021). Land use in Rockville generally consists of residential areas; agriculture, including croplands and livestock grazing; and public land. Geologic Setting The Rockville study area is in the transition zone between the Colorado Plateau and Basin and Range physiographic provinces. The study area lies within a structural block formed by the Hurricane fault to the west and Sevier fault to the east. The down-to-the-west Hurricane fault exhibits a higher slip rate relative to the Sevier fault, leading to increased uplift and erosion rates within the structural block, e.g., the formation of Zion Canyon to the northeast of Rockville. Bedrock in the study area includes the Triassic Moenkopi, Chinle, and Moenave Formations, and the Jurassic Kayenta Formation and Navajo Sandstone (Figure 2). Unconsolidated deposits include Quaternary alluvial fans, stream terraces, river and stream deposits, eolian deposits, and landslide deposits (Willis et al., 2002). Hydrogeologic Setting Two aquifers are utilized in the study area, both in Trias- sic strata. Regionally, these strata are typically viewed as confining units, but they contain some members that act as aquifers locally. The Shinarump Conglomerate Member of the Chinle Formation crops out on the Rockville Bench and South Mesa and acts as the main aquifer for the town, sup- plying several public wells and feeding Rimrock Spring. The Shinarump Conglomerate is a medium to coarse-grained, pebbly sandstone and pebbly conglomerate that has rela- tively high-quality fresh water (Cordova, 1981; Biek et al., 2010). Overlying this aquifer is the Petrified Forest Mem- ber of the Chinle Formation, consisting chiefly of mudstone, claystone, and siltstone. This unit mostly acts as a confining unit, especially where it contains bentonite clay (Biek et al., 2010). Quaternary landslide deposits on the South Mesa are primarily composed of the Petrified Forest Member. The two uppermost members of the Moenkopi Formation are exposed in Rockville: the upper red member and the Shnab- kaib Member. The upper red member is siltstone, mudstone, and fine-grained sandstone, and is possibly a water-bearing member in hydrologic connection with the overlying Shi- narump Conglomerate (Inkenbrandt et al., 2013). There are currently no wells in Rockville completed in the upper red member. The Shnabkaib Member of the Moenkopi is a gyp- siferous mudstone and siltstone and is typically observed as a confining unit having poor quality water (Inkenbrandt et al., 2013). However, this unit, along with overlying Qua- ternary deposits, is utilized for stock and domestic water in Rockville along the Virgin River. SEPTIC-SYSTEM DENSITY ANALYSIS Land-use planners use soil maps and septic-tank suitability maps to determine where effluent from septic-tank systems is likely to percolate at a rate that will promote treatment in the soil zone. However, studies show that percolation alone does not remediate many constituents found in wastewater, including nitrate. Under aerobic conditions, ammonium from septic-tank effluent can convert to nitrate, contami- nating groundwater and posing potential health risks to humans (primarily very young infants; Comley, 1945; Fan et al., 1987; Bouchard et al., 1992). Studies involving lab rats ingesting a combination of nitrate and heptamethyle- neimine in drinking water reported an increase in tumor occurrence (Taylor and Lijinsky, 1975). However, epidemio- logical investigations involving human beings have shown conflicting evidence. Stomach cancer in humans associated with nitrate from drinking water was reported in Colom- bia and Denmark (Cuello et al., 1976; Fraser et al., 1980). Conversely, investigations in the United Kingdom and other countries indicate no correlation between nitrate levels and cancer incidence (Forman, 1985; Al-Dabbagh et al., 1986; Croll and Hayes, 1988; Taneja et al., 2017). The U.S. En- vironmental Protection Agency (EPA) maximum contami- nant level (MCL) for drinking water (and Utah groundwater quality standard) for nitrate as nitrogen is 10 mg/L (U.S. EPA, 2022). With continued population growth and installation of sep- tic tank soil-absorption systems in new developments, the potential for nitrate contamination will increase. One way to evaluate the potential impact of septic-tank systems on groundwater quality is to perform a nitrate mass-balance calculation (Hansen, Allen, and Luce, Inc., 1994; Wallace and Lowe, 1998a, 1998b, 1998c, 1999; Zhan and McKay, 1998; Lowe and Wallace, 1999; Lowe et al., 2000, 2002, 2003; Bishop et al., 2007a, 2007b; Jordan et al., 2019; Schlossnagle et al., 2022; Wallace et al., 2024), which will allow planners to more effectively determine appropriate average septic-system densities. Data for the mass-balance model include total population and density, area acreage and minimum lot size, ground- water flow volume, number of existing septic-tank systems, and present-day nitrate concentrations. The 2020 popula- tion of Rockville was 226 (U.S. Census Bureau, 2021) with an average of 2.0 people per household (pph). The current minimum lot size in use for Rockville is 0.5-acres (Vicki Bell, Rockville Town Clerk, written communication, 2023). We used nitrate as a proxy for dispersion and dilu- tion of most common septic-tank effluent constituents be- cause it is soluble, mobile, and less expensive to test than other constituents. We determined groundwater hydraulic conductivity from aquifer tests and determined hydraulic gradient from existing potentiometric surface maps to esti- mate groundwater flow volume. Then, using the estimated Utah Geological Survey4 Qm Qm Qm QmQm Qm QmQmQm Qm QmQm Qa Qm Qm Qm Qm Qa Qm Qm Qm Qm Qe Qm Qe Qe Qm Qm Qm Qm Qm Qm Qm Qe Qm Qm Qe Qm Qe QmQm QmQm Qa QaQa Qa Qa Qm Qm Qa Qa Qa Qm QaQa Qm Qa Qa Qm Qa Qe Qm Qv JkJTRm TRcp JTRm Jk JTRm Jn Jk Jk JTRm Jk JTRm Jks TRcp TRcp TRcp JkJTRm Jks TRcpTRcp TRcp Jn TRcp JTRm JksJTRm Jk TRcs TRmu TRcp TRmu TRcp TRcs TRcp TRmu TRmu TRcs TRmu TRcp TRcp TRmu TRcp TRcs TRmuTRcpTRmu TRcp TRmu TRmu TRcs TRcs TRmu TRms TRms TRcp TRms TRcp TRms TRcp TRcpTRms TRmu TRcs TRms Jks TRcp TRcp TRcp TRmu TRcs TRms TRms TRcs TRcs TRms TRcs Qm Jks Qm Qm 37 ° 1 0 ' 37 ° 9 ' 37 ° 8 ' 37 ° 7 ' -113°1'-113°2'-113°3'-113°4'-113°5'-113°6' Project Area Utah Virgin Ri v e r 9 Figure 2. Simplified geologic map of the Rockville area, modified from Biek et al. (2010). 0 0.5 1 Miles 0 10.5 Kilometers TRms Triassic Shnabkaib Member Moenkopi Fm TRmu Triassic Upper red member Moenkopi Fm JTRm Jurassic-Triassic Moenave Formation Triassic Shinarump Conglomerate Member Chinle Fm TRcp Triassic Petrified Forest Member Chinle Fm Jks Jurassic Springdale Sandstone Member Jk Jurassic Kayenta Formation Jn Jurassic Navajo Sandstone Qv Quaternary volcanic rocks Qm Quaternary mass movement deposits Qe Quaternary eolian deposits Qa Quaternary alluvial and colluvial deposits Geologic units Rockville boundary Jn ± TRcs Figure 2. Simplified geologic map of the Rockville area, modified from Biek et al. (2010). 5Analysis of septic-tank density for Rockville, Washington County, Utah amount of wastewater and accompanying nitrogen load introduced per septic-tank system, we projected nitrogen loads based on increasing numbers of septic-tank systems. By projecting allowable degradation of groundwater nitrate concentration by approximately 1 mg/L (a common amount of water-quality degradation determined to be acceptable by local government officials), we were then able to derive septic-tank density projections. Groundwater Contamination from Septic-Tank Systems As the effluent from a septic tank soil-absorption system leaves the drain field and percolates into the underlying soil, it can have high concentrations of pathogens, such as viruses and bacteria. Organisms such as bacteria can be mechanically filtered by fine-grained soils and are typically removed after traveling a relatively short distance in the unsaturated zone. However, pathogens can travel up to 40 feet in the unsaturated zone in some soils (Franks, 1972), and in coarse-grained soils, or soils containing preferential flow paths such as fractures, worm burrows, or root holes, these pathogens can reach the water table. Some viruses can survive longer than 250 days (U.S. Environmental Protection Agency, 1987), which is the minimum required groundwater travel time for public water- supply wells or springs to be separated from potential biologi- cal contamination sources. Many household and industrial chemicals are commonly dis- posed of through septic systems that, unless they volatilize easily, are not remediated by percolation through soils in the unsaturated zone. Contamination from these chemicals can be minimized by reducing their disposal via septic-tank sys- tems, maximizing the potential for dilution of those chemicals (Lowe and Wallace, 1999). Community awareness and educa- tion can also help prevent or reduce the amount of this type of waste entering the septic system. Phosphate, typically derived from organic material and some detergents, is discharged from septic-tank systems. Although phosphate (and phosphorus) is a major factor in causing eutro- phication of surface waters, it is generally not associated with water-quality degradation from use of septic-tank systems (Fetter, 2001). Phosphates are removed from septic-tank sys- tem effluent by adsorption onto fine-grained soil particles and by precipitation with calcium and iron. In most soils, complete removal of phosphate from septic-tank discharge is common (Franks, 1972). Ammonia and organic nitrogen, mostly from the human uri- nary system, are present in wastewater in septic tanks. Typi- cally, almost all ammonia is converted into nitrate before leaving the septic-tank system drain field. Once nitrate passes below the zone where aerobic bacteria and plant roots are present, attenuation is negligible as it travels farther through the soil (Franks, 1972). Once in groundwater, nitrate becomes mobile and can persist in the environment for long periods of time. Areas having high densities of septic-tank systems risk elevated nitrate concentrations reaching levels greater than the EPA’s maximum contaminant level. In the early phases of groundwater quality degradation associated with septic- tank systems, nitrate is likely to be the only pollutant detected (Deese, 1986). Regional nitrate contamination from septic- tank discharge has been documented in densely populated areas where sewer systems do not exist (Fetter, 2001). Groundwater having less than 0.2 mg/L nitrate is assumed to represent natural background concentrations; groundwater having nitrate concentrations between 0.21 and 3.0 mg/L is considered transitional and may or may not represent human influence (Madison and Brunett, 1985). Groundwater having concentrations exceeding 3 mg/L is typically associated with human- or animal-derived sources, but higher concentrations have also been identified with natural sources (Green et al., 2008), albeit less commonly. Changes in land-use practices in arid regions in the western U.S. have been linked to trends of water quality degradation from nitrate contamination (Xu et al., 2007). Distances between septic tank soil-absorption system drain fields and sources of culinary water must be sufficient for dilu- tion of nitrate in effluent to levels below the groundwater qual- ity standard. We consider nitrate to be the key contaminant for use in determining the number or density of septic-tank sys- tems allowed in Rockville. Projected nitrate concentrations in all or parts of aquifers can be estimated for increasing septic- tank system densities using the mass-balance approach. The Mass-Balance Approach General Methods The mass-balance approach for water-quality degradation as- sessments is a practical method to apply under time, budget, and data availability/acquisition constraints, and it provides a quantitative basis for land-use planning decisions. To compute projected nitrate concentrations, we added the average nitrogen mass expected from projected new septic tanks to the existing mass of nitrogen in groundwater and then diluted with the es- timated groundwater flow available for mixing, plus water that is added to the system by septic tanks. We used an average es- timated discharge of 198 gallons (749 L) of effluent per day for a domestic home based on a per capita indoor usage of 60 gal- lons (227 L) per day (Utah Division of Water Resources, 2010) multiplied by an average 2-person household in Rockville (U.S. Census Bureau, 2021). We used an estimated nitrogen loading of 64 mg/L nitrate as nitrogen in effluent per domestic septic tank based on (1) average nitrogen loading of 17 grams nitrogen per capita per day (Kaplan, 1988), (2) 227 L per capita per day water use, and (3) an assumed retention of 15% of the nitrogen in the septic tank (to be later removed during pumping) (An- dreoli et al., 1979). Our nitrogen loading estimate is similar to Utah Geological Survey6 Bauman and Schafer’s (1984) nitrogen concentration in septic- tank effluent of 62 ± 21 mg/L based on the averaged means from 20 previous studies. For our mass-balance calculations, we allowed a 1 mg/L degradation above current baseline levels of nitrate (a value adopted by other Utah counties and munici- palities as an acceptable level of degradation to be protective of water quality [Hansen, Allen, and Luce, Inc., 1994]) as a refer- ence point to evaluate the potential impact of increased num- bers of septic-tank systems. We also include results of several other degradation scenarios, as local government officials may choose a different nitrate concentration as an acceptable level of degradation to be protective of water quality. We determined groundwater flow available for mixing—the major control on nitrate concentration in aquifers when using the mass-balance approach (Wallace and Lowe, 1999)— us- ing aquifer test data compiled from drinking water source protection documents in the region. We obtained the num- ber of septic-tank systems based on data provided by the Southwest Utah Public Health Department and the Town of Rockville. To establish baseline nitrate concentrations, we collected samples for nitrate plus nitrite analysis from repre- sentative wells and springs in each major aquifer using stan- dard sampling procedures for water-quality sampling. These samples were analyzed by the Utah Department of Health, Chemical and Environmental Services Division of the Utah Public Health Laboratory. We used the following equation to determine the projected nitrate concentration resulting from additional septic sys- tems, and thus how many septic-tank systems can be added before exceeding a designated target nitrate concentration: [(STT - STC)QST] * NST + [NA(QGW + [STT * QST])] [STT * QST] + QGW where: NP is the projected nitrate concentration (mg/L), NA is the ambient (baseline) nitrate concentration for the domain (mg/L), NST is the estimated average nitrate concentration from septic tanks (mg/L), STT is the total number of septic tanks in the domain (variable, unitless), STC is the current number of septic tanks (constant, unitless), QST is the flow from each septic tank in liters per sec- ond (L/s), QGW is the groundwater flow available for mixing (L/s). To determine a septic-system density, we divided the domain area acreage by the total number of septic tanks (STT) that would exist at the projected nitrate concentration (NP). Groundwater Flow Calculations We calculated groundwater flow available for mixing as: Q=KbLI (2) where: Q is the volume of discharge (ft3/s), K is the hydraulic conductivity (ft/s), b is the vertical mixing zone thickness (ft), L is the width of cross section (ft) where flow occurs, I is the hydraulic gradient (ft/ft). We used data from aquifer tests compiled from drinking wa- ter source protection documents (Diedre Beck, Utah Division of Drinking Water, written communication, October 2023) to derive hydraulic conductivity from reported transmissivi- ties. To supplement these data, we also estimated hydraulic conductivity from information obtained from water well logs. We estimated transmissivity using specific capacity data from well logs using the TGUESS algorithm of Bradbury and Rothschild (1985), which utilizes the Cooper and Jacob (1946) solution of the Theis (1935) equation. We used poten- tiometric surface maps from existing publications and data- sets to determine groundwater flow directions and hydraulic gradients. We used a mixing zone thickness of 50 feet based on aquifer thickness; we assumed uniform and complete mix- ing/dilution of septic-tank effluent occurs within this layer. The upper part of the aquifer is where nitrate associated with septic-tank systems is most likely to degrade water quality. Bauman and Schafer (1984) found that mixing zone thickness has minimal impact on nitrate concentrations in aquifers hav- ing low groundwater velocities like those commonly found in Utah. Environmental Tracers To help conceptualize groundwater flow in Rockville’s aquifers, we utilized environmental tracer data. Environ-mental tracers are naturally occurring or anthropogenic chemicals or isotopes that can indicate water sources and flow processes such as recharge, flow rate, geologic subsurface interactions, residence times, and mixing be- tween sources (Kendall and Caldwell, 1998). Ideal trac- ers have well-defined input sources and input histories, are inert (no reactions) or geochemically conservative (lim- ited reactions), have transport mechanisms identical to water, and are detected precisely and economically. The use of multiple tracers provides a more comprehensive understanding of the groundwater system. For this study we used tritium and radiocarbon as tracers and analyzed water samples from a well in each aquifer for concentra- tions of each. Tritium (3H) provides a qualitative age of groundwater for determining the relative time when water NP = (1) 7Analysis of septic-tank density for Rockville, Washington County, Utah entered the groundwater system (Clark and Fritz, 1997). Tritium is an unstable isotope of hydrogen with a half-life of 12.32 years, therefore tritium concentration in ground- water isolated from other water will decrease by one-half after 12.32 years. Tritium concentrations in water are re-ported in tritium units (TU). Carbon-14 (14C) is a natu- rally occurring radioactive isotope of carbon that has a half-life of about 5730 years, which allows the determina- tion of groundwater residence times of up to 40,000 years (Kalin, 2000). Carbon-14 data are expressed as percent modern carbon (pmC) relative to A.D. 1950 levels. Car- bon-13 (13C) is a naturally occurring stable isotope of car- bon that is used to evaluate chemical reactions involving carbon (Clark and Fritz, 1997). Carbon-13 is expressed as an isotopic ratio (13C/12C), reported as delta (δ) values in units of parts per thousand (per mil or ‰). Additionally, we collected samples for major ion analysis to aid in inter- pretation of radiocarbon data, which were analyzed by the Utah Department of Health, Chemical and Environmental Services Division of the Utah Public Health Laboratory. Tritium concentrations were measured at the University of Utah Department of Geology and Geophysics Dissolved and Noble Gas Laboratory in Salt Lake City, Utah and car- bon-14 samples were analyzed by accelerator mass spec- trometer at the University of Georgia Center for Applied Isotope Studies in Athens, Georgia. Detailed methods and theory are beyond the scope of this report; for a detailed discussion of environmental tracer methods and theory, see Wallace et al. (2024). Limitations There are many limitations to any mass-balance approach (see, for example, Zhan and McKay, 1998; Lowe et al., 2000). We identified the following limitations to our application of the mass-balance approach: • Calculations of groundwater available for mixing are based on data compiled from isolated aquifer tests and regional potentiometric surface maps. • Baseline nitrate concentration is attributed to natu- ral sources, agricultural practices, and use of septic- tank systems, but projected nitrate concentrations are based on septic-tank systems only and do not include nitrate from other potential sources (such as fertilizer or livestock). • The approach assumes uniform geologic conditions within a given area, and thus does not account for lo-cal variation that may cause flow variability that can reduce or enhance mixing. • Calculations do not account for localized, high-con- centration nitrate plumes associated with individual or clustered septic-tank systems. • Calculations assume that the septic-tank effluent from existing homes is in a steady-state condition with the aquifer. • Calculations assume negligible denitrification and in- stantaneous groundwater mixing for the entire mixing zone below the study area. Additionally, calculations do not account for changes in groundwater conditions due to groundwater withdrawal from wells, are based on aquifer parameters that must be extrapo- lated to larger areas where they may not be entirely represen- tative, and may be based on existing data that do not represent the entire study area. Although many caveats exist in applying this mass-balance approach, we believe it is the best available method in land- use planning because it provides a general basis for making decisions on septic-tank system densities. In addition, the approach is cost-effective and can be applied in areas with limited information. Region-Specific Septic-Tank Density Evaluations We partitioned the study area into two subdomains, the Vir- gin River corridor and the South Mesa, based on differing ge- ology and aquifer characteristics (Figure 3). The Virgin River corridor refers to low-lying land within Rockville along the Virgin River and State Route 9, where most residents and agriculture exist. The South Mesa area has few current resi- dents but has potential for future development. Virgin River Corridor Groundwater flow variables: Due to the lack of public water supply wells along the Virgin River in Rockville and nearby communities, available aquifer test data is lacking. Therefore, we calculated a groundwater flow scenario using hydraulic conductivities derived from specific capacity data from well logs (Table 1). The resulting geometric mean hydraulic con- ductivity is 47 ft/day. We used a hydraulic gradient of 0.007 based on potentiometric surface contours of Inkenbrandt et al. (2013), a mixing depth of 50 feet, and a 3750-foot-wide transect (a typical width of the Virgin River corridor in Rock- ville) resulting in a groundwater flow available for mixing of 0.71 ft3/s (Table 2). Environmental tracers: Tritium concentration from the Vir- gin River corridor well (Site ID WL-MAYL) is 1.80 TU (Table 3). Using methods to calculate thresholds for defining modern and premodern groundwater from Lindsey et al. (2019), we de- termined this sample to be modern groundwater (i.e., entered the aquifer after 1952). Carbon-14 concentration from this well is 93.9 pmC with a carbon-13 value of -13.6 ‰, which is also consistent with modern groundwater. This aquifer, composed of gypsiferous mudstone and siltstone overlain by thin alluvial deposits, is most likely recharged by the Virgin River. Baseline nitrate concentrations: We collected groundwater samples from seven wells in the Virgin River corridor area for Utah Geological Survey8 Rockville 0.2770.1 0.84 0.2220.897 0.1 0.10.10.1 0.1 0.175 37 ° 1 0 ' 37 ° 9 ' 37 ° 8 ' -113°1'-113°2'-113°3'-113°4'-113°5' Nitrate (mg/L) Subdomain Virgin River corridor South Mesa South Mesa subdivision Groundwater flow Flow transect Flow direction Rockville boundary 0 0.5 1 Miles 0 10.5 Kilometers Project area Utah ± 9 Figure 3. Nitrate concentrations in groundwater and groundwater flow direction and flow transects used to evaluate each subdomain in the Rockville study area. Figure 3. Nitrate concentrations in groundwater and groundwater Àow direction and Àow transects used to evaluate each subdomain in the Rockville study area. Site ID WIN1 or Water Right Screened or Open Interval (ft) Yield (gpm) Pump Duration (hr) Well Diam. (in) Well Depth (ft) Method Transmissivity (ft2/day) Hydraulic Conductivity (ft/day) Rockville area WL-GRAV 81-3607 50 200 0.33 16 100 TGUESS2 3407 68 WL-TFN 431101 5 35 1 8 45 TGUESS 832 166 WL-SPIT 81-4217 55 25 4 8 105 TGUESS 1910 35 WL-FORD 431047 5 30 1 8 60 TGUESS 697 139 -81-424 5 12 1 6 40 TGUESS 72 14 -434322 20 50 24 8 110 TGUESS 193 39 -434350 8 6 1 8 120 TGUESS 500 100 -81-1117 40 7 7.75 6 220 TGUESS 92 5 WL-RPC3 3220 10 --6 100 estimated 70 1.34 Apple Valley area (used for South Mesa calculations) -439070 144 69.5 24 6 567 Cooper and Jacob3 61 0.42 -429022 48 138 24 8 290 Cooper and Jacob 193 1.93 -428423 100 162 10 8 304 Cooper and Jacob 447 9.31 1 XQLTXHZHOOLGHQWL¿HUXVHGE\WKH8WDK'LYLVLRQRI:DWHU5LJKWV 2 Bradbury and Rothschild, 1985 3 Cooper and Jacob, 1946 Table 1. $Tuifer SroSerties determined from water well log sSeci¿c caSacity data and aTuifer test data. 9Analysis of septic-tank density for Rockville, Washington County, Utah analysis of nitrate plus nitrite (Figure 3). Nitrate plus nitrite concentrations ranged from <0.1 mg/L (non-detect) to 0.897 mg/L (Table 4), with a geometric mean of 0.26 mg/L. The highest nitrate concentration, although still low, is downgra- dient from agricultural fields and a small livestock operation, suggesting a possible connection to fertilizer or animal waste. Results: We plotted the projected nitrate concentrations ver- sus the number of septic-tank systems (Figure 4A). We used an average baseline nitrate concentration of 0.26 mg/L based on data collected for this study. The Virgin River corridor is 1 unique well identifier used by the Utah Division of Water Rights 2 total dissolved solids 3 percent modern carbon 4 tritium unit 1 unique well identifier used by the Utah Division of Water Rights 2 results in italics denote sample reporting limit Table 2. Aquifer parameters used to compute groundwater flow available for mixing. Table 3. Major ion and environmental tracer data. . Table 4. Nitrate data and field parameters. . Subdomain Hydraulic Conductivity (K)Mixing Zone Thickness (b) Cross-section Width (L) Hydraulic Gradient (I) Groundwater Flow Available for Mixing (Q) Virgin River corridor 47 ft/day 50 ft 3750 ft 0.007 0.71 cfs South Mesa 1.96 ft/day 50 ft 5000 ft 0.027 0.15 cfs South Mesa subdivision 2000 ft 0.06 cfs Site ID WIN1 Latitude Longitude TDS2 Ca Mg K Na Cl SO4 HCO3 CO2 δ13C Carbon-14 Tritium mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L ‰pmC3 TU4 WL-MAYL 21759 37.1562 -113.0416 2990 477 106 10.4 282 202 1560 423 90.4 -13.62 93.92 ± 0.29 1.80 ± 0.10 WL-RPC3 3220 37.1498 -113.0257 362 29.1 15 4.84 111 19.1 25.8 364 106 -9.14 60.38 ± 0.20 0.03 ± 0.01 Site ID WIN1 Latitude Longitude pH Temperature (°C) Specific Conductance (μS/cm) Nitrate (mg/L) Virgin River corridor WL-MCG -37.16093 -113.07536 7.04 17.1 3420 0.175 WL-TFN 431101 37.16074 -113.05549 6.83 15.8 2590 0.277 WL-TFS 434322 37.16041 -113.05506 6.89 14.9 3280 0.1 WL-MAYL 21759 37.15621 -113.04164 6.83 16.2 3860 0.84 WL-FORD 431047 37.15721 -113.03962 7.44 15.8 957 0.222 WL-SPIT 18130 37.15720 -113.05001 6.92 16.8 3420 0.897 WL-GRAV 1493 37.16874 -113.07613 6.66 16.2 3280 0.1 South Mesa SP-RPC1 -37.15156 -113.02476 7.39 12.4 743 0.1 WL-RPC3 3220 37.14984 -113.02571 7.46 16.9 666 0.1 WL-RPC4 1920 37.14873 -113.02623 7.61 16 610 0.1 WL-MESA 429959 37.14719 -113.03462 6.94 17.1 832 0.1 approximately 1300 acres, excluding federal land, and has 61 septic-tank systems, making the current septic-tank system density 21 acres/system. Using the groundwater parameters described above, estimated groundwater flow available for mixing in the Virgin River corridor is 517 acre-feet/year. To limit nitrate concentration to 1.26 mg/L, an increase of 1 mg/L, the total number of septic-tank systems should not exceed 124. This result corresponds to an increase of 63 new septic-tank systems and a septic-tank system density of 11 acres/system. Results of calculations based on nitrate concentration increases of 2 and 5 mg/L are also included on Figure 4A and in Table 5. Utah Geological Survey10 21 11 7 3 acres/system 0 1 2 3 4 5 6 0 100 200 300 400 500 Vi r g i n Ri v e r co r r i d o r pr o j ecte d ni t r a t e co n c e n t r a t i o n (m g / L ) Number of septic tanks 37 13 8 3 acres/system 8 4 1.5 acres/system 0 1 2 3 4 5 6 0 20 40 60 80 100 So u t h Me s a pr o j e c t e d ni tr a t e co n c e n t r a t i o n (m g / L ) Number of septic tanks South Mesa Subdivision A B Figure 4. Projected nitrate concentration versus septic-system density for A) the Virgin River corridor subdomain and B) the South Mesa subdomain, including the South Mesa subdivision. Acres/system shown for each projected nitrate concentration. Figure 4. Projected nitrate concentration versus septic-system density for A) the Virgin River corridor subdomain and B) the South Mesa subdomain, including the South Mesa subdivision. Acres/system shown for each projected nitrate concentration. 11Analysis of septic-tank density for Rockville, Washington County, Utah South Mesa Groundwater flow variables: Although Rockville utilizes public supply wells on the South Mesa, there are no available aquifer test data from these wells. Drinking water source pro- tection plans for these wells estimate hydraulic conductivity at 1.34 ft/day. However, several supply wells located approximate- ly eight miles to the south-southwest in Apple Valley are com- pleted in the same Shinarump Conglomerate aquifer. We used aquifer test data from three of the Apple Valley supply wells to derive a geometric mean hydraulic conductivity of 1.96 ft/day for calculating groundwater flow scenarios (Table 1). We used a hydraulic gradient of 0.027 based on potentiometric surface contours of Inkenbrandt et al. (2013), a mixing depth of 50 feet, and a 5000-foot-wide transect, resulting in a groundwater flow available for mixing of 0.15 ft3/s (Table 2). We also calculated groundwater flow for a smaller subdivision of the South Mesa composed of ~60 parcels using the same parameters, with the exception of a 2000-foot-wide transect, resulting in a ground- water flow available for mixing of 0.06 ft3/s (Table 5). Environmental tracers: Tritium concentration from the South Mesa well (Site ID WL-RPC3, Table 3) is 0.03 TU. Using methods to calculate thresholds for defining modern and premodern groundwater from Lindsey et al. (2019), we determined this sample to be premodern groundwater (i.e., entered the aquifer prior to 1952). Carbon-14 concentration from this well is 60.4 pmC with a carbon-13 value of -9.1‰, which reflects an uncertain modernity, most likely indicating the water is a mixture of modern and premodern. This age suggests a slow travel time, which is supported by the rela- tively low hydraulic conductivity we derived, and/or a long flow path from recharge (Eagle Crags and similar uplands to the south) to discharge (Rimrock Springs). Baseline nitrate concentrations: We collected groundwater samples from three wells and one spring located in or down- gradient from the South Mesa area for analysis of nitrate plus nitrite (Figure 3). Nitrate plus nitrite concentrations were all <0.1 mg/L (non-detect) (Table 4). Results: We plotted the projected nitrate concentrations versus the number of septic-tank systems (Figure 4B). We used an average baseline nitrate concentration of 0.1 mg/L based on data collected for this study. The South Mesa is ap- proximately 260 acres, excluding federal land, and has seven septic-tank systems, making the current septic-tank system density 37 acres/system (Table 5). Using the groundwater pa- rameters described above and shown on Table 2, estimated groundwater flow available for mixing in the South Mesa is 111 acre-feet/year. To limit nitrate concentration to 1.1 mg/L, an increase of 1 mg/L, the total number of septic-tank sys- tems should not exceed 20. This result corresponds to an in- crease of 13 new septic-tank systems and a septic-tank sys- tem density of 13 acres/system. The South Mesa subdivision is approximately 40 acres and has no existing septic-tank systems. Estimated groundwater flow available for mixing through this subdivision is 44 acre-feet/year. To limit nitrate concentration to 1.1 mg/L, an increase of 1 mg/L, the total number of septic-tank systems in this subdivision should not exceed five. This result corresponds to an increase of five new septic-tank systems and a septic-tank system density of 8 acres/system. Results of calculations based on nitrate concen- tration increases of 2 and 5 mg/L are also included on Figure 4B and in Table 5. If every parcel in the South Mesa subdivi- sion were developed with an individual septic-tank system, the EPA 10 mg/L MCL for nitrate would be exceeded. SUMMARY AND CONCLUSIONS Rockville is a small rural town in southwestern Utah that is experiencing an increase in residential development. Like many rural parts of Utah, development often relies on individual septic tank soil-absorption systems. Groundwater from an isolated aquifer is the primary source of drinking water for most Rockville residents. Septic-tank effluent carries constituents which undergo little to no natural remediation during percolation toward the aquifer. Attenuation of these constituents is typically achieved via dilution upon reaching Study Subdomain Area (acres) Discharge (cfs) Current Density (acres/system) Current Septic Tanks Projected Nitrate Concentration (mg/L) Projected Total Septic Tanks Calculated Density (acres/system) Virgin River corridor 1300 0.71 21 61 1.26 124 11 2.26 188 7 5.26 396 3 South Mesa 260 0.15 37 7 1.1 20 13 2.1 34 8 5.1 78 3 South Mesa subdivision 40 0.06 0 0 1.1 5 8 2.1 11 4 5.1 28 1.5 Table 5. Results of mass balance analysis for different nitrate concentration projections in Rockville study subdomains. Utah Geological Survey12 the aquifer. We used nitrate, a common and mobile septic-tank effluent constituent, to evaluate dilution of wastewater in the upper zones of two aquifers in Rockville. Our evaluation used a mass-balance method based on the volume of groundwater flow available for mixing with septic-tank effluent in each aquifer. Discharge, projected number of septic-tank systems, and septic-tank density for each study domain are summarized in Table 5. The mass-balance approach indicates that the most conservative maximum septic-tank densities for the Virgin River corridor, South Mesa, and South Mesa subdivision are 11 acres per system, 13 acres per system, and 8 acres per system, respectively. These average densities are based on hydrogeologic parameters used to estimate groundwater flow volume. Connection to existing sewer systems may be a better alternative to septic-tank systems where feasible. The use of advanced onsite wastewater treatment systems could also reduce groundwater contamination. ACKNOWLEDGMENTS This project was funded in part by the Utah Division of Water Quality and the Town of Rockville. 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