Groundwater Nitrogen Budget for Carson Valley
Twenty-seven domestic water supply wells in Carson Valley have shown more than a 50 percent increase in nitrate concentrations from 1985 to 2001. Although the main source of increasing nitrate concentrations is now relatively well defined, it is not known how high nitrate concentrations will change in the future and if the increase will go higher than the MCL for nitrate set by the USEpA. In order to address these issues, an estimate of the amount of nitrate entering the system and a nitrogen budget to assess gains and losses from the aquifer system is needed.
Determining a nitrogen budget for Carson Valley will help Douglas County manage existing water-quality problems, plan development of sewer systems to maximize reductions of nitrate to groundwater, and help provide information for different management scenarios in the basin. The study will show the areas that are of greatest concern and will allow County officials to concentrate efforts on those areas where nitrate contamination is most likely and will have the greatest detrimental effect on the aquifer.
In order to determine a nitrogen budget for the Carson Valley aquifer, nitrogen concentrations from different locations and depths within the aquifer will be needed, source terms will need to be evaluated, flow paths and travel times of water in the aquifer need to be determined, and ET and hydrologic properties of the aquifer need to be estimated by
- sampling as many wells in Carson Valley as possible drawing water under different land uses (a minimum of 100 wells will be sampled each time) during two summer periods using field nitrate ion selective probes and comparing these results with selected analyses from the USGS National Water Quality Laboratory
- monitoring selected wells where nitrate concentrations are elevated for nitrogen isotopes, and tritium and CFC age determinations,
- incorporating GIS analysis to determine land use within a 500 m buffer around each well using aerial photographs,
- derive estimates of nitrogen source terms from other studies that will measure inputs from septic tanks, and estimates from natural background sources and other human sources,
- estimate ET from nearby ET measurements and other data to provide nitrogen budget input to different modeling scenarios, and
- develop a nitrogen budget model based on the groundwater flow model being developed in another project for the CWSD. These data will be combined to determine a nitrogen budget for all inputs and outputs to the basin. Those inputs and outputs that will not be measured directly will be estimated from the literature.
Twenty-seven domestic water supply wells in Carson Valley have shown more than a 50 percent increase in nitrate concentrations from 1985 to 2001. Rosen (2003) and Shipley and Rosen (2005) concluded that nitrate increases were due to the increasing use of septic tank systems in the valley over this time. The locations of the wells with increasing trends are throughout the valley. Examination of a subset of the 27 wells used in the Rosen (2003) study show continued increases in nitrate concentrations in some wells. In order to address these issues, an estimate of the amount of nitrate entering the system and a nitrogen budget to assess gains and losses from the aquifer system is needed. This study will determine a nitrogen budget for Carson Valley by monitoring groundwater nitrogen concentrations, estimating groundwater travel times, using source term information from other studies and combining this information with a groundwater flow model that is being developed by another study in Carson Valley and a new contaminant transport model. This nitrogen budget will be used to develop a management plan for the basin in order to determine what effect different water use scenarios will have on nitrogen concentrations over time in the basin. The study builds on a previous study that determined trends in nitrate concentrations over time and determined the likely cause of increasing trends. This study will utilize results of a water quantity investigation being conducted by the USGS in Carson Valley for Douglas County and a similar nitrogen budget being researched in Spanish Springs Valley for Washoe County.
Rosen, M.R., Kropf, Christian, and Thomas, K.A., 2006, Quantification of the contribution of nitrogen from septic tanks to groundwater in Spanish Springs Valley, Nevada: U.S. Geological Survey Scientific Investigations Report 2006-5206, 12 p.
Kropf, C.A., Dragan, D., and Rosen, M.R., 2004, Septic nitrate contamination of municipal water supply wells—trends and remedial efforts: Geological Society of America, Abstract with programs, Vol. 36, no. 5, p 330.
Rosen, MR, 2003, Trends in nitrate and dissolved-solids concentration in groundwater, Carson Valley, Douglas County, Nevada 1985-2001: U.S. Geological Survey Water Resources Investigations Report 03-4152.
Location: Carson Valley, western Nevada
Start Date: 2008
End Date: 2011
Michael Rosen, PhD.
USGS Nevada Water Science Center
2730 N. Deer Run Rd.
Carson City, NV 89701
phone: (775) 887-7683
CWSD: Carson Water Subconservancy District
GIS: Geographic Information Systems
MCL: Maximum Contaminant Level
USEPA: U.S. Environmental protection Agency