Distribution and Transport of Nitrate in an Unconfined Alluvial Aquifer, Carson Valley, Douglas County, Nevada, 2009.

Abstract

Residents of Carson Valley, Douglas County, Nevada rely on groundwater from an unconfined alluvial aquifer for drinking water and agricultural irrigation (although the Carson River is also used for irrigation). Since the 1970's, there has been a rapid increase in population in several parts of the valley with little economic incentive for commercial builders to construct centralized wastewater disposal and drinking water systems. This has resulted in several areas having high septic tank densities (>1.0 septic tanks/ha) with the majority of residents in these areas relying on private domestic wells as a source of drinking water. The majority of these domestic wells are screened in an unconfined alluvial aquifer with most groundwater levels within 120 m of the land surface. In order to determine the distribution and future impacts of nitrate concentrations on wells, over 200 groundwater samples were collected and two contaminant transport models were constructed. An ion selective electrode (ISE) was used to measure nitrate, chloride, and bromide concentrations for the sampled wells. Nitrate-N concentrations ranged from below detection (<0.05 mg/L) to 18.3 mg/L with a mean of 2.2 mg/L and a median of 1.2 mg/L. Three of the sampled wells had a nitrate-N concentration that exceeded the USEPA's MCL (10 mg/L). Chloride concentrations ranged from below detection (<0.3 mg/L) to 238.0 mg/L with a mean of 12.2 mg/L and bromide concentrations ranged from below detection (<0.05) to 0.19 mg/L, although values above 0.2 mg/L were considered inaccurate because of interferences with chloride during analysis. Results from quality control samples show that the ISE can be used to efficiently and accurately measure nitrate, chloride, and bromide concentrations for groundwater in Carson Valley.A spatial comparison of land use data and nitrate concentrations show a weak correlation between elevated nitrate concentrations, single-family land use, and septic tank density. Wells with more than 50% single family land use, in a 500 m buffer area, had nitrate concentrations two times higher than areas with less than 50% single-family land use. Areas with septic tank densities greater than 1.14 septic tanks/ha had nitrate concentrations two to three times higher than areas with densities less than 1.14 septic tanks/ha. In addition, there was a correlation between elevated nitrate concentrations and the amount of time that septic tanks have been in use in an area. Areas with high septic tank densities (>0.6 septic tanks/ha), where septic tanks have been in use for more than 20 years, had nitrate concentrations two to three times greater than areas with similar septic densities but the tanks have been in use for less than 20 years. Lower nitrate concentrations in the areas where septic systems are less than 20 years old are likely caused by storage of septic tank leach field nitrogen in the unsaturated zone. In order to determine the source and transport of nitrogen in the aquifer, 30 CFC, 37 tritium, 5 tritium/helium, and 37 nitrogen isotope samples were collected from wells throughout the valley. Mean residence times from CFC, tritium, and tritium/helium results show groundwater age ranges from 10 to greater than 50 years old with most of the results between 20 and 40 years old. A correlation between high nitrate concentrations and younger water shows the potential for elevated nitrate, from septic tank leach fields, to affect large parts of the aquifer and potentially public water supply wells for a long time. Thirty-four of 37 nitrogen isotope results fell in both the soil and manure and septic waste range. However, a correlation between high nitrate concentrations and heavier isotope values indicates that septic systems are the primary source of nitrogen. Contaminant transport models were constructed for two areas that have elevated nitrate concentrations (>3.5 mg/L) and high septic tank densities (>1.0 septic tanks/ha). Simulated nitrate concentrations were similar to observed concentrations at the subdivision scale for samples collected during this study. Results for the predictive models indicate that with a sufficient amount of time, nitrate concentrations will continue to increase both horizontally and vertically and may exceed the USEPA's MCL over extended areas within the subdivisions. Increases in nitrate concentrations in the horizontal directions were limited because of pumping from domestic wells that retards lateral groundwater flow. Vertical profiles of the predictive models show nitrate moving deeper into the groundwater system, potentially affecting public supply wells. Simulated nitrate concentrations in the shallow groundwater system are high compared to simulated concentrations at depths where most of the domestic wells are screened. This could suggest that even if septic tanks are removed, there could be a large amount of nitrate stored in the shallow groundwater (above the screened interval of most wells) that could continue to increase nitrate concentrations in the future. These results are consistent with groundwater and septic tank age results. Hypothetical models, simulating the removal of septic tanks and wells, show the largest decrease in nitrate concentrations occur when septic tanks are removed and wells continue to pump. With continued growth and change in land use in the valley, nitrate concentrations will continue to increase in areas that are dependent on septic tank systems.