FATE AND TRANSPORT OF SEPTIC TANK DERIVED NITROGEN IN AN ALLUVIAL AQUIFER, CARSON VALLEY, NEVADA

Residents of Carson Valley, Nevada are dependent on groundwater as a source of drinking water and agricultural irrigation. Municipal and domestic wells obtain water from a mostly unconfined alluvial aquifer that underlies most of the developed areas of Carson Valley. Over the past few decades there has been a rapid increase in population and because of a lack of municipal infrastructure for wastewater disposal, there has been a degradation of groundwater quality in some areas. There are over 4000 septic tanks in the valley, which have increased nitrate and dissolved solids concentrations in parts of the aquifer. A statistical analysis using spatial data from this and other studies suggests that elevated nitrate concentrations are related to single-family land-use and septic tank density. In order to determine the fate and transport of nitrogen in the aquifer, 27 nitrogen isotope, 30 CFC, 37 tritium and over two hundred nitrate samples were collected from wells throughout the valley between October 2008 and August 2009. Nitrate concentrations were relatively low (1.9 mg/L), but ranged from below detection (0.16 mg/L) to 18.3 mg/L. Areas with high septic tank densities (one septic tank per acre) had average nitrate concentrations between 4 and 6 mg/L, two to three times the average for the rest of the valley. All 27 nitrogen isotope samples fell in the range of natural soil and septic/animal waste. Correlation between high nitrate concentrations and heavier isotope values may indicate that septic systems are the primary source. CFC and tritium samples showed mean residence times, from the time water enters the aquifer to the time it reaches the well, between 20 and 40 years. 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 community water supply wells for a long time. A nitrogen budget will be calculated for the aquifer using collected data and a contaminant transport model, constructed in MT3DMS, to determine how nitrate loading may change over time and space.