A GEOSPATIAL METHOD FOR IDENTIFYING NITRATE AND DISSOLVED-SOLIDS SOURCES IN GROUND WATER: A CASE STUDY FROM CARSON VALLEY, NEVADA, USA

Douglas County, in western Nevada, has experienced a population growth rate of nearly 500 percent over 30 years; the result is land-use change and greater demand for high-quality ground water. Data from a 16-year period (1985-2001) were used to investigate ground-water quality and its relation to nearby land uses in the Carson Valley area of Douglas County. Within a 500-meter radius of ground-water wells with increasing nitrate concentrations, properties on septic systems accounted for 40 percent of the area, whereas, agricultural areas accounted for 8.5 percent. Agricultural lands comprise 18 percent of the areas with stable trends and 40 percent with decreasing trends; an indication that land conversion from agriculture to nonsewered properties may be related to increases in nitrate concentrations in the valley. Older septic systems were not correlated with high nitrate or dissolved-solids concentrations; however, a positive correlation (R²=0.42) was observed between increasing trends in nitrate concentrations and high numbers of nearby septic systems. The mean size of parcels with septic systems near wells with increasing nitrate concentrations was about half (1.1 hectares) compared to those near wells with stable nitrate concentrations (2.0 hectares). For wells with increasing nitrate and dissolved-solids trends, 50-55 properties utilized septic systems within a 500-meter radius, compared to less than 25 properties near stable trends. A comparison of tests from two long-term data sets (1976-1988 and 1985-2001) indicates the number of wells with increasing concentrations rose from 2 in 1988 to 15 in 2001 for nitrate, and from 1 to 14 for dissolved solids; an indication that it takes a minimum of 20-30 years before increasing trends can be identified. This amount of time is comparable to age determinations of 37-58 years for natural recharge to occur, as interpreted from tritium activities analyzed from well water in Carson Valley. The procedures used in this study to delineate land use and identify septic-tank characteristics can be applied in any unconfined aquifer system. Analysis of land use in relation to trends in ground-water quality can be used by water managers to protect water quality in vulnerable aquifers.