TEMPORAL TRENDS IN GROUND-WATER NITRATE CONCENTRATIONS, DOUGLAS COUNTY, NEVADA

Douglas County, located in western Nevada on the Nevada/California border, contains 3 general areas where ground water is used as a drinking water supply; these areas are the Carson River Basin (CRB), Lake Tahoe Basin (LTB) and Topaz Lake Basin (TLB). The population of the county has increased greatly over the past 30 years. This has placed increasing pressure on ground-water quality and quantity in the county. The county began monitoring water levels and quality in 1985 to measure the effects of population growth on ground-water resources in the county. Elevated nitrate concentrations have been the primary water-quality issue and there are a number of anthropogenic sources of nitrate in the county. These include nitrate from agriculture, irrigation using wastewater, septic tanks, and domestic fertilizer application. However, only contributions from septic tanks and home gardens and lawns are significant in the LTB and TLB. Distinguishing between the different sources of nitrate in the CRB is the main focus of this study.

Analysis of 37 monitoring wells in all three basins, which have long-term records (>5 years of data from 1980 to the present) of nitrate concentrations, indicates that 43% of these wells show increasing trends over time, 22% of wells show decreasing trends and 35% haven’t changed during the sampling period. All but three of the wells that show increasing trends are located in the CRB, while half of the wells that show decreasing trends are located in the LTB. Only 4 wells in the TLB have long-term records. Two of these wells show increasing trends and one shows a decreasing trend.

The number of septic tanks has increased in Douglas County over the last 30 years. In 1970, there were 610 registered septic tanks in the county. This figure has grown to 5157 in 2002, an increase of approximately 1500 septic tanks every 10 years. Most of this increase has been in the CRB; some of this has been at the expense of agricultural land.

Nitrate concentrations in the ground water do not show depth related trends in wells from the CRB and TLB (R²=0.01) or the LTB (R²=0.06). For example, ground-water samples taken from a well with a static water level 75 m below land surface has a nitrate concentration of >4 mg/L, which is double the mean nitrate concentration for the 37 wells sampled. Analysis presently is ongoing to better define the observed trends.