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