Data from a 16-year period were used to investigate ground-water
quality and its relation to nearby land uses in the Carson Valley area of
Douglas County, Nevada. Within 500 m of ground-water wells with increasing
nitrate concentrations, properties on septic systems accounted for 40% of
the area, whereas agricultural areas accounted for 8.5%. Agricultural
lands comprised 18% of the area near wells with stable contamination
trends and 40% near wells with decreasing trends, indicating that lands
converted from agriculture to nonsewered properties may be related to
increases in nitrate concentrations. The mean size of parcels with septic
systems near wells with increasing nitrate concentrations was about half
(1.1 ha) that of those near wells with stable nitrate concentrations (2.0
ha). For wells with increasing nitrate and dissolved-solids trends, 50 to
55 properties utilized septic systems within 500 m, compared to fewer than
25 properties near stable trends. The number of wells with increasing
trends 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 to 30 years
before increasing trends can be identified. Geoprocessing tools and
Geographic Information System (GIS) methods in this study can be used for
efficient interpretation of ground-water contaminant location and trend,
relative to surrounding land uses.