Although concentrations of nitrate, dissolved solids, and chloride in most wells generally meet EPA drinking-water standards or guidelines, nationally, the proportion of samples exceeding the limits for nitrate and dissolved solids increased from 12 to 15 percent over the decade. Most of the increases in concentrations occurred for shallow monitoring wells installed in urban or agricultural areas. The implications of the findings are that concentrations of these contaminants in deep aquifers are likely to increase during the next decade as shallow groundwater with elevated concentrations moves downward. The potential for contamination of the deep aquifers requires attention because these aquifers are commonly used for public water supply, and because restoration of groundwater is difficult and expensive.
In areas where large amounts of road salt are used for deicing, groundwater contributions of chloride to stream base flow is contributing to exceedances of the chronic aquatic life criteria of 230 mg/L. This finding has the ecological community very concerned because even if loadings to the land surface were significantly reduced, improvements in stream quality may not be apparent for decades because of the slow rate of groundwater movement to streams.
USGS believes that continuing the long term, consistent monitoring of nutrients and other constituents in ground water; improved accounting of contaminant sources; and improved tracking and modeling of climate and landscape changes will be essential during the next decade of NAWQA. This work will help distinguish trends in groundwater quality, understand the causes of trends, and accurately track the effectiveness of strategies to manage and protect this critical resource. If you have any questions regarding the findings presented in this report or interactive maps, please contact Bruce Lindsey at blindsey@usgs.gov or at 717-730-6964.