Decadal Changes in Agricultural Contaminants in Groundwater in the United States, 1988-2015
U.S. Geological Survey
The U.S. Geological Survey, National Water-Quality Assessment (NAWQA) Project has been evaluating groundwater quality with respect to agricultural and other contaminants for more than 25 years. A key element of these studies has been resampling of networks of wells to evaluate changes in water quality on a decadal scale. Results from the period of 1988-2000 are compared to results from the period of 2000-2012. An interactive web-based mapping tool has been developed to illustrate these changes. This tool allows the user to display the statistical results for groundwater networks for 24 constituents, including agricultural contaminants such as nitrate, phosphorus, and several pesticide compounds. In addition to the comparison of data between the first and second decade of sampling, data from a third decade of sampling are available for those networks that have been sampled from 2013 to 2015. Highlights of the findings from the comparison of the first 2 decades include significant increases in concentrations of nitrate and the atrazine degradate deethylatrazine in about 21 percent and 25 percent of the 67 networks, respectively. Dissolved solids and chloride concentrations also increased in a large percentage of networks but have many sources in addition to agricultural activity. Other sampling and analysis has been done to enhance the understanding of the decadal-scale changes. Biennial sampling took place for a 10-year period on a subset of wells in each network, and can be used to help determine whether the decadal changes are part of a long-term trend, or to illustrate the timing of a trend reversal. Groundwater age-dating and flow modeling also provide perspective on the time frame in which changes might be expected, given a change in source inputs. The NAWQA project has also implemented continuous (daily) monitoring at a subset of wells across the country in order to understand changes that take place on a time scale that is less than a decade. In addition, wells are being sampled along a vertical profile in selected areas to help understand changes that take place on a time scale greater than a decade.