An Overview of Studies of Agricultural Contaminant Trends in Groundwater in the United States
U.S. Geological Survey
The U.S. Geological Survey (USGS), National Water-Quality Assessment (NAWQA) program began in 1991. As part of the NAWQA program, groundwater-quality data from several thousand wells across the United States has been collected, including pesticide and nutrient analyses and other data related to agricultural activity. Over the previous two decades, many study wells have been sampled multiple times. These data, along with groundwater age-dating results, present an opportunity to analyze trends in groundwater quality in many regions of the United States. The approaches to analyzing the groundwater-quality trends data vary depending on the goals of the analysis. Determining whether or not a constituent has increased in concentration on a decadal basis is the simplest question that can be answered; however, this is typically not the only relevant question. What caused the apparent trend or non-trend in concentration? Will the same trend continue into the future? What management practices could affect future trends? Answering some of these questions requires supporting data from a variety of sources within and outside of the USGS. The NAWQA program has formed a team of scientists to investigate groundwater-quality trends at national and local scales. Statistical approaches are utilized for data analysis at the national scale. Groundwater modeling and geochemistry are utilized in local- and regional-scale studies to gain additional insight into the factors leading to groundwater-quality trends, and to allow predictive modeling of groundwater-quality trends into the future. For example, data on age-dating tracers have been collected to improve the understanding of travel times and to reconstruct groundwater trends. These data are being used to evaluate trends in nitrate concentrations and to understand the fate of atrazine in the vadose zone and shallow groundwater in multiple aquifers throughout the United States. Additionally, the groundwater trends team is conducting detailed studies using flow and transport modeling methods to simulate the long-term fate of nitrate concentrations in agricultural regions. These efforts will include detailed analysis of the existing distribution of nitrate and accompanying nitrate-attenuation factors (such as oxidation-reduction processes) as well as forecasting of long-term trends in nitrate concentrations. The combination of data and studies from both within and outside of the NAWQA program are used in order to provide the best explanation of trends in groundwater quality.