Nitrate and Chloride Data for evaluating Decadal Changes in Groundwater

The U.S. Geological Survey’s National Water Quality Network (NWQN) is the primary source of data to monitor trends in contaminants in groundwater in the United States. However, other data sources are available to evaluate trends. The Water Quality Portal (WQP) and the National Groundwater Monitoring Network (NGWMN) portal provide data that can be used for groundwater quality trends in areas that are not covered by the NWQN or can be used to compare results in areas that are covered by the NWQN. Using the WQP and NGWMN datasets (and excluding sites in the NWQN), networks of wells were created for trend analyses based on the distribution of available data that passed screening criteria and based on the aquifer the well is in. The samples were grouped into three periods for the analyses (referred to as decades) to make results comparable to the NWQN; 1988-2001, 2002-2012, and 2013-2022, and one sample per site was selected within each decade. About 7,230 wells in 59 networks were available for analysis from the WQP and NGWMN. In some cases, aquifers had sites covering large regional areas, and networks were created to represent different regions of that aquifer. In other cases, aquifers had sites covering small, local areas. The sites and sampling frequency were not originally designed for trend analysis or as networks and therefore do not take into account factors such as the spatial distribution, well depth, land uses, and purpose of sampling.