Water quality data is freely available through the USGS National Water Information System (NWIS) and the Water Quality Portal (data from multiple agencies, including the USGS). Sometimes these data are hard to understand, so you can also contact the USGS Water Science Center for the state in question to request data and reports from studies that might have been conducted on water quality.
The USGS collects water-quality information for both groundwater and surface water. When staff visit a site to collect data, they collect information in the field and collect samples to send to laboratories for analysis. In addition, some USGS monitoring locations collect water quality data using sensors. See a list of all monitoring locations that are currently transmitting these data in near real-time.
If data collected by the USGS exceeds water quality benchmarks, we do not indicate that on our websites; we encourage you to use other tools, such as EPA’s How’s My Waterway, to understand more about the data we collect.