The USGS National Water Quality Network: Tracking America's Rivers Over Decades

From a U.S. Geological Survey fact sheet of April 2021.

Understanding the water quality of U.S. streams and rivers takes consistent data collection and analysis over decades. The USGS National Water Quality Network (NWQN) collects comparable data in large rivers and small streams across different regions and land uses, for the federal, state and local agencies that manage the nation's water.

It is designed to answer two questions:

• What is the water quality of the nation's streams and rivers?
• How is it changing over time?

Its data support assessments and models linking landscape, climate and water quality, and forecasting how water quality may change.

Built on decades of monitoring

• 1962. The USGS set up the Hydrologic Benchmark Network to track changes in the flow and quality of streams in undisturbed watersheds.
• The following decades added other national networks: the National Stream Quality Accounting Network (NASQAN), the National Water-Quality Assessment Project (NAWQA) and the National Monitoring Network (NMN).
• 2013. The USGS created the NWQN to keep consistent monitoring going at selected sites from those networks — sites upstream of key receiving waters such as the Gulf of Mexico, sites with long records, and sites that broaden the range of regions and land uses covered.

The network in 2021

The NWQN had 110 sampling sites on large coastal and inland rivers and on small streams in agricultural, urban and reference (minimally disturbed) settings, sampled with consistent methods so water quality can be compared across the country and through time.

• Sampling is typically 12 to 24 times a year, depending on the type of site.
• Every sample is analysed for physical properties, major ions, and selected trace elements and nutrients; samples from coastal rivers, large inland rivers and agricultural and urban sites are also analysed for suspended sediment and pesticides.
• Streamflow. Every site is paired with a streamgage run by the USGS or another agency, giving continuous (hourly to daily) flow data. Some sites also have continuous sensors reporting hourly water temperature, pH, specific conductance, turbidity and dissolved nitrate.

Figure 2. Sampling the Delaware River at Trenton, New Jersey (USGS station 01463500). Photograph by Molly L. Schreiner.

Methods

Where possible, samples are collected across the width and depth of the stream, so data are comparable between sites and over time. They are analysed at the USGS National Water Quality Laboratory using documented methods. Field blanks, replicates and spikes are analysed each year at every site for quality control, and the data are analysed statistically with published methods and updated each year.

Tracking water quality

Combined with streamflow, landscape and other data, NWQN results show how much of each constituent rivers carry to receiving waters, how concentrations and loads are changing, and how pesticide levels compare with health benchmarks. The USGS Tracking Water Quality website presents these results, updated annually:

• Loads and trends. Observed concentrations and estimated annual loads, plus statistical estimates of trends that remove the effect of year-to-year swings in streamflow, with tables quantifying the trend over each decade up to the latest year.

Figure 3. Annual phosphorus loads and trends, Mississippi River near St. Francisville, Louisiana (USGS station 07373420). Credit: U.S. Geological Survey.

• Pesticides. The number of pesticides analysed and detected, compared with human-health and aquatic-life benchmarks. At Contentnea Creek at Hookerton, North Carolina (station 02091500), in 2019:

Sources

• Riskin, M.L., and Lee, C.J., 2021, USGS National Water Quality Monitoring Network: U.S. Geological Survey Fact Sheet 2021–3019. https://doi.org/10.3133/fs20213019
• The photograph, graph and pesticide figures are taken from the fact sheet's PDF. Its map of the network, on an Esri basemap, is not reproduced.
• Rewritten in hubnx's own words.