Changing Water Quality in the Delaware River Basin, 1972–2012

The Delaware River from a drone. Joe Adams (pilot) and Joseph Duris (frame), USGS.

A pilot for water availability

In 2019 the U.S. Geological Survey began a pilot Integrated Water Availability Assessment (IWAA) in the Delaware River Basin, to test ways of judging how much water is available for people and ecosystems, and why. Water quality is central: it shapes ecological health and whether water is fit for drinking, recreation, farming and industry. Seeing how it has changed with the landscape and climate shows the challenges ahead.

Of the sites in the USGS's national trend study, 22 are in the basin, across the Appalachian Plateau, Valley and Ridge, Piedmont and Coastal Plain provinces. They give a first look at how nutrients (total phosphorus, total nitrogen, nitrate) and salinity indicators (specific conductance, sulfate, chloride) changed from 1972 to 2012.

Figure 1. The basin's trend sites by physiographic province; none are in the New England province. USGS.

What changed through 2012

Figure 2. Annual mean concentrations at the basin's trend sites. The number of sites varies by measure. USGS.

• Saltier water: specific conductance and chloride began rising as early as 1972, with some of the biggest increases at the sites that were already highest.
• Less sulfate: sulfate fell steadily.
• Nutrients: changes were smaller and less consistent. Total phosphorus and total nitrogen fell slightly; nitrate barely changed.
• Where: some of the highest concentrations were at sites in the Piedmont and Coastal Plain.

Nutrients: a good sign

Over the past decade or more, nutrient concentrations and loads fell slightly or held steady — encouraging, since many mid- and North Atlantic estuaries are prone to eutrophication and the population keeps growing. Similar small declines elsewhere in the region bode well for limiting harmful algal blooms, which are becoming common in large rivers and estuaries.

Algae in a river. Jennifer Murphy, USGS.

Salt and corrosion

Rising specific conductance and chloride with falling sulfate suggest rivers and streams may be growing more corrosive.

• Urban, snowy areas like the Delaware basin saw bigger chloride increases since the early 1980s and 1990s than other parts of the country and less urban basins.
• What that means for drinking-water systems is unclear — but lead action-level exceedances tend to be higher where water carries more chloride relative to sulfate.
• The likely source is road salt. Balancing winter safety against the quality of drinking-water sources has no clear solution.

What it means for water use

• Drinking-water standards: in 2012, chloride, sulfate and nitrate were far below them. Even rising chloride stayed well under the national secondary standard of 250 mg/L.
• Ecosystems: total nitrogen and total phosphorus were near or above EPA's ecoregional nutrient criteria — 0.12–0.18 mg/L for total nitrogen and 0.01–0.7625 mg/L for total phosphorus, depending on the ecoregion — levels at which streams may face eutrophication.
• Looking ahead: some trends, or the lack of change, could become problems as urbanization continues and the climate brings warmer temperatures and heavier rain.
• Streamflow: across the Northeast, high flows have grown more frequent, changing stream chemistry, especially chloride and other major ions. In urban basins with harsh winters, like the Delaware, road-salt chloride may be worsened by climate-driven changes in flow.

Sources

Based on A Historical Look at Changing Water Quality in the Delaware River Basin, USGS Fact Sheet 2020–3007 (March 2020), by Jennifer C. Murphy and Megan E. Shoda, Integrated Water Availability Assessments Program, U.S. Geological Survey; a work of the United States government in the public domain. The map, charts and photographs are taken from the fact sheet's PDF.