The Delaware River Basin supplies drinking water to 13.3 million people. Its rivers were badly polluted before the mid-20th century and have been cleaned up a great deal since. A U.S. Geological Survey analysis of trends through 2018 found two stories running in opposite directions: nutrients are falling, and salt is rising — faster than before.
Figure 1: sites with a trend for at least one measure. USGS
How it was measured
| Sites | 124 river sites across the basin |
| Measures | salinity (specific conductance, total dissolved solids), major ions, nutrients, suspended solids |
| Data | eight monitoring organizations — the USGS, the Delaware River Basin Commission, and the state agencies of Delaware, New Jersey, New York and Pennsylvania, plus New York City and the New Jersey Pinelands Commission |
| Periods | the longest record at each site: starting between 1968 and 2009, ending 2017 or 2018 |
| Methods | Seasonal Kendall trend tests and, where possible, the WRTDS model — which agreed closely; significant at p ≤ 0.1 |
Pooling eight organizations' records gave far more sites, and richer records per site, than any one could alone.
Salt: up, and speeding up
Salty water tastes wrong, can harm health, and corrodes pipes.
- Across sites, the median rise over each record was 27.5% to 76.4% for total dissolved solids, specific conductance, sodium and chloride.
- Specific conductance rose at 73% of sites and fell at 8%; total dissolved solids rose at 50% and fell at 7%.
- Sodium and chloride rose almost everywhere: only 1 and 6 sites respectively went down.
- It is accelerating. The median 5-year rise in specific conductance was 8 µS/cm in 1993–98 and 14 µS/cm in 2013–18. Dissolved solids, sodium and chloride follow the same pattern.
- Sites whose watersheds changed more on the surface tended to salt up more — a starting point for finding the causes.
Figure 2: change from start to end of each record — salinity and ions (A), nutrients (B). USGS
What to watch. At current rates, chloride at most sites would not pass the secondary drinking-water standard within 100 years. Total dissolved solids is the nearer concern: it already exceeds the standard at four sites, and is rising toward it at another 43%.
Nutrients: down, but not far enough
Excess nitrogen and phosphorus feed algae, and algae can strip water of oxygen. Here the trend is good:
- Median changes across sites ranged from −20% to −52%.
- Falls in total nitrogen sped up between 2003 and 2008, then held at the larger rate.
- Unlike salt, nutrient trends showed no link to how much the land surface changed.
- But most sites with significant nitrogen and phosphorus trends are still above the EPA's nutrient criteria for their ecoregion — the improvement is real, the job is not done.
Figure 3: 5-year changes — specific conductance rising faster (A), total nitrogen falling faster (B). USGS
Figure 4: salinity tracks land-surface change (A); nitrogen does not (B). USGS
Sediment
Total suspended solids had the most unclear results — 68% of trends not significant. Where there was a trend, it was down, a sign of less soil erosion in the basin.
Sources
Written from River Water Quality in the Delaware River Basin—Concentrations and Trends Through 2018, U.S. Geological Survey Fact Sheet 2023–3014; a work of the United States government in the public domain. It cites the Delaware River Basin Commission (2019), the EPA (2021), Murphy and others (2020), Shoda and others (2019), Shoda and Murphy (2022), and Kummer (2020).
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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