The Delaware River Basin provides freshwater to 8.3 million people living in the basin and supplies an additional 5 million people outside of the basin with drinking water. The basin also provides recreational opportunities to millions of residents and visitors every year, and its waters are essential for supporting ecological communities, industry, and transportation. Before the early 20th century, water quality of the basin had been degraded to the point of interfering with its use for human and ecological purposes. Over decades, the dedicated and coordinated efforts by Federal and state governments and non-governmental organizations improved the quality of water throughout the basin. Many organizations currently monitor surface-water-quality to track changes in conditions that might warrant a change in how water quality is assessed, managed, or regulated. In support of this tracking, over 260,000 water-quality records provided by 8 different organizations were compiled, screened, and used to generate surface-water-quality trends in the Delaware River Basin.

This report presents trend results in 16 constituents at 124 sites, using 2 different methodologies and data collected by 8 different organizations. The results reveal generally upward trends in specific conductance, total dissolved solids, chloride, and sodium. The increases in these constituents were some of the greatest changes observed in the basin and occurred at faster rates over time. By contrast, trends in nutrients (ammonia, nitrate, orthophosphate, total nitrogen, and total phosphorus) tended to decline, although many sites still exceeded the U.S. Environmental Protection Agency ecoregional criteria for total nitrogen and total phosphorus. Investigating long-term trends in water quality can help the Delaware River Basin water management community quantify the success of management practices and identify potential threats to water availability.

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