Spatial patterns and seasonal timing of increasing riverine specific conductance from 1998 to 2018 suggest legacy contamination in the Delaware River Basin

• Published in: The Science of the total environment 2023-02, Vol.858, p.159691, Article 159691
• Main Authors: Rumsey, Christine A., Hammond, John C., Murphy, Jennifer, Shoda, Megan, Soroka, Alexander
• Format: Article
• Language: English
• Subjects: Delaware River Basin; Exceedance criteria; Specific conductance; Trend analysis; Water quality; WRTDS
• ISSN: 0048-9697
• DOI: 10.1016/j.scitotenv.2022.159691

Increasing salinization of freshwater threatens water supplies that support a range of human and ecological uses. The latest assessments of Delaware River Basin (DRB) surface-water-quality changes indicate widespread salinization has occurred in recent decades, which may lead to meaningful degradation in water quality. To better understand how and when salinity transport occurs and implications for DRB streams, this study: 1) explores the variability of specific conductance (SC) trends spatially and seasonally from 1998 to 2018, and 2) investigates how trends relate to streamflow, land disturbance, and impervious surface area to better understand regional salinization drivers. We find widespread increases in SC across the DRB, with several sites in the lower basin exceeding thresholds for aquatic life and experiencing increasing frequencies of exceedance over time. In general, the greatest basin wide increases in SC occurred during low flow conditions, indicating that a legacy component resulting from subsurface retention and transport processes has driven observed changes in riverine SC. For a subset of sites in the lower basin, where impervious area and cumulative land disturbance are higher, the greatest SC increases occurred during high flow conditions in winter months. Given the patterns of SC and watershed changes across the basin, as well as strong relationships between SC trends and sodium and chloride trends, deicing salt appears to be a likely driver of observed SC change. Even if deicing salt application plateaus or declines in coming years, the continued release and transport of the legacy subsurface component may still contribute to elevated DRB riverine SC. [Display omitted] •Widespread increases in SC in the DRB 1998–2018, most often during low streamflow.•Greatest SC changes in lower DRB occurred most frequently during winter high flows.•Predicted exceedance of chronic aquatic life criteria more frequent in lower DRB.•Evidence for increasing role of subsurface in delivering legacy salt to streams.•Winter stormflow runoff contributes additional pulses of SC to surface/groundwater.