Chlorides are found in water and wastewater and at elevated concentrations can impart a salty taste to drinking water. Chloride concentrations also indicate the degree to which ocean derived saltwater has moved into the freshwater portion of the upper estuary. DRBC has adopted criteria and monitors chlorides to ensure water quality in the upper estuary remains suitable as a source of drinking water and protective of aquatic life.

One important metric for understanding salinity concentrations in the Delaware Estuary is the seven-day average location of the salt line, the 250 mg/L chloride concentration based on drinking water quality standards. The salt line's location fluctuates along the tidal Delaware River as streamflows increase or decrease in response to changing inflows, which either dilute or concentrate chlorides in the river. It is used by DRBC as an indicator of salinity intrusion in the Delaware Estuary and is tracked as part of the commission's flow and drought management program, which focuses on controlling the upstream migration of salty water from the Delaware Bay during low-flow conditions. As salt-laced water moves upriver, it increases corrosion control costs for surface water users, particularly industry, and can raise the treatment costs for public water suppliers. Salinity levels also affect aquatic living resources.

Over the past several years, instream monitoring of the non-tidal Delaware River has shown an upward trend in chloride concentrations. This trend is not just seen in the Delaware River's freshwater, it is becoming commonplace in areas of the U.S. with significant roadway de-icing activity. While concentrations are still below criteria for drinking water and aquatic life use, the trend is of concern. Studies suggest that chloride concentrations in winter could increase as much as a hundred-fold over summertime levels, and that mean annual levels increase as a function of impervious surface. Additional monitoring and investigation into sources, mitigation measures, and de-icing alternatives to salt and brine are needed.

DRBC is also studying the potential affects of sea level rise on chloride concentrations in the Delaware River and Bay. Will more freshwater coming downstream be needed to repel the salt front in the estuary? And, to ensure these sufficient freshwater flows into the estuary, will additional storage be needed to make reservoir releases? Staff are using rainfall-runoff models and the latest climate research to help plan for a sustainable water supply to meet future water demands in the basin.

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