NYC DEP, 21 March 2025
Croton System Chloride Levels Have Tripled in the Last 30 Years and Could Exceed Maximum Salinity Levels By 2108; Road Salt and Human Activities are Key Contributors
Minor Salinity Impacts in City’s Larger Reservoirs Throughout the More Sparsely Populated Upstate Catskills Region
The New York City Department of Environmental Protection (DEP) today released a new, comprehensive study on rising salinity levels in the City’s water supply, which show salinity levels in the Croton System reservoirs—one of the City’s primary water sources—have been steadily rising due to road salt runoff and other human activities. If the trend continues, the New Croton Reservoir could exceed the State’s maximum allowable chloride levels at the turn of the century. Salinity levels in the system’s main New Croton Reservoir have tripled in just a 30-year time frame.
The study also found that the City’s main reservoirs in the rural Catskills region further upstate have low levels of salinity; those salinity levels are also increasing, but at a much slower rate.
DEP’s “Salinity Management Assessment” study reviewed nearly 33 years of data and found steadily increasing salinity levels in all 12 reservoirs and three controlled lakes in the water supply’s Croton System, the oldest and smallest of the reservoir systems which typically provides about 10 percent of the City’s water supply. All of the Croton System’s reservoirs and lakes throughout Westchester and Putnam counties feed into the New Croton Reservoir about 22 miles north of the city line. The study found that at the current rate of increase the New Croton Reservoir would reach the state’s maximum contaminant level for chloride, a main component of salt, by the year 2108.
By analyzing data from 1987 through 2019, DEP found a sustained increase in chloride concentrations in all of reservoirs serving the water supply system, though the City’s largest reservoirs in the Catskills showed only negligible increases and contain relatively low levels of chloride. This is not unexpected given the lower population, pavement and property parcel densities in that region, though the study includes recommendations to help protect those reservoirs in the future.
The study found winter de-icing of roadways and parking lots, wastewater treatment plant discharges and private water softening systems as likely main contributors to increased salinization. Water filtration plants cannot remove salt from water. Instead, salinity can only be reduced through a costly and heavily energy dependent desalinization process.
Recommendations to reduce salinity included supporting stakeholder efforts to develop best practices for reducing man-made salinity contributions throughout the water supply region, expanding studies on specific sources, and building public and stakeholder awareness to the trends and impacts of salinity infiltration of fresh water supplies.