SALINIZATION OF STREAMWATER IN EXURBAN AND SUBURBAN WATERSHEDS OF SOUTHEASTERN PENNSYLVANIA, 1999-2019

Roadway deicing agents, including rock salt and brine containing NaCl, can have a profound impact on the water quality of rivers and streams. Specifically, the “freshwater salinization syndrome” (FSS), which pertains to long-term increases in concentrations of major ions and metals in fresh waters, has been linked to road salt application in urbanized areas. In addition to increased Cl^- in runoff and recharge, Na⁺ may displace Ca²⁺, Mg²⁺, K⁺, and trace metals from clay minerals by reverse cation exchange, resulting in FSS indicators and Na⁺ to Cl^- molar ratios <1.0. These impacts have rarely been explored in exurban and suburban watersheds with relatively low yet increasing impervious surface cover (<15%). We used long-term (1999-2019) monthly streamwater quality data combined with daily streamflow for six watersheds in southeastern Pennsylvania to evaluate FSS conditions. Specific conductance (SC) increased by more than 20% in all six streams over the 20-year study. To determine trends in various ions and their contributions to SC and salinity, we examined flow-normalized concentrations of Cl^-, Na⁺, Ca²⁺, Mg²⁺, K⁺, SO₄^2-, and HCO₃^-. Annual, baseflow months (August-November), and winter (December-March) concentrations of Cl^-, Na⁺, Ca²⁺, and Mg²⁺ increased in all six streams, providing evidence of FSS impacts. A Cl^- mass-balance model for all watersheds indicated that road salt accounted for most inputs. Although annual flow-normalized Cl^- concentrations are below the USEPA chronic threshold value for impacts to aquatic organisms, based on current trends, future concentrations may exceed thresholds before the end of this century. In 2019, the flow-normalized Na⁺ concentrations in all watersheds exceeded the USEPA 20 mg L^-1 threshold for individuals restricted to a low-sodium diet. Additionally, indices used to evaluate the corrosivity of source waters to drinking water infrastructure, such as the Chloride to Sulfate Mass Ratio and Larson Ratio, increased between two- and seven-fold over the study period. These ratios currently exceed thresholds known to cause corrosion of lead, steel, iron, and copper pipes. Collectively, these results indicate the prevalence and increasing magnitude of the FSS in exurban and suburban watersheds and the importance to water resource managers.