Sensors track mobilization of 'chemical cocktails' in streams impacted by road salts in the Chesapeake Bay watershed
Increasing trends in base cations, pH, and salinity of freshwaters have been documented in U.S. streams over 50 years. These patterns, collectively known as Freshwater Salinization Syndrome (FSS), are driven by a multitude of processes, including applications of road salt deicers and human-accelerated weathering of impervious surfaces, and other anthropogenic legacies of changes in the region. FSS mobilizes chemical cocktails of multiple elemental mixtures via ion exchange, shifts in pH and solubility, and other biogeochemical processes. We analyzed impacts of FSS on streamwater chemistry across 5 urban watersheds in the Baltimore-Washington, USA metropolitan region. Through combined grab sampling (2-week intervals) and high-frequency monitoring by USGS sensors (15-minute intervals), regression relationships were developed among specific conductance and major ion and trace metal concentrations. These linear relationships were statistically significant in most of the urban streams (e.g., R2 = 0.62 and 0.43 for Mn and Cu, respectively), and show potential as proxies for predicting the behavior of major ions and trace metals as chemical cocktails. Groupings of major ions and trace metals analyzed via linear regression and principal component analysis (PCA) showed co-mobilization (i.e., correlations among combinations of specific conductance, Mn, Cu, Sr, and all base cations during certain times of year and hydrologic conditions). Co-mobilization was strongest during peak snow events but could continue for over 24 hours after specific conductance peaked, which suggests that there were lag times and legacies in contaminant mobilization associated with road salt use. Modeled predictions of metals concentrations using specific conductance as a proxy for Mn and Cu indicated acceptable goodness of fit for predicted vs. observed values, but only when model calibration data included peak concentrations of metals during snow events representing the highest ranges in concentrations (Nash-Sutcliffe Efficiency > 0.28). Interestingly, observed metals concentrations still remained elevated for weeks after specific conductance decreased, which suggested lag times and legacies in mobilization following road salt use. Our results show that: (1) proxies derived from sensors can advance our monitoring of FSS but need to be calibrated and validated on a site by site basis, (2) contaminant co-mobilization from FSS occurs as chemical cocktails, and (3) these chemical cocktails vary significantly across seasons and during and after road deicing events. High-frequency monitoring of nonpoint source pollution associated with FSS is critical for predicting magnitude and duration of contaminant pulses in response to salinization and impacts on aquatic life, infrastructure, and drinking water supplies.