Tracking ‘chemical cocktails’ in the Chesapeake Bay watershed using routine monitoring and high-frequency measurements

Abstract:

Increasing trends in base cations, pH, and salinity of freshwaters have been documented in U.S. streams over the past 50 years. This effect, known as Freshwater Salinization Syndrome, can be driven by a multitude of processes, including application of road salt deicers and human-accelerated weathering of impervious surfaces. “Chemical cocktails”, or mixtures of base cations (Ca²⁺, K⁺, Mg²⁺, and Na⁺) and metals (Mn, Cu, and Sr), are often released in high concentrations in temperate regions following seasonal road salting. Through a combination of repeated grab sampling (2-week intervals) and high-frequency USGS sensor monitoring (15-minute intervals) over three years, relationships were developed between specific conductance and metal concentrations. These linear relationships (e.g. R² = 0.61 and 0.59 for Mn and Cu, respectively) show potential as proxies for understanding the behavior of metals in chemical cocktails over great temporal and spatial scales. Groupings of major and trace elements analyzed via linear regression and principal component analysis (PCA) show co-mobilization (i.e., correlations among combinations of specific conductance, Mn, Cu, Sr, and all base cations). Co-mobilization is strongest during the height of storm events but can continue for 24+ hours after specific conductance has peaked, suggesting there are lag times in contaminant mobilization. In the future, high-resolution monitoring may provide information on pulses, lag times, and sources of chemical cocktails. While proxies derived from sensors may need to be calibrated on a site by site basis, their use as a predictor of dissolved metal concentrations and relationships among elements as chemical cocktails shows promise. As the cost of specific conductance sensors decreases over time, their use to improve understanding of these relationships may become more tenable.