Tracking ‘chemical cocktails’ in the Chesapeake Bay watershed using routine monitoring and high-frequency measurements
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Increasing trends in base cations, pH, and salinity of freshwaters have been documented in the streams of the United States for at least the past 50 years. This process, known as Freshwater Salinization Syndrome, can be driven by a multitude of processes, including application of road salt deicers exacerbated by human-accelerated weathering of impervious surfaces. “Chemical cocktails”, or mixtures of base cations (Ca2+, K+, Mg2+, and Na+) and metals (Cu, Mn and Sr), often are released in high concentrations immediately after seasonal road salting in temperate areas. Through a combination of regular grab sampling (2-week interval) and high-frequency USGS sensor monitoring (15-minute interval), relationships were developed among specific conductance and metal concentrations. 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 long-term temporal and large 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 continues for at least 24 hours after peak specific conductance, which suggests 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 from sensors may need to be calibrated on a site by site basis, they show promise as a predictor of dissolved metal concentrations and relationships among elements as chemical cocktails. With specific conductance sensors becoming less expensive over time, improved understanding of these relationships may be useful for monitoring where there is limited equipment or funding.