Citation:

Galella, J., S. Kaushal, K. Wood, J. Reimer, AND P. Mayer. Sensors track mobilization of 'chemical cocktails' in streams impacted by road salts in the Chesapeake Bay watershed. Environmental Research Letters. IOP Publishing LIMITED, Bristol, Uk, 16(3):035017, (2021). https://doi.org/10.1088/1748-9326/abe48f

Impact/Purpose:

Urbanization has increased the concentrations of contaminants such as heavy metals and nutrients in urban streams compared to reference conditions due to increased inputs from anthropogenic sources, accelerated weathering of the built environment, and increased runoff through urban water conveyance systems. These processes create chemical cocktails that may have novel negative and synergistic effects on human health and the environment. This presentation describes how these chemical cocktails form, examines fate and transport, and discusses the use of real-time sensor data to characterize behavior and trends of nutrients and metals in urban streams in the Chesapeake Bay watershed. We also discuss future research directions to better diagnose and manage chemical cocktails that will be more cost effective and can elucidate the complex spatial and temporal scales of chemical cocktails.

Description:

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.

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