Road salt chloride exposure in urban streambeds and links to groundwater – surface water interactions and salt sources

Groundwater transport of chloride (Cl) containing road salt deicers is an important contributor to salinization of fresh surface waters in temperate climates. While mass loading of salt to streams via groundwater has received greater recognition lately, only a few studies have demonstrated the unique risk posed by the direct discharge of salt-laden groundwater to aquatic life residing in the benthic zone (e.g., macroinvertebrates, mussels, plant roots, fish eggs). These studies revealed high Cl concentrations in stream porewater but provided limited information on its spatio-temporal variability and factors influencing it. To address this gap, this study conducted a detailed 2-year field investigation on 100-m scale reaches of two streams adjacent to salt-receiving roads in London, Ontario, applying year-round streambed porewater sampling, streambed surface electrical conductivity mapping, and electrical imaging (geophysics) techniques. Areas of probable groundwater upwelling and surface water downwelling were identified using streambed temperature mapping and hydraulic head gradients. Benthic porewater Cl concentrations were commonly above water quality guidelines, but also varied substantially over small spatial scales, ranging from <100 to >5000 mg/L over ≤10 m distances. Generally, porewater Cl concentrations were relatively stable seasonally compared to the stream water, leading to benthic areas with year-round exposure to elevated Cl and limiting prospects for benthic zone refugia. Key factors affecting this spatial and temporal variability included proximity to roads, impacts from point sources (snow storage locations), and surface water downwelling (hyporheic flows). These observations have direct implications for the health of urban stream benthic communities and for designing management and remediation strategies.