INVESTIGATING THE WHEN AND WHERE OF AQUATIC ECOSYSTEM EXPOSURE TO POLLUTANTS IN GROUNDWATER DISCHARGING TO SURFACE WATERS

Groundwater can be contaminated by a host of anthropogenic activities or sources, as well as through some natural processes. When contaminated groundwater discharges to a surface water body, it can lead to contaminant exposure of the aquatic ecosystem, including organisms residing in the discharge zone within the sediments (endobenthics) and on the sediments (epibenthics), and in the overlying water. Water circulation or flow can also transport these contaminants away from the discharge zone, leading to distant exposure. The range of studies investigating this groundwater-sourced-contaminant exposure are quite limited, especially concerning the influence of spatial and temporal variability in groundwater – surface water interactions. This study reports on full-year investigations of two sites, each with groundwater contaminated by a historic landfill discharging to a nearby surface water body; one being a stream, the other a pond (with an outlet stream). Monitoring of various leachate tracers (artificial sweeteners, ammonium, chloride, electrical conductivity) were performed for shallow discharging groundwater and surface water at the sediment bed interface and the overlying water column. Measurements of stream discharge, piezometer head level, sediment temperature profile, and hydraulic gradient via potentio-manometers provided insight into the variation in groundwater fluxes and allowed calculation of off-site mass fluxes. The endobenthic zone experienced the highest concentrations, with similar exposure in summer and winter. However, variations in concentrations and mass fluxes within the receiving surface waters were observed at diurnal, event, and seasonal scales, which might not be captured by typical annual/biannual monitoring programs. The findings of this work will have implications for ecological assessment and monitoring of sites with groundwater contamination reaching surface water bodies.