Ecological effects from groundwater contaminated by volatile organic compounds on an urban stream’s benthic ecosystem

While the discharge of groundwater contaminated with volatile organic compounds to surface waters is widely reported, assessments of its ecological threat are rare in the scientific literature, largely being restricted to comparisons of riparian groundwater concentrations to water quality guidelines for protecting aquatic life. Here we investigated potential negative impacts on the benthic community from overlapping petroleum hydrocarbon – chlorinated solvent plumes discharging to an urban stream using in-field ecological assessment methods (targeting both meio- and macrobenthos) over multiple scales. Several lines of evidence were suggestive of detrimental impacts from the plumes, including reduced total abundance and richness of benthic taxa, reduced or enhanced abundances of individual benthic taxa, and an altered benthic community structure. However, the findings were not conclusive, as the evaluation was complicated by substantial small-scale (<20m) spatial variation in concentrations of many other groundwater contaminants, making it difficult to determine reference areas (i.e., those with similar substrate and chemistry, but lacking exposure to plume compounds) and isolate impacts from a single factor. Furthermore, detections of the wastewater indicator acesulfame also demonstrated the issue of uncertain influences from unmeasured contaminants (e.g., pharmaceuticals). Some of the assessment methods showed greater promise than others (e.g., sampling along a gradient rather than having separate impacted and reference sites), which may provide guidance on future field applications. Overall, the study findings suggest a need for further inquiry into the most appropriate application of ecological and ecotoxicological assessment methods to groundwater-based contamination hazards.