POTENTIAL THREAT OF GROUNDWATER CONTAMINANTS TO AN URBAN STREAM ECOSYSTEM

The discharge of groundwater to urban streams may be accompanied by land-sourced contaminants. However, there is little information available on the potential threat these substances pose to aquatic ecosystems, and especially to the benthic community, which may experience higher contaminant concentrations than the stream itself due to fewer losses from sorption, degradation and volatilization processes. Typically, this contaminant pathway has been addressed on a site-specific and aquifer-focused basis, following the discovery of groundwater contaminant plumes. In this study, monitoring for groundwater contaminants in the deeper benthic zone was performed at the stream reach-scale, as a means of screening for areas of potential ecological concern and identifying possible sources of groundwater contamination. In the first part of the study, groundwater samples from below the stream bed (e.g. typically 50 cm) were collected using a drive-point technique at intervals of about 12 m along a 650-m stretch of an urban stream in eastern Canada. These were then analyzed for a wide range of common urban contaminants and general chemistry. Identified contaminants at this stream reach included benzene and other petroleum hydrocarbons, fuel oxygenates (e.g. MTBE), and various chlorinated solvent compounds. In addition, elevated levels of nitrate, phosphate, some heavy metals, including arsenic, and elevated chloride (likely indicating road salt) were detected. In combination, these groundwater contaminants covered nearly the entire length of this stream reach, with many areas experiencing multiple contaminants. In the second part of the study, specific areas were targeted using the same drive-point technique but with an expanded set of analyses, including environmental (isotopic and chemical) tracers, to better constrain potential pollutant sources (e.g. lawn fertilizer vs. septic system nitrate) and to monitor contaminant profiles with depth below the stream. Future work will attempt to determine the effects of these contaminants on the benthic community structure of this urban stream reach.