RISK TO BENTHIC ORGANISMS, SUCH AS THE FRESHWATER MUSSEL LAMPSILIS SILOQUEODEA FROM GROUNDWATER CONTAMINATED WITH ROAD SALT AND DISCHARGING TO AN URBAN STREAM

Road salt applied in urban environments may infiltrate with melt water and be transported by groundwater to local streams, potentially exposing aquatic organisms to toxic levels of chloride throughout the year. Aside from the snow melt period, chloride concentrations will often be greater for discharging groundwater than for the stream, thus leading to greater concentrations with depth in the sediment. Therefore, burrowing invertebrates, such as juvenile freshwater mussels may be especially at risk. In this study, chloride concentrations in shallow groundwater below an urban stream were measured in summer and compared to results from acute (96 h) sodium chloride toxicity tests performed with newly released (<1 week) juvenile Lampsilis siloqueodea (Fatmucket). Toxicity tests revealed an EC50 of 1507 (1383-1648) mg chloride/L, demonstrating that these early life stage freshwater mussels are among the most chloride sensitive invertebrates. The EC10 was 734 mg/L; chronic effects would also be expected to occur at levels below the EC50. Screening of groundwater at 10-m intervals along a 800-m section of stream (76 sample locations), at depths of 25-75 cm below the sediment surface, revealed chloride concentrations ranging between 20 and 3600 mg/L, with an average of 860 mg/L, compared to 20-30 mg/L for the stream water. These were highly correlated with sodium. About 17% and 42 % of the groundwater locations had chloride concentrations at or above the laboratory measured EC50 and EC10. Further detailed sampling in several of the identified chloride hot spots indicated that salt concentrations were often in a similar range at 5 to 10-cm depth, which corresponds to the zone inhabited by juvenile mussels and other endobenthic organisms, as at the deeper depths mentioned above. These findings suggest that discharge of salt-impacted groundwater may impair mussel habitat across substantial portions of this urban stream.