Assessing the impact of VOC-contaminated groundwater on surface water at the city scale

摘要

This study is believed to be one of the first to assess the impact of urban VOC-(volatile organic compound) contaminated groundwater on river-water quality at the city scale. A network of riverbed piezometers was used to study the 7.4-km urbanised reach of the River Tame that flows across the groundwater-effluent unconfined Triassic sandstone aquifer underlying the city of Birmingham (UK). Aquifer groundwater contained significant chlorinated VOC contamination due to the city's industrial heritage. Chlorinated VOC-contaminated baseflow was widespread along the reach with trichloroethene (TCE) dominant. VOC concentrations in riverbed piezometers were in the range 0.1-100 μg/l with typical regulatory limits occasionally exceeded by an order of magnitude. Although anaerobic biodegradation products such as cis-dichloroethene were widespread, they were unlikely to have formed in the generally aerobic riverbed. The lack of anaerobic conditions was ascribed to insufficient accumulation of low-permeability, organic-carbon rich riverbed sediments in this medium-high energy river. Assumptions a priori that natural attenuation of chlorinated VOCs will occur via reductive dechlorination in urban riverbeds are likely in error, particularly where deposits of medium-high permeability exist transmitting much of the baseflow. Surface-water quality impacts were nevertheless still low with in-river TCE increasing by just 2 μg/l over the 7.4-km reach. Agreement of baseflow contaminant flux estimates based on five flow-concentration product methods was achieved to within an order of magnitude with 22-200 kg/ yr of TCE estimated to discharge to the 7.4-km reach (equivalent to 0.8-7.5 mg/d/m~2 of riverbed). Such uncertainty was not regarded as unreasonable when the large measurement scale and geological and chemical heterogeneities are considered. Improved flux estimation methods and greater monitoring densities are nevertheless warranted. Considering Birmingham's long industrial history and known incidence of VOC-contaminated groundwater, the city-scale impact of VOC-contaminated groundwater upon surface-water quality was judged to be relatively modest.