Abstract

 Despite the proximity of most hazardous waste sites to surface water, regulatory programs are only recently recognizing the importance of understanding groundwater-surface water interactions. This case study indicates how a passive collection technique was successfully used to evaluate a chlorobenzene plume suspected of entering a gravel-bed river in New England. Bed porewater concentrations were determined using an innovative, cost-effective sampling procedure [passive vapor diffusion (PVD)] recently developed by the USGS. This reconnaissance tool avoids use of more invasive and costly sampling procedures and compares favorably with sediment sampling because of its lower quantitation limits and cost. Sixty PVD samplers were constructed and installed in a systematic grid pattern. Upon retrieval two weeks later, only 4 of 60 samples contained detectable chlorobenzene concentrations [> 0.02 milligrams per cubic meter (mg/m3)]. Three of the four detections were clustered in an area bounded by the limits of the suspected plume. The highest reported PVD vapor-phase concentration was 5.5 mg/m. The vapor-phase results require conversion based on Henry's Law to determine the equivalent equilibrium porewater concentration to compare with available standards. The maximum 5.5-mg/m3 vapor-phase concentration corresponds to an equilibrium porewater concentration of 50 ug/L. This concentration is an order of magnitude below the state standard of 500 ug/L for chlorobenzene in groundwater discharge to surface water and significantly below surface water quality criteria for chlorobenzene. The results indicate that the discharge of chlorobenzene into the river did not constitute a significant environmental risk. These results preclude the need for a more extensive investigation of the river. As well, PVDs are being considered for investigation of other nearby sites.