Tuesday, April 30, 2013: 8:00 a.m.
Regency West 6 (Hyatt Regency San Antonio)
At many sites, groundwater is a significant pathway for the contamination of stream bed sediments and surface waters by petroleum hydrocarbons and other organic chemicals. Here we detail the reasoning, design, implementation, and validation for an upland groundwater source control hydraulic containment system at a manufactured gas plant located along a tidally influenced river. Both upland groundwater remediation and river sediment cleanup actions are being designed and conducted at the site. Upland subsurface contamination associated with this site includes petroleum hydrocarbons, chlorinated solvents, and cyanide. To prevent recontamination of the river sediment, source control of the upland groundwater discharge must be implemented prior to the sediment remedy. The alluvial aquifer is more than 200 feet deep at this site, making hydraulic containment using extraction wells necessary. Step-tests at upland pilot extraction wells have shown the extraction system will control groundwater seepage to the river. A network of 21 extraction wells will be used to maintain reversal of hydraulic gradients on 0.45 mile of shoreline along the site. Using field pump test data, the site MODFLOW models have shown that hydraulic containment is achievable across the full spectrum of environmental conditions, including low water events due to tidal and river discharge fluctuations. Water level measurements obtained from piezometers screened in the river sediment have shown that the upland extraction wells will effectively control seepage up to 500 feet from the river shoreline. By mid-2013, the extraction system will be completed, and the system mechanics will be validated. Validation of hydraulic containment will be done through the assessment of field parameters in transition zone piezometers, seepage rate measurements, temperature profiling, and water level gauging. Once upland source control of groundwater has been achieved, the in-stream sediment and upland groundwater remedies can be implemented.