Upland Groundwater Hydraulic Containment: A Means to Facilitate Sediment Capping and a More Cost-Effective Sediment Remedy

摘要

Background/Objectives. The selection of a groundwater source control technology should evaluate potential impacts and benefits to any contemplated sediment remedy and the potential for significant associated cost savings. Such an evaluation was conducted at the former Gasco manufactured gas plant site on the Willamette River in Portland, Oregon, where upland groundwater and river sediment contamination exists, including dense nonaqueous-phase liquid (DNAPL); cyanide; benzene, toluene, ethylbenzene, and xylene (BTEX); and polycyclic aromatic hydrocarbons (PAHs). This evaluation identified a groundwater source control remedy that will facilitate less costly sediment capping and monitored natural recovery (MNR) in large areas that may have otherwise not been protective. Approach/Activities. Existing offshore groundwater transitional zone water (TZW) seepage rates were estimated using data from offshore seepage meters, as well as MODFLOW modeling based on upland well data, and both estimation methods closely concurred. Future effects on offshore seepage rates by potential upland groundwater containment systems were estimated by installing pilot shoreline extraction wells, upland monitoring wells, and in-water piezometers. Short- and long-term pump testing was completed to directly map offshore areas of groundwater seepage control. These data were used in the MODFLOW model to estimate the extent of offshore TZW seepage reduction or reversal and conditions not directly measured by the pilot testing. The modeled future expected TZW capture extents and seepage rates were used to support sediment cap modeling and MNR evaluations. The projected seepage rates were also input into one-dimensional cap models and a comprehensive sediment contaminant fate and transport model (FTM) to determine cost-effective cap designs and the effectiveness of sediment MNR. This information was then used to develop and evaluate sediment remedy alternatives in a feasibility study (FS). Results/Lessons Learned. Reversal of TZW seepage rates is projected throughout the sediment cleanup area except in two small areas where seepage rates would be greatly reduced. Cap modeling indicated that an armored, very thin sand layer would effectively isolate contaminated sediments in seepage reversal areas, and in the reduced seepage areas a 1-foot-thick sand cap amended with 10 to 15%, by weight, organoclay would be effective. The FTM indicated that MNR, supported by upland source control, would be effective outside capping areas in reducing groundwater-related, sediment risks over time. The FS showed that alternatives focusing on mostly capping and MNR minimize construction-related releases and water-quality impacts; achieve the remediation goals