Operable Unit (OU) 2 includes contaminated groundwater, explosives contaminated soil not remediated during OU1 which could act as a source of explosives contamination of groundwater, and soil contaminated with volatile organic compounds. The major components of the selected remedy for OU2 include:
• Hydraulically contain contaminated groundwater exceeding the Final Target Groundwater Cleanup Goals;
• Focused extraction of groundwater in areas with relatively high concentrations of TCE and explosives;
• Treat all extracted groundwater using granular activated carbon adsorption, Advanced Oxidation Process, and air stripping, any of which may be applied individually or in combination;
• Dispose of the treated groundwater by beneficially reusing it or through surface discharge;
• Provide a potable water supply to local groundwater users whose water supply contains RDX exceeding the Health Advisory and/or TCE exceeding the Maximum Contaminant Level;
• Monitor the groundwater elevations and water quality; and
• Excavate and treat explosives-contaminated soil which could act as a source of explosives contamination of groundwater and which does not meet the OU1 excavation criteria.
Hydraulic Containment and Focused Extraction
In place at the site is a network of containment and focused extraction wells for remediation. Eight extraction wells are currently in operation to hydraulically contain contaminated groundwater exceeding the Final Target Groundwater Cleanup Goals and three extraction wells are currently in operation as focused extraction wells. One groundwater circulation well is also currently in operation to treat and area of relatively high TCE concentrations. Four extraction wells that no longer contributed to containment and/or may have impacted the direction of flow of groundwater were removed from the groundwater extraction system, but may be turned on at a later time if needed.
Groundwater Treatment and Disposal
Four groundwater treatment plants are currently in operation at the site to treat extracted groundwater. Two of the treatment plants operate by air stripping, one operates by the Advanced Oxidation Process, and the other operates using granular activated carbon to remove contamination from the extracted groundwater. Extracted groundwater is treated at the plants and then properly discharged and, in some cases, made available for beneficial reuse.
Water Supply Well Sampling and Alternate Water Supplies.
A number of private residential water supply wells are sampled every year within one mile to the south or to the east of the former Nebraska Ordnance Plant groundwater contaminant plumes. A granular activated carbon unit and/or bottled water is provided to any local groundwater user whose water supply at any point contains TCE above 5 µg/L and/or RDX 2 µg/L. The results of the Water Supply Well Sampling Program determine if an alternate water supply will be provided to a user. Currently five residential water supply wells are included as part of the alternate water supply program.
Water Quality and Groundwater Elevation Monitoring
Monitoring well and surface water sampling is conducted quarterly as part of the Groundwater Monitoring Program. A network of over 300 active monitoring wells is present at the site and each year the Corps of Engineers along with the federal and state regulators determine which wells should be sampled. In addition, surface water samples are collected from three creeks adjacent to the site. Groundwater samples from monitoring wells and surface water are used to monitor and evaluate potential changes in the contaminant concentrations and monitor and evaluate the concentration of groundwater and surface water contamination. Groundwater data are also used to provide data for evaluating whether the plumes are being contained by the groundwater extraction well network.
Site water level measurements are collected periodically at wells prior to monitoring well sampling or during other investigation activities. Regional water level measurements are also collected semi-annually in conjunction with other agencies at over 500 wells located by the former Nebraska Ordnance Plant and surrounding area. The water level data are used to prepare maps that show groundwater flow direction across the area, update the groundwater flow model and identify abnormalities in water levels across the site, and help document capture and/or drawdown related to the extraction well network.
Soil Excavation and Treatment
The excavation and thermal treatment of explosives contaminated soil was completed for OU2 during the OU1 Remedial Action with a removal of approximately 13,607 cubic yards of contaminated soil, thus, reducing the potential for a continued source of explosives contamination migrating to groundwater. No further action for explosives-contaminated soil is required for OU2.