Supplemental feasibility study for remedial action for the Groundwater Operable Unit at the Chemical Plant Area of the Weldon Spring Site, Weldon Spring, Missouri

Site data evaluated indicate that after source removal, dilution and dispersion appear to be the primary processes that would further attenuate groundwater contaminant concentrations. On the basis of these attenuation processes, the calculations presented in Chapter 2 indicate that it would take several years to decades (approximately 60 to 150 and 14 years, respectively, for Zones 1 and 2) for TCE concentrations in Zones 1 and 2 to attenuate to the MCL (or ARAR) of 5 pg/L. The estimates for Zones 1 through 3, where the higher nitrate concentrations are clustered, indicate that it would likely take at least 80 years for nitrate concentrations to attenuate to the MCL (or ARAR) of 10 mg/L. Costs for implementing NINA for groundwater at the chemical plant area are primarily associated with those incurred for monitoring contaminant concentrations and the replacement costs for monitoring wells. Cost estimates are relatively high because a rather lengthy period of monitoring would be involved. Calculations performed to evaluate the feasibility of groundwater removal and subsequent treatment of the extracted water included determinations for the number of extraction wells needed, required number of pore volumes, and the number of years of implementation required to attain bench marks. The calculations were performed per zone of contamination, as discussed in Chapter 1. Several observations can be made about the results presented in Chapter 3 regarding Alternative 4. The first is that by looking at the results for Zones 1 and 2 evaluated under Alternative 4, one can also assess the feasibility of Alternative 7, because Alternative 7 addresses this particular subset of Alternative 4 (i.e., Zones 1 and 2). TCE contamination has been observed in Zones 1 and 2, but has not been reported in any of the remaining five zones. Nitrate, nitroaromatic compounds, and uranium have also been reported in Zones 1 and 2. The present-worth costs for implementing the pump and treat alternative in Zones 1 and 2 constitute the major component of the overall present-worth cost for Alternative 4, which indicates that the cost for Alternative 7 would be similarly high. Another observation is that although estimated times are shorter for the pump and treat approach than those for MNA, pump and treat for Zones 1 and 2 likely would take several decades (at least 30 years) to attain ARARs or bench marks. The cost estimates (in present-worth costs) for Alternatives 4 and 7 are much higher (approximately an order of magnitude higher) than those for Alternative 3