Formatted Title

Eating the Elephant: Development of a Remedial Strategy in a Complex Karst Bedrock Setting

Background/Objectives

The complexities involving storage and movement of contaminants in karst aquifers often lead project teams to conclude that groundwater remediation is impracticable. Meaningful progress may be achieved by systematically attacking source zones in the residuum and epikarst, focusing effort where remedial technologies have demonstrated effectiveness in other terranes. This presentation describes a case study of remedial prioritization, selection, and approach for a 6,600-acre groundwater operable unit at Redstone Arsenal (RSA) located in Huntsville, Alabama. The operable unit (RSA-146) contains 23 source areas impacting groundwater quality. Primary sources consist of chlorinated solvents and perchlorate from former rocket engine manufacturing, and explosives from an ordnance plant. The primary contaminants are TCE and perchlorate. The site is atop a silty clay residuum overlying limestone bedrock. The limestone comprises a significant saturated epikarst, and deeper zone of karst conduit networks. Groundwater velocities of hundreds of feet per day have been documented. The scale and complexity of the study area requires a systematic approach to both evaluate and prioritize remedial actions (RAs) with a focus on source areas, where meaningful mass reductions are more likely to be achieved within the limitations of the karst setting.

Approach/Activities

The first step in the remedial approach was to define discrete areas where remedial alternatives could be evaluated. Each source area was evaluated separately. RAs that could be applied to the residuum and epikarst to reduce the mass flux to the deeper karst bedrock and springs where that groundwater discharges were considered. Direct remediation of groundwater in the deeper karst conduit networks was deemed impracticable. Remedies evaluated comprised 5 categories and 16 technologies. The applicability of each to address DNAPL and dissolved impacts in the residuum and the epikarst was considered and ranked. Three preferred remedial alternatives were identified: in situ bioremediation (ISB), in situ biogeochemical reduction (ISBGR), and MNA. The optimal schedule for implementing the RAs was determined through a prioritization process. Factors evaluated included “Contribution to Off-Post Migration” and “Presence of a Continuing Source.” This prioritization process provided a logical roadmap for remedy implementation.

Results/Lessons Learned

The stepwise approach described above yielded a clear strategy for the Army to implement RAs at RSA-146. Lessons learned during pilot implementation of ISB and ISBGR at two priority source areas will be presented, where investigation activities and CSM refinement has been performed. The pilot studies involve injection of ISB solution or ISBGR slurries to the residuum, and gravity injection of ISB solution to the epikarst. Because important transmission zones in the epikarst cannot be verified remotely, injection point placement will be based both in relation to observed source geometry and to conditions observed during drilling. Specifically, field transmissivity testing will be performed to select locations that are hydraulically well-connected to the epikarst. Because monitoring wells alone may not adequately monitor important flow paths in the karst aquifer, injectates will include fluorescent tracers, one tracer will be introduced into the residuum and a second into the epikarst. Monitoring wells, springs, and surface streams will be monitored for key analytes, including the tracers, to better characterize injectate distributions and velocities. Collected data will be evaluated to determine the efficacy of the technology and design of a full-scale remedy if warranted. Treatment benefits of the injections on the underlying and surrounding karst bedrock aquifer will be presented. Results will be applicable to the broader RSA-146 remedial strategy and will be used to optimize the performance of future remedial work.