Formatted Title
Lessons from DNAPL Remediation in Bedrock by Chemical Oxidation: Concentrations Had to Go Up to Come Down
Background/Objectives
Background/Objectives. In situ chemical oxidation (ISCO) for source area treatment often fails by not accounting for non-aqueous phase contaminant mass that can be sorbed to soil or as dense non-aqueous phase liquids (DNAPL), and the non-aqueous phase mass constitutes a much larger fraction than the mass dissolved in groundwater. Effective remediation of bedrock source areas is one of the most challenging issues facing groundwater remediation practitioners due to incomplete understanding of groundwater flow and contaminant transport and difficulty of injection into bedrock. This presentation examines a Superfund Site in Region 2 where 100 years of manufacturing and dry-cleaning operations, tetrachloroethene (PCE) resulted in dense non aqueous phase liquid (DNAPL) in overburden and carbonate bedrock composed of interbedded and fractured limestone, dolomitic limestone.
Approach/Activities
Approach/Activities. An initial ISCO program (persulfate with limited base for activation) was performed using a network of 89 injection wells to depths of up to 75 feet below grade. During the initial injection, high injection pressures were used, and significant daylighting occurred. Consequently, most wells did not receive a meaningful volume of oxidant and, as a result, limited PCE treatment occurred in bedrock. Subsequent ISCO injection events incorporated numerous modifications including re-installing bedrock casings, utilizing straddle packers targeting discrete fractures, installing additional injection wells with identified fracture depths, reducing injection flow rates, adjusting persulfate activation based on additional base buffer demand testing. A remedial optimization incorporated specifically for DNAPL remediation was the inclusion of a combined oxidant approach with Modified Fenton’s Reagent (MFR). MFR reactions solubilize DNAPL through a combination of bubble agitation and free radicals created.
Results/Lessons Learned
Results/Lessons Learned. Successful remediation was achieved with optimized techniques and modifications made to each ISCO injection event. The objective of the remediation modifications was to increase the quantities of oxidant delivered to desired subsurface zone and to improve delivery of reagents to locations containing residual PCE mass. MFR was introduced to increase dissolution of PCE DNAPL. Subsequently many wells experienced significant increases in PCE groundwater concentrations, which benefits the overall treatment by converting non-aqueous phase PCE to a form that is more readily oxidized. Overall, the program has reduced PCE in many wells by 90% to >95%, demonstrating that site optimized ISCO is an effective remediation strategy for DNAPL in bedrock. The presentation will share the lessons learned from bedrock investigation and characterization, remediation design optimization, and implementation of injections.