Formatted Title

Demonstration of Multiple Amendment Delivery Technologies for DNAPL Remediation in Fractured Crystalline Bedrock

Background/Objectives

Background/Objectives. Characterization and remediation of DNAPL in fractured crystalline bedrock can be the most challenging and expensive of remediation efforts. At a municipal site with chlorinated solvent contamination in both overburden and bedrock, high concentrations (270,000 ug/L) of cis-DCE detected in an “upgradient” well initiated efforts to identify the source, characterize the plume, and generate a remedial strategy. Recognizing that potential DNAPL in fractured bedrock would present technical, regulatory, and financial challenges for the municipality, an efficient, adaptive, phased bedrock characterization, design, and pilot testing program was implemented, resulting in a robust remedial design of an in situ chemical and biological reduction (ISCBR) permeable reactive barrier (PRB) at the plume boundary. While site characterization results have been previously presented, the current discussion will focus on results of the pilot testing in both the overburden and bedrock.

Approach/Activities

Approach/Activities. Following the adaptive site characterization, bench-scale testing was performed to demonstrate that chemical and biological reduction could treat the contaminants present at the site, and to establish the optimum combination of amendments for the PRB. Pilot testing was then performed to demonstrate and validate amendment delivery approaches and to test technology performance in both the bedrock and overburden. For the bedrock, the full-scale design basis calls for delivery of ISCBR amendments into existing fractures, rather than attempting to create new fractures where none currently exist. The reasoning for this is that 1) creating new fractures in the crystalline bedrock would require extremely high pressures and would risk mobilizing contamination and 2) the existing fractures represent the zones where contaminant transport and groundwater flow are already occurring at the site.

To this end, permeability enhancement (hydraulic fracturing) was used to deliver an amendment slurry consisting of zero valent iron, emulsified vegetable oil (EVO), and a sand proppant into seven existing fractures (previously mapped during site characterization activities) in three boreholes. Tiltmeter monitoring was performed to assess fracture propagation and amendment distribution, and groundwater monitoring was conducted to assess performance. A separate pilot was performed to assess amendment delivery in the overburden, where a mixture of EVO, lactate, and tracers was injected using a traditional injection well approach.

Results/Lessons Learned

Results/Lessons Learned. Results from the bedrock pilot test indicate that amendment was distributed at least 20 ft from all fractures (the pilot design was a minimum of 15 ft), and in some case up to 50 ft or more. Results from the overburden pilot indicate that amendment was delivered at least 10 ft from injection points (design was a minimum of 6 ft). In terms of remedy performance, results show substantial increases in total organic carbon, creation of strongly reducing conditions, and widespread VOC degradation within two months of amendment emplacement. Preliminary results from the 12-month post-emplacement monitoring event show substantial VOC concentration decreases, with orders of magnitude increases of ethene and ethane. Additional pilot monitoring data and a complete assessment of ISCBR performance will be presented.