Formatted Title
Confirmation Sampling Case Study: Validating a 2.3 Million Gallon ISBR Injection Project
Background/Objectives
A large-scale, 2.3 million gallon in situ biological reduction (ISBR) injection project was completed to remediate 1,2-dichloroethane in groundwater. Accurately confirming reagent distribution is difficult due to aquifer heterogeneity, rapid temporal changes in the treatment zone after injections, access limitations, and the high cost of data collection.
ERM implemented a robust confirmation sampling program to provide real-time data to optimize injections, ensure that reagent was adequately distributed in the treatment area, and confirm concentrations were high enough to stimulate reductive dechlorination.
Approach/Activities
During the project, a total of approximately 2.3 million gallons of reagent was injected into 161 injection points. The total treatment area was approximately 188,000 square feet. The target treatment zone thickness ranged from 10 feet (55-65 feet below ground surface [bgs] range) to 25 feet (45-70 feet bgs range). The injected reagent included a total of 147,200 pounds of ELS (carbon substate) and 482 liters of bioaugmentation material (each). Injection point spacing was approximately 30 feet and locations were advanced using a direct-push drill rig.
Sampling was performed immediately after injections were completed in each portion of the treatment area to measure reagent before significant total organic carbon (TOC) fermentation began and allow time for design changes if warranted based on results.
A direct-push drill rig was used to collect hydropunch grab groundwater confirmation samples near 11 injection points. Three samples were collected at 5, 10, and 15 feet from each point, resulting in 39 samples collected total. Each sample was analyzed for TOC by U.S. Environmental Protection Agency (USEPA) Method 5310B.
Results/Lessons Learned
Collecting TOC confirmation samples 5, 10, and 15 feet from injection points allowed precise quantification of radius of influence (ROI) and TOC concentration across the treatment area. The confirmation sampling results allowed real-time optimization of injection parameters, including demonstrating that overall reagent volumes could be decreased and determining that one area needed tighter injection spacing.
Eighty-five percent of the samples reported a TOC value greater than the estimated background (4 mg/L) and 79 percent reported TOC greater than the recommended minimum value for reductive dechlorination to occur (20 mg/L) (USEPA 1998). The average TOC concentration of all 39 samples was 228 mg/L. Seventy-seven percent of the samples collected 15 feet away from the injection points reported a TOC concentration greater than the estimated background value (4 mg/L). This indicates the average ROI of injected reagent was greater than 15 feet from injection points, meeting the ROI design objective.
This systematic and rigorous confirmation sampling approach can be used as a model for similar injection programs to identify and fill treatment gaps and confirm design objectives have been met.