Theme: 01. Remediation Technology Innovations

Topic Area: 01m. In Situ Technologies: Lessons Learned

Formatted Title:

Successful Cleanup of Low-Level Chlorinated Propanes in Groundwater Using an In Situ Bioreactor

Background/Objectives:

Chlorinated propanes, predominantly 1,2,3-trichloropropane (TCP) and 1,2-dichloropropane (DCP), were solvents and carriers in legacy pesticide formulations and are emerging groundwater contaminants characterized by high subsurface mobility, recalcitrance to most conventional remedies, and low regulatory levels relative to other organic compounds. For example, the State of California promulgated a maximum contaminant level (MCL) of 0.005 micrograms per liter (µg/L) for TCP and 5 µg/L for DCP. TCP and DCP have been detected in groundwater near chemical manufacturing facilities and military bases and TCP is often present in low-level diffuse plumes resulting from widespread application of agricultural chemicals, including fumigants and pesticides. Since the mid-2000s, proof-of-concept tests have demonstrated in situ biological reduction (ISBR) as a promising technology for TCP and DCP remediation at both high and low concentrations. The objectives of this abstract are to present the findings of a first-to-field case study of an ISBR treatment cell utilizing bioaugmentation for low-level TCP and DCP remediation at a former agricultural chemical facility.

Approach/Activities:

ISBR utilizing bioaugmentation was first implemented as a pilot study in 2016 at a former agricultural chemical manufacturing facility with low level TCP concentrations (up to 70 µg/L), collocated DCP concentrations (up to 730 µg/L), and elevated nitrate concentrations (up to 380 mg/L) in groundwater with no noticeable natural attenuation in concentrations. A sequenced approach to the remedy with biostimulation using a commercial buffered emulsified vegetable oil product followed by bioaugmentation with a microbial culture enriched in Dehalogenimonas (Dhg) species was implemented. The Dhg bioaugmentation culture was raised on TCP to facilitate acclimation to site conditions. Based on the results of the pilot study, a full-scale ISB remedy was implemented in 2018. The full-scale remedy consisted of an ISBR treatment cell with groundwater recirculation using strategically placed extraction and injection wells to enhance amendment distribution and manage COC degradation. Strategically placed monitoring wells provided adequate performance resolution and allowed streamlining of operations and maintenance needs.

Results/Lessons Learned:

Post-injection performance monitoring during the pilot demonstrated sustained reduction of TCP to below analytical method detection limits (0.005 µg/L) after an initial lag period of approximately six months. DCP concentrations concurrently decreased to near analytical method detection limits detection limits (1 µg/L) along with detections of propene, a degradation product. An increase in the Dhg population was reported after the initial lag period, concurrent with onset of TCP and DCP degradation. While prior laboratory scale studies demonstrated higher degradation efficiencies and kinetics with TCP concentrations, the lag period observed during the pilot appears to be independent of TCP concentrations and potentially influenced by competition from elevated nitrate and acclimation of the bioaugmentation culture to site geochemistry. During the full-scale remedy, the effectiveness of the ISBR treatment cell voided the need for an above ground treatment or polishing step using granular activated carbon (GAC), minimizing waste disposal requirements. After a full-scale ISBR treatment period of approximately 4 years, TCP concentrations have decreased to below the MCL across the site in the targeted aquifer, and DCP concentrations have decreased by over 95%. This presentation will discuss the elements of groundwater hydrogeology, geochemical conditions, and adaptive operations management that influenced the efficacy of TCP and DCP treatment using the ISBR remedy.

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Melissa Asher