Theme: 01. Remediation Technology Innovations

Topic Area: 01i. In Situ Chemical Oxidation: Optimized Design Approaches and Lessons Learned

Formatted Title:

Case Study: Using Recirculation Injection Methodology to Improve Efficacy of In Situ Chemical Oxidation Remediation

Background/Objectives:

Successful in situ injections require achieving sufficient reagent distribution across the target treatment area within the oxidatively active lifetime of the selected reagent. Site-specific considerations such as geologic heterogeneity, hydrogeology, and difficult drilling conditions can pose limitations to achieving adequate reagent distribution and thus could limit contaminant mass reduction.

Three in situ chemical oxidation pilot studies were conducted to remediate total petroleum hydrocarbons (TPH) and chlorinated ethenes in groundwater. The treatment interval was 5 to 20 feet below ground surface and consists of silty sand fill. The first two pilot studies utilized direct injection of activated sodium persulfate (ASP). To enhance ASP contact with site contaminants, the third pilot study utilized recirculation by extracting groundwater from a well on one side of the treatment area, dosing with ASP, and reinjecting on the other side. Recirculation creates a convection cell in the subsurface, which enhances pore flushing and uniformly distributes ASP solution across the recirculation flow path. This presentation presents pilot study results to showcase the benefits recirculation provided over previous direct injection methods to effectively distribute reagent and remediate site contaminants.

Approach/Activities:

Groundwater was pumped from one extraction well using a submersible pump and pumped through granular activated carbon (GAC) to remove potential contaminants. Following GAC, the water was mixed with Klozur® SP (sodium persulfate) and base activator (sodium hydroxide). The reagent solution was then injected in one injection well using a double diaphragm pump. Injection rates were maintained at approximately 4 gallons per minute at an average injection pressure of 4 pounds per square inch. A total of 5,244 gallons (10 percent of pore volume) of 9 percent weight per weight reagent solution was injected and recirculated.

A field test kit was used during injections for daily monitoring of persulfate concentrations in groundwater at the extraction well and two monitoring wells located directly in between the injection and extraction wells. Monthly performance monitoring was performed for 3 months following injections, including persulfate testing to directly measure reagent migration.

Results/Lessons Learned:

Persulfate concentrations increased to approximately 630 ppm in monitoring wells located between injection and extraction wells and in the extraction well after exchanging 8 percent of the pore volume, indicating that sufficient reagent distribution across the treatment area was achieved rapidly. Persulfate concentrations were greater than 10,000 ppm (above target concentration of 3,000 ppm) during subsequent monitoring events 1 week through 4 months after injections, indicating favorable reagent contact and longevity.

TPH and chlorinated ethene concentrations within the treatment area have significantly decreased below site cleanup levels and performance monitoring data indicate overall stable or downward trends in monitoring wells. Optimizing ASP remediation using recirculation achieved cleanup to treatment goals where previous direct injections could not. Remediation practitioners should consider implementing recirculation, where feasible, to increase reagent distribution and treatment effectiveness.

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Jule Ferguson

Biography