Formatted Title
How to Remediate a Hexavalent Chromium and Volatile Organic Plume in a Single Application Using Injection Flow Through Barriers
Background/Objectives
A release of hexavalent chromium (metal finishing plating and process wastewater) and volatile organics (degreaser pit) resulted in impacts to soil and groundwater. Groundwater data reported Cr+6 concentrations of 38 mg/L at the source, 10 mg/L approximately 1,500 feet downgradient, and discharging some 3,000 linear feet to a stream. The release of Cr+6 was from leaks from historic cypress wastewater holding tanks and through the plating line concrete floors. The remedial objective was to chemically reduce the more mobile/toxic Cr+6 to an intermediate less mobile/toxic Cr+3 then to immobile Cr(OH)3 while concurrently achieving reductive dichlorination of the commingled VOCs. This presentation will focus on the results of the in situ chemical reduction (ISRC) of hexavalent chromium concentrations by understanding the migration of the contaminate plume and mass flux by designing ISRC barriers walls resulting in achieving the remedial objective of less than 500 µg/L (many locations were non-detect), under a single application, followed by monitored natural attenuation.
Approach/Activities
The use of high-resolution site characterization (HRSC) techniques was invaluable in understanding 1) the subsurface geology and 2) contaminant transport mechanisms as the release occurred above a hydraulic divide which bifurcated the release into two plumes. Subsurface conditions consisted of a fine to coarse sand (water table) overlying a deltaic fine sand, silt, and/or silty/clay deposit overlying a lacustrine clay aquitard. A strong downward vertical gradient was present at the hydraulic divide which aided in the vertical migration of the plumes deep into the deltaic deposit (50 to 80 feet below grade or 35 to 65 feet below the water table), to an underlying lacustrine boundary unit. Because the deposits were within the distal component of the delta, historical glacial lake levels permitted thin clay units to dry (descanted) before subsequent deltaic deposits were deposited. Because of the descanted nature of the clays and the strong vertical gradients, the Cr+6 and cVOCs migrated deep into these deltaic sequences. The release was also strongly acidic and oxidizing (chromic, nitric, and sulfuric acids (plating line) altering the geochemistry to basic conditions (pH – 9.5). These geochemical and abiotic conditions were not conducive to natural biological degradation and permitted the plumes to migrate with only limited dilution of Cr+6 and cVOCs.
The remedial strategy began with bench scale microcosm studies to define amendment selection. A field pilot studies was implemented and identified that by combining amendments methanogenesis was observed 10-years after the initial field pilot. These data were then used to design barrier walls permitting the natural groundwater flow to migrate through the ISCR barriers to reduce the contamination. This approach resulted in fewer direct injection points (lower costs) and, due to the HRSC, the injections were targeted to the zones with the greatest contaminant mass prior to implementing a monitored natural attenuation (MNA) remedy.
Results/Lessons Learned
The injection program was successful in reducing the concentration of hexavalent chromium after a single application through the development of barriers by direct injection. A total of 5 acres were treated with 155-injection points. The alternative would have been a traditional direct injection approach (every 30 feet on center with a radius of influence of 15 feet, based upon injection studies) requiring a total of 1,230 points to achieve the same coverage.
Concentration in the downgradient plume after approximately 1 year were less than the remedial objective of 500 µg/L and in many monitoring wells less than 100 µg/L or non-detect. At the source area, similar reductions were observed; however, contaminated groundwater that had migrated beneath a downgradient building (not treated) subsequently migrated beyond the building footprint resulting a downgradient well becoming recontaminated by the residual plume beneath the building. Additional targeted injections have been completed and new data set will be presented post injections.
The use of HRSC proved to be invaluable reducing the volume of the amendment and targeting the treatment zone of greatest mass which resulted in both significant cost savings (second application was planned but was not required) and reduction in time to achieve the state approved remedial goals prior to a long-term MNA program. In addition, by targeting the upgradient mass, the down gradient plume, approximately 1,000 feet downgradient of the barriers was reduced at least 50 percent after 1 year even though the seepage velocity was only 35 to 70 feet per year. The take-away is that with good site characterization along with bench scale and pilot testing, successful ISCR injection programs can be designed to treat areas with the greatest mass of contamination with a single application.