Remediating the Good Earth
An eight-million-dollar, 20-year soil remediation project at a defunct plating shop is cleaned up at half the cost in a fraction of the time.
Its status as a typical, middle-American community earned Muncie, Ind., the moniker “Middletown” and made it the focus of a series of scientific cultural studies spanning a period of 80 years.
Like many typical, middle-American communities, Muncie thrived on industry, starting in the late 1800s. However, also like many American cities, Muncie experienced tumultuous times in the 1980s when many large manufacturing facilities closed. Manufacturing in the city declined greatly, and what did remain were primarily smaller, independent, mom-and-pop-style businesses.
It was during this period that a chrome plating and grinding shop opened its doors, with clients primarily in the food container and service industries.
The company began chrome plating operations in the basement of an older, existing house. Business grew quickly, and the building was expanded to about 13,000 sq ft. However, the chrome plating continued in the original basement using a “wet floor” method whereby the parts were washed off above a rinsewater collection sump and chromic acid solutions occasionally dripped off of the workpieces and onto the wet floor.
The floor was periodically rinsed with water and the residual chromic acid rinse water solution drained into the collection sump, where it was returned to the plating process as chromic acid bath makeup water. Although the floor was overlaid with an acid-resistant liner membrane, the acidic solutions were able to penetrate beneath the liner material, migrate through the concrete and into the underlying soil and groundwater.
As in many typical communities, a local plating shop that was once a financial asset now sat on property that represented millions of dollars of potential environmental liability. Not only had hexavalent chromium, arsenic, nickel and antimony impacted on-site soil and groundwater, but the groundwater contaminants had migrated offsite under a newly constructed housing project and community park.
Following extensive legal proceedings associated with the release of the contaminants, two nationally recognized environmental consulting firms were asked to submit competing bids to remediate the impacted soil and groundwater. They submitted bids covering traditional remediation methods, commonly referred to as “hog-and-haul” (excavation and off-site disposal of impacted soil) and “pump-and-treat” (extraction of impacted groundwater with treatment on the surface prior to reinjection). Their proposed cleanup by these methods would cost in excess of $8 million, and the project duration was expected to range from 15 to 20 years.
Acuity Environmental Solutions (AcuityES) then presented the city with an innovative in-situ approach to remediating the metals-impacted soil and groundwater—at half the price and a fraction of the cleanup time.
Thinking Outside the Box
In-situ remediation is simply in-place treatment of impacted soil or groundwater. Successful in-situ remediation is dependent on two equally important factors:
A remediation reagent that takes into consideration both the contaminants and the site geochemistry.
A reagent delivery system that takes into consideration both the reagent characteristics and the site geohydrology.
These two critical factors can be decided on the basis of relatively simple laboratory and field tests that provide the data necessary to design the full-field implementation activities involved in a full-scale site remediation.
The selection of the remediation reagent at the Muncie site was based on laboratory bench testing conducted on soil and groundwater collected from the contaminated Site. AcuityES identified a number of bench test reagents to geochemically reduce the hexavalent chromium to trivalent chromium, as well as geochemically stabilize the arsenic, nickel and antimony. The bench test results identified the best candidate as a newly available reagent consisting of nano-scale ferrous sulfide (FeS) slurry.
It’s a Contact Sport
Reagent delivery methods are typically dependent on the site lithology (physical makeup of the land) and geohydrology (distribution of groundwater in the soil), and the reagent and contamination need to come into intimate contact for the remediation reaction to occur. To treat groundwater at sites that are generally uniformly permeable, it’s common to do the traditionally slow pump-and-treat method. However, for projects like the Muncie site that are characterized by groundwater zones with moderate permeability, or highly varied lithology, it is more effective to establish a closely spaced, grid-based injection network and then inject the reagent through drill rods at high pressure to allow for even distribution.
Injection delivery methods aren’t typically used to treat unsaturated soils that are impacted with contaminants because it’s difficult to create the necessary chemical contact with the contaminant. In cases where the site consists of uniformly permeable soils, infiltration galleries can be used. However, unsaturated soils at the Muncie site consisted of metal-impacted, low-permeability clay, silt and sand as much as 15 feet below the surface. While the traditional remediation method in this scenario would be hog and haul, AcuityES identified the site as a perfect candidate for in-situ soil blending using a dual-axis blender (DAB) capable of mixing the soil and reagents to depths of 15 feet, or greater if the excavation was “benched” or protected from cave-in by stair-stepping the sides.
Get There, Get it Done
The Muncie site had languished for a couple of years due to the performance of a previous consultant and the complicated legal matters that surrounded the project. But within 10 months of AcuityES assuming the project responsibilities, the site was entered into the Indiana State Voluntary Remediation Program, a remediation work plan was submitted and approved by the state, and AcuityES was mobilizing to the site to initiate remediation activities.
The source-area soil that had been contaminated by metal was blended on site with the DAB in a primary treatment area of 100 × 70 × 15 feet and a secondary adjacent shallow area extending 30 feet outside the primary source area to a depth of 5 feet. A total of 400 tons of the nano-scale FeS slurry was mixed into the source area soils. This in-situ soil mixing was completed in 17 days.
Soil sampling confirmed the effectiveness of treatment. Lab results indicated that toxic levels of hexavalent chromium at concentrations as high as 7,900 mg/Kg were effectively converted to the immobile and non-toxic trivalent chromium, with no residual hexavalent chromium detected above the laboratory detection limit. In addition, confirmation soil sampling indicated that other concerning metallic constituents were geochemically stabilized and rendered immobile.
Groundwater was encountered at the site at a depth of about 15 feet below grade, ranging in saturated thickness of 5 to 20 feet. The lithology of the saturated zone consisted of low permeability clay and silt, cut with channels of sand and gravel. Thus, there was a high variation of soil permeability, with the contamination being primarily present in the sand and gravel channels.
Hexavalent chromium was detected in on-site groundwater at concentrations as high as 9,700 mg/L and offsite at as high as 59 mg/L. The state’s clean-up objective was 0.1 mg/L. Other groundwater contaminants included antimony at concentrations as high as 30 mg/L, arsenic as high as 0.36 mg/L and nickel as high as 50 mg/L.
The FeS slurry was injected into the groundwater using 155 injection points based on a 30 foot on-center grid. A total of 35,385 gallons of FeS slurry was advanced through the drill stem of the drill rig in five-foot vertical intervals. This groundwater injection program took approximately six weeks to complete.
Quarterly post-treatment performance monitoring was conducted on groundwater for 10 consecutive quarters following the treatment. Laboratory analytical results reported within eight quarters of sampling that the hexavalent chromium was below the laboratory detection limit of 0.01 mg/L for both the on-site and off-site monitoring wells. Antimony was remediated to below laboratory detection limits within four quarters, nickel in the first quarter, and arsenic by the 10th quarter.
Based on these post-treatment performance monitoring results, a closure report was submitted to the state regulatory agency and the site currently is pending regulatory approval for “No-Further-Action.”
The traditional approach to remediating the plating and grinding facility in Muncie would have required more than 15 years to complete and cost in excess of $8 million. Through AcuityES’ in-situ approach, however, the project was completed within three years and at a cost of less than $4.5 million. The environmental liability is off the company ledger and the property is ready to be returned to useful service again.
The success of the Muncie project was based on selecting the appropriate chemical reagent for the job and identifying the most effective delivery method.
Jim Rouse is the principal geohydrologist with Acuity Environmental Solutions in Fishers, Ind., and can be reached at 317-570-4919 or firstname.lastname@example.org. Richard H. Christensen, Jr., PhD, senior hydrogeologist, and Steve Irvin, principal engineer also contributed to this article.