Story

Project

EHC Pilot Study for Oregon DEQ's Dry Cleaner Program at a Former Dry Cleaner Site, OREGON USA

Project manager/regulatory contact: Don Hanson, Oregon Department of Environmental Quality

Summary

Groundwater at the former Serry's dry cleaner site in Corvallis, Oregon, was impacted with chlorinated volatile organic compounds (CVOCs). The key CVOCs found at the site include PCE, TCE, DCE, and VC, as high as 22,000, 1,700, 3,100, and 7 ppb, respectively.  Field scale pilot tests were performed for the Oregon DEQ's Dry Cleaner Program to evaluate the performance of EHC®, an in situ integrated biological and chemical reduction (ISCR) technology.  The method of injection was also evaluated to determine the effectiveness of the injection method in distributing EHC, given the low permeability of soils at the site (sandy silt and silty clay). Subsequent field monitoring has shown greater than 99.9% removal of total CVOC concentrations two years after the injections, without the accumulation of catabolites.
                                        
The Challenge

The groundwater flow direction at the site changes with seasonal conditions, and is either toward the south-east or the north-west. The groundwater velocity is unknown, but assumed to be low due to the type of soils at the site. No confirmed source of PCE has been found, however groundwater concentrations suggest that there may be two hot spots on site: the north-west corner and south-east corner of the building. The building is currently occupied and access is difficult to obtain. The primary goal of the pilot-scale treatment is to reduce groundwater concentrations at the southeastern hot-spot that may have historically contributed to indoor air vapor intrusion at nearby residences.

Test Injection

A test injection was conducted prior to full-scale implementation to evaluate the effectiveness of direct push methods in distributing EHC in the low permeability soils. A total of 450 lbs of EHC was injected from 13 to 25 ft bgs (4 discrete layers, spaced 4 ft apart) using GeoProbe's pressure activated injection tip. A high pressure pneumatic grout mixing and pumping system was used to mix and pump the EHC to the injection tip.  Six soil cores were obtained around the injection point (from 0.5 to 5 ft from the injection point) to assess the radius of influence of the injection (Figure 1). EHC fractures were found at the farthest sampling points indicating that the radius of influence was up to at least 5 ft. Figure 2, 3, 4 and 5 show some of the EHC bands or fractures observed in the soil cores.

The following lithology was observed from the soil sampling:

• Brown sandy, clayey silt from 8 to 20 ft bgs;
• Stiff brown silty clay from 20 to 22 ft bgs;
• Stiff gray clay below 22 ft bgs;
• Water bearing and loose sand layers were encountered at approximately 26 ft bgs.

Field-Scale Pilot Study

In August 2006, a total of 10,250 lbs of EHC was injected into an area measuring approximately 825 ft2 x 20 ft deep (from 10 to 30 ft bgs), which resulted in an average application rate of 0.6% to soil mass. The EHC was supplied as a dry powder in 50-lb bags and mixed with water on site into slurry containing about 29% solids (Figure 6). Using conventional direct push technology, the EHC slurry was injected at 3 to 6 GPM at a pressure of approximately 200-400 psi.  The depth interval targeted was from the groundwater table (ca. 10 ft bgs) to approximately 30 ft bgs, where a less permeable layer was encountered. The EHC was injected in discrete layers using the Geoprobe pressure-activated injection tip and high pressure grout mixing unit.

The EHC was added to a total of 32 injection points (Figure 7), including nine additional points south of the building. The additional points were added due to issues with surfacing: it was not possible to achieve the originally planned application rate of 1% to soil mass in the tight soil formation at the targeted flow rate of approximately 5 GPM due to pressure build-up in the subsurface. The treatment area was therefore increased resulting in a lower application rate of 0.6% to soil mass within the treatment area.  In addition, a relatively tight injection spacing of 5 ft was used for the EHC injections based on the results from the test injection. Approximately 18 lbs of EHC was added per vertical foot on average.

Figure 7 also shows the locations of four new monitoring wells (MW-6 to MW-9) installed in January 2007. Previous monitoring performed in February 2005 and August and September 2006 was conducted via direct push probe exploration locations (HC-22 to HC-29).

The Result

Figure 8 shows the groundwater concentration of PCE and daughter products at the four sampling clusters within the EHC injection zone. Total CVOCs decreased from a maximum of 25,606 ppb to below 100 ppb at all locations within 12 months. A small rebound was observed during the rainy season (18-month data) due to the introduction of CVOCs from the smear zone as a result of a 6 to 7 ft increase in the groundwater table. While the groundwater table remained high through June 2008 (22-month data), total CVOC concentrations decreased suggesting continued treatment. By the latest sampling event, conducted in August 2008, PCE and TCE have decreased to below the detection limit of 1 ppb at all locations. The maximum DCE level has decreased by greater than 99.9% (from a baseline of 8,800 ppb to current maximum of 4.6 ppb). Similarly, no accumulation of VC was observed (maximum concentration of 7.4 ppb measured at the latest sampling event).  Furthermore, an increase in ethene levels confirms that complete degradation is occurring; ethene levels of up to 760 ppb were measured in July 2007 which constitutes an increase of 96% compared with the maximum concentrations measured in August 2006.

The Cost

The material cost of using EHC was $1.24/ft3 ($44/m3). The injections were completed in 5 days. Results of this pilot study have shown that ISCR using the EHC technology offers a safe, effective and cost-efficient remedial solution for dry cleaning and related sites.

For more information, see the Oregon DEQ Project Description at: http://www.deq.state.or.us/wdr/?p=20107