Story

Project: Former Unregulated Solid Waste Management Unit, Ohio

Lead Consultant – Hull & Associates, Inc.

Summary

Groundwater at a former unregulated solid waste management unit is impacted by various chlorinated volatile organic compounds (CVOCs), mainly chlorinated ethenes. Groundwater collection trenches totaling approximately 330 ft in length were previously constructed downgradient of the suspected source areas. In February 2006, EHC® was injected into the gravel trenches to convert them into permeable reactive barriers (PRBs). A microbial culture of Dehalococcoides species (KB-1 inoculant; SiREM) was also applied.

Subsequent field monitoring showed that trichloroethene (TCE) and cis-1,2-dichloroethene (cis-DCE) decreased below the detection limit within the trenches after just 6 months, without the accumulation of recognized catabolites, such as vinyl chloride (VC). Continued monitoring has shown that the PRBs remained active over 18 months and that the concentration of total chloroethenes is decreasing downgradient of the center of the PRB (over 99% removal since November 2006). A positive correlation between the presence of EHC breakdown products (TOC and dissolved Fe) and removal rates has been observed downgradient of the PRB.

The Challenge

Site groundwater proximal to a known area is impacted by TCE (maximum of 750 pb; remedial objective = 5ppb) and its recognized anaerobic daughter products 1,2-DCE (maximum of 5,200 ppb; remedial objective = 70 ppb) and VC (maximum of 630 ppb; remedial objective = 2 ppb). In an effort to mitigate transport of groundwater CVOC's, collection trenches measuring approximately 250 long were previously installed along the southern and western boundaries of the former unregulated solid waste management unit. Another collection trench measuring approximately 80 ft long was installed inside of a building (Figure 1 - red lines).

The goal of this current project was to turn these existing collection trenches into permeable reactive barriers (PRB's) to passively treat the groundwater as it flowed through the newly created reactive treatment zones. The groundwater table is generally at 6 ft below ground surface (bgs) and the trench extends down to approximately 12 ft bgs. The trenches measures 2 ft wide and are filled with washed river gravel up to a foot above the groundwater table.

The Solution

Using conventional direct push technology, 11,850 lbs of EHC were injected into the gravel zone to form a reactive zone thus creating an in situ PRB that would treat the groundwater as it flows through the reactive zone (Figures 2 and 3). The EHC product was delivered as a dry powder and consisted of solid organic carbon and zero-valent iron at 42%. The EHC powder was mixed with water on site into a slurry containing approximately 20 to 25 percent solids using a grout mixer.

The injection points were spaced 10 ft apart, resulting in a total of 30 injection points. A total of 400 lbs of EHC (ca 200 USG of slurry) was designed for each injection point. Immediately following the EHC injections, KB-1 inoculant was added to accelerate removal of cis-1,2 dichlorethylene DCE. Monitoring Program Following the installation of the EHC PRB, the groundwater was samples for CVOC's on a quarterly basis at the following locations (Figure 4): - Manholes (CB-1 and CB-2), - Downgradient wells (MK-5 and MK-6) - Sitegradient well (MK-7), and - Upgradient and downgradient piezometers (P1-P5).

In addition, groundwater quality field parameters including pH, DO, temperature, water level and ORP have been collected monthly. Nine months following the installation of the PRB's the groundwater was analyzed for total iron, ferrous iron, and TOC to verify which sampling locations are within the EHC zone of influence. Samples from MK-5, MK-6 and P5 were also analyzed for Dehalococcoides species.

The Result

Figures 5 and 6 shows the concentration of total chloroethenes in groundwater collected from the collection manholes at the downgradient end of the trenches. The first round of performance monitoring (May 2006) was conducted approximately three months after the EHC injections and did not show any discernible response.

However, subsequent monitoring 6 , 9, 12 and 18 months post EHC injection showed almost complete removal of all CVOCs, without the accumulation of catabolites. In the latest monitoring event, conducted in August 2007, total CVOCs decreased by 99.8% at the trench along the property line (from 2,700 to 6 ppb) and 97.0% at the building trench (from 1,950 to 58 ppb) compared to the initial concentrations measured in September, 2004.

In addition, a steady decline in CVOCs was observed at monitoring well MK-6, which is located 13 ft downgradient from the center of the trench (Figure 7). Eighteen months following the EHC injections total CVOCs had decreased by 99.7%; TCE decreased from 370 ppb to ND (<1 ppb), DCE from 2,800 to 4.5 ppb, and VC from 510 ppb to 4.8 ppb. No consistent CVOC removal has been observed in the other monitoring locations outside of the trench.

To verify whether these points were within the direct zone of influence of the EHC, the groundwater was sampled for TOC and total and ferrous iron nine months following the injections. Following the injection of EHC into the subsurface a significant increase would be expected in these parameters within and directly downgradient of the injection zone. The extent of the zone with elevated levels of EHC breakdown products will depend on the linear groundwater velocity and direction. Elevated levels of TOC and iron were measured within the trenches and at MK-6, directly downgradient from the lowest point of the trench which correlates with the CVOV removal achieved at these locations.

No Significant increase of EHC breakdown products was observed at the remaining sampling locations and it could therefore be assumed that the EHC PRB has not directly impacted those locations et. This could possibly be explained by the flow-pattern created by the gravel trenches installed into the low-permeability clayey soil; the groundwater would be expected to be directed through the gravel collection trenches (as designed) and to be primarily released at the lowest point b collection manhole CB-1. This would also be supported by the significant and fast removal of CVOC's at MK-6 directly downgradient from manhole CB-1 (Figure 8).

Nine months after the EHC injections, the groundwater was also analyzed for Dehalococcoides at MK-5, MK-6 and P5. There appeared to be a positive correlation between the presence of EHC breakdown products, Dehalococcoides levels and CVOC degradation (Table 1 and Figure 9).

The Cost

he material cost of using EHC was $12/ft2 ($129/m2) of PRB cross-section (a total of 11,850 lbs of EHC injected into PRBs measuring a total of 330 ft long x 6 ft deep on average).

Regulatory Contact OHIO EPA