Description
Project
EHC® Treatment of Chlorinated Ethenes, Ethanes, and Methanes in Saprolite Soils and Partially Weathered Rock, Manufacturing Facility - Western NC
Summary
Field scale pilot tests were performed to evaluate the performance of enhanced reductive dechlorination (ERD) of chlorinated aliphatic hydrocarbons (CAHs) at a manufacturing facility in western North Carolina. The key CAHs found in groundwater at the site include 1,1,2,2 tetrachloroethane (TeCA), trichloroethene (TCE), and chloroform (CF), as high as 350, 1,100, and 1,500 ppb respectively. Adventus employed its EHC® in situ integrated biological and chemical reduction (ISCR) technology yielding safe, rapid, and effective treatment. Eleven weeks after injection, concentrations of TeCA, TCE, and CF in a nearby well reduced by 86%, 98% and 66%, respectively. After approximately 26 months, the key CAHs in the monitoring wells in the silty-clay (saprolite) zone indicated an overall decrease of up to 95%.
The Challenge
The site is located in the Appalachian
Piedmont physiographic province (mountainous) of North Carolina and
consists of saprolite soils overlying a partially weathered rock (PWR)
zone. Fractured bedrock underlies the PWR zone. Groundwater
concentrations have rebounded since the historical treatment, indicating
the potential for continuing sources, desorption, or complicated
hydrogeology and geochemistry, or microbial conditions at the site.
Presence of low permeability soils, low groundwater velocities and a
mixture of CAHs presented a unique challenge for remediation.
The Solution
EHC® is a patented combination
of controlled-release carbon and zero valent iron (ZVI) particles used for
stimulating reductive dechlorination of otherwise persistent organic
compounds in groundwater. At this site, EHC was modified with BASF's
micronscale ZVI (carbonyl iron powder). Two pilot-scale permeable
reactive barriers (PRBs) were installed in January 2005 to evaluate the
efficacy of EHC to control and treat the chlorinated solvents plume in two
areas of the site. The first injection PRB was installed into the
saprolite between 10 and 25 ft below ground surface (bgs) and the second
injection PRB was installed into the partially weathered rock (PWR)
between approximately 30 and 35 ft bgs. Each PRB measured an estimated 40
ft long x 20 ft wide. Approximately 3,000 pounds of EHC was injected in
two Geoprobe boreholes in the saprolite zone and 3,500 pounds of EHC was
injected at four drilled locations in the PWR zone.
The presence of low permeability soils and the
PWR zone required hydraulic fracturing to inject EHC at the site.
Injection pressures and ground uplift were monitored, and soil boring and
monitoring wells were installed to verify fracture propagation and
distribution of the EHC. Soil samples collected around the injection
points indicated the presence of EHC material. Based on field
observations, the influence of EHC injection extended in a zone
approximately 10 to 15 feet around the injection point in the
saprolite. The injection in the PWR zone was controlled by the
fracture orientation.
The presence of high concentrations of total
organic carbon and metabolic acids and negative oxidation-reduction
potential in the nearby monitoring wells indicated that the injected EHC
created anaerobic zones favorable for CAH reduction.
The Result
Eleven weeks after injection, concentrations
of TeCA, TCE, and CF in a nearby well in the PWR zone reduced by 86%, 98%
and 66%, respectively. After approximately 26 months, these
compounds in the same well reduced over 98%. Potential degradation
products of these CAHs such as 1,1,2- trichloroethane, methylene chloride,
cis-1,2-dichloroethene, trans-1,2-dichloroethene, vinyl chloride, ethene,
ethane and methane were detected in low concentrations, as shown in
Figure 1. The concentrations of TeCA, TCE and CF
initially increased in one downgradient well in the saprolite zone and
gradually decreased. The test results indicated that reductive
dechlorination was delayed in the aquifer despite the presence of
dechlorinating bacteria such as Dehalococcoides, Dehalobacter and
Methanogens. The likely reasons for the slow treatment
response are the slow rate of groundwater flow, desorption of CAHs and due
to presence of multiple CAHs (ethanes, ethenes, and methanes). After
approximately 26 months, the key CAHs in the monitoring wells in the
saprolite and PWR zone indicated a decrease of up to 99% (see
Figures 1, 2, and 3). Despite the reductions in CAH
concentrations, organic carbon is still present supporting the reductive
dechlorination process in the saprolite and PWR zone.
Companies On This Project
ADVENTUS GROUP
