Alta Environmental is at the forefront of technology developments in remediation for some of the most challenging contaminated properties. Based on our depth of experience and ongoing work with various technology providers, we’ve assembled a brief overview of current issues and emerging techniques for addressing chlorinated solvents that may affect groundwater supplies.
Chlorinated solvent compounds such as tetrachloroethene (PCE) and trichloroethene (TCE) are commonly used in cleaning and manufacturing processes at various industrial/commercial facilities, including aerospace, dry cleaning, metal plating and finishing, and other facilities where solvents are used. PCE and/or TCE and associated degradation compounds such as cis-1,2-dichloroethene (DCE) and vinyl chloride are often released or spilled into the underlying soils and groundwater. If groundwater is affected, the uppermost aquifer is usually impacted. However, because chlorinated solvent compounds are denser than groundwater, the compounds may sink to the bottom of the upper aquifer and possibly may affect lower aquifers that are used for local groundwater supplies.
Remediation of chlorinated solvent compounds, particularly if groundwater is affected, is usually regulated by the California Regional Water Quality Control Board or the Department of Toxic Substances Control. The regulatory agencies require preparation and implementation of a Remedial Action Plan (RAP) which outlines various remedial alternatives that are applicable for an impacted site, and describes the scope of work, costs, and schedule of the selected remedial alternative. Effective implementation of the RAP is critical because not only is it directed by the regulatory agency, but also allows the affected aquifer to be restored to productive use and minimizes impact to drinking water wells if those wells are threatened.
Construction of permeable reactive barriers (PRBs) within the impacted aquifer is an emerging remedial alternative for control and capture of chlorinated solvents in groundwater. This technology is not exactly new, but is becoming more prominent and cost-effective as an accepted and viable remediation approach. The PRB wall is constructed along a line across the PCE/TCE plume, within a downgradient portion of the aquifer (such as along the property line thereby preventing further migration offsite) or where control and capture of the contaminants is needed (such as upgradient of sensitive receptors). The PRB is constructed by installing a mixture of dechlorinating products, including include zero-valent iron (ZVI), emulsified vegetable oil (EVO), and dehalococcoides (DHC). For shallow aquifers (i.e, less than 25 feet below grade), the PRB wall is constructed with the use of an excavator, and the width of the PRB is therefore equal to the width of the excavating bucket (typically 1 to 2 feet wide). For deeper impacted aquifers, the PRB is constructed via hydro-fracturing by drilling a row of injection points with a direct-push or hollow-stem auger drilling rig, and injecting the materials into the borings with an injection pump. The injection points are spaced at equidistant points along the planned PRB wall location, and the PRB is usually about 3-6 inches wide.
The ZVI/EVO/DHC mixture causes dechlorination of dissolved-phase PCE/TCE at the PRB wall. Specifically, the ZVI provides an electron donor which is attached to the PCE/TCE compound and breaks down the bonds in the compound, the EVO provides a substrate (food) for microorganisms thus nourishing the microbial population, and the DHC increases the rate of bioremediation – allowing a more rapid transformation of PCE to TCE, TCE to DCE, and DCE to vinyl chloride, and then to the final by-product (ethane).
It is important to note that likely within the next 2-3 months, DHC will be accepted by the RWQCB on the general WDR permit, thus decreasing permitting time. Currently, a site-specific permit for injection of DHC is required, in which case it can take a year or longer for use of DHC to be approved.
If you are interested in learning more about this technology, or if you wish to discuss other remedial technologies which are used to remediate chlorinated solvent compounds, please contact Steve Ridenour (firstname.lastname@example.org) or Mike Cassidy (email@example.com), or call us at (562) 495-5777.
GeoSierra Environmental, Inc.