Obtaining High-Resolution Data to Demonstrate Proprietary Remediation Method Performance in a Large TCE Plume with Extensive DNAPL Present
Thomas A. Harp (
tharp@LTenv.com) (LT Environmental, Inc., Arvada, Colorado, USA)
ABSTRACT: A high-resolution, quantitative-data approach was imple- mented at a large, urban industrial facility where Trichloroethene, used extensively as a cleaning solvent, con- taminated underlying riverine sedimen- tary deposits and bedrock. Subtle facies changes resulted in solute concentra- tions that varied by orders of magnitude over distances of only centimeters. This inherent complexity was the impetus for obtaining quantitative, high-resolution geologic and chemical data to charac- terize site conditions and design, imple- ment, and demonstrate the performance of BOS 100®, a proprietary remediation amendment. The high-resolution pro- gram for this site consisted of analyzing 1,291 continuous soil samples from 186 borings and 5,515 groundwater samples from 1,349 monitoring wells.
Pre-treatment data were used to cal- culate mass flux and mass discharge to develop an accurate conceptual site model, evaluate source and plume strength (contaminant mass moving in groundwater per unit time (The Interstate Technology & Regulatory Council, 2010), and calculate injectate loadings. The “added value” of high- resolution data is that it enhances the ability to identify zones which transmit- ted the bulk of contaminant mass, thus facilitating a more economically and environmentally effective cleanup. Post- treatment data were used to evaluate injectate distribution to ensure adequate coverage of the remedy.
INTRODUCTION
A large quantity of trichloroethene (TCE) was stored and used at a manufacturing facility located in a major metropolitan area. Decades of
10 TPG •
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tankage spills and product-line releases resulted in “source areas” totaling approximately 230 square meters (m2) with interspersed dense non- aqueous phase liquid (DNAPL) and an approximately 6,200 m2 dissolved- phase plume (Figure 1). TCE in soil was observed at concentrations up to
solute mobilization, reagent depletion, and insufficient residence time.
Instead, a carbon-based technology (BOS 100®) was selected because geo- chemical and/or bacterial deficiencies in the plume are not a factor and remediation is complete following a single application. Retreatment is only necessary if slurry distribution is insuf- ficient, not because of rebound, i.e., back diffusion when lower-permeability strata become sources rather than stor- age sinks of solutes in the exchange with higher-permeability strata.
Figure 1 - TCE Plume
25,477,000 micrograms per kilogram (μg/kg) and TCE in groundwater was observed at concentrations up to 1,280,000 micrograms per liter (μg/L).
An extensive technical feasibility review was conducted to select an appro- priate remedy. Biotic methods were dismissed because there was very little if any apparent natural biodegradation in the subsurface. Conventional abiotic methods such as, chemical oxidation or chemical reduction, were dismissed because of inherent deficiencies, e.g.,
In extremely heterogeneous settings such as this site quantitative, high- resolution data are needed to accurately characterize and map solute distribu- tion in soil and groundwater. Dynamic- workplan-assessment technologies such as the Membrane Interface Probe (MIP) can be effective qualitative screening tools, but are ineffective for accurate injection design or performance monitor- ing because only total volatile organic compound concentrations are detected, not the concentrations of specific ana- lytes. Data gathering does not stop at design, but is integral to the implementa- tion phase as well. Successful injection programs need to be flexible and sup- ported by robust performance monitor- ing that allows using “where you have been” to adjust for “where you need to go”.
Site Description: The site was underlain by river deposits and sedi- mentary bedrock (Figure 2 on the follow- ing page). Impacted alluvium included approximately 5 meters (m) of interbed- ded sand and clay-rich deposits. Beneath the source area were approximately 15 m of well-graded sands and gravels underlain by a low-permeability zone of silt and silty clay where DNAPL pooled
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