Assessing the impact of source-zone remediation efforts at the contaminant-plume scale through analysis of contaminant mass discharge

Abstract

The long-term impact of source-zone remediation efforts was assessed for a large site contaminated by trichloroethene. The impact of the remediation efforts (soil vapor extraction and in-situ chemical oxidation) was assessed through analysis of plume-scale contaminant mass discharge, which was measured using a high-resolution data set obtained from 23 years of operation of a large pump-and-treat system. The initial contaminant mass discharge peaked at approximately 7 kg/d, and then declined to approximately 2 kg/d. This latter value was sustained for several years prior to the initiation of source-zone remediation efforts. The contaminant mass discharge in 2010, measured several years after completion of the two source-zone remediation actions, was approximately 0.2 kg/d, which is ten times lower than the value prior to source-zone remediation. The time-continuous contaminant mass discharge data can be used to evaluate the impact of the source-zone remediation efforts on reducing the time required to operate the pump-and-treat system, and to estimate the cost savings associated with the decreased operational period. While significant reductions have been achieved, it is evident that the remediation efforts have not completely eliminated contaminant mass discharge and associated risk. Remaining contaminant mass contributing to the current mass discharge is hypothesized to comprise poorly-accessible mass in the source zones, as well as aqueous (and sorbed) mass present in the extensive lower-permeability units located within and adjacent to the contaminant plume. The fate of these sources is an issue of critical import to the remediation of chlorinated-solvent contaminated sites, and development of methods to address these sources will be required to achieve successful long-term management of such sites and to ultimately transition them to closure.

Assessing contaminant-removal conditions and plume persistence through analysis of data from long-term pump-and-treat operations

“…Dissolved contaminant associated with lower-permeability zones is prominent for all of the sites and, hence, back diffusion is anticipated to be a significant common factor contributing to the observed plume persistence. This is supported by the results of prior investigations conducted at the AFP44 site to examine source-zone remediation efforts and associated plume behavior (Nelson and Brusseau, 1996; Blue et al, 1998; Brusseau et al, 1999; Zhang and Brusseau, 1999; Nelson and Brusseau, 2003; Brusseau et al, 2007, 2011a,b). Multiple methods, including forced-gradient contaminant elution tests, multiple-solute tracer tests, sediment coring, analysis of historic pump-and-treat operations data, laboratory experiments, and mathematical modeling, were used to characterize the relative impacts of plume-scale back diffusion, plume-scale sorption/desorption, and dissolution of organic liquid trapped in the source zones on contaminant removal and plume persistence.…”

“…However, these data are rarely used for purposes other than to monitor the mass of contaminant removed. These data sets constitute a source that can be mined to provide additional information to enhance site characterization activities and remediation-performance assessments (Brusseau et al, 2007, 2011a,b, 2013). The information obtained from mining of the data can be used to update the site conceptual model, to revise the design and operation of the remediation systems, and to support decision-making concerning remedy modification and closure.…”

Characterizing long-term contaminant mass discharge and the relationship between reductions in discharge and reductions in mass for DNAPL source areas

“…The use of contaminant-mass-discharge or mass-flux measurements to characterize site conditions and assess remediation performance is becoming more widespread (Rosenbloom et al, 1993; Butcher and Gauthier, 1994; Freeze and McWhorter, 1997; DiGiulio et al, 1999; Einarson and Mackay, 2001; Rao et al, 2002; Newell et al, 2003; Bockelmann et al, 2003; Stroo et al, 2003; Brooks et al, 2004; Soga et al, 2004; NRC, 2005; SERDP, 2006; Brusseau et al, 2007; Basu et al, 2009; ITRC, 2010; Brusseau et al, 2011a, 2011b). Almost all applications to date have been based on obtaining one or two discrete measurements of contaminant mass discharge (e.g., collected before and after a remedial action), using multi-point transects or short-term pumping tests that are often applied within the contaminant plume.…”

Validation of a multilevel sampling device to determine the vertical variability of chlorinated solvent in a contaminated aquifer

“…Chlorinated solvent monitoring with the multilevel sampler showed that the most contaminated layer was located at a depth between 4 and 6 m. However, this does not mean that the flux of contaminant is maximum at this depth [19]. Indeed, the flux in the most contaminated layer may be low and consequently limit the decontamination efficiency [20].…”