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

Abstract

Historical groundwater-withdrawal and contaminant-concentration data collected from long-term pump-and-treat operations were analyzed and used to examine contaminant mass discharge (CMD) and mass-removal behavior for multiple sites. Differences in behavior were observed, and these differences were consistent with the nature of contaminant distributions and subsurface properties of the sites. For example, while CMD exhibited a relatively rapid decline during the initial stage of operation for all three sites, the rate of decline varied. The greatest rate was observed for the PGN site, whereas the lowest rate was observed for the MOT site. In addition, the MOT site exhibited the lowest relative reduction in CMD. These results are consistent with the actuality that the MOT site likely contains the greatest proportion of poorly accessible contaminant mass, given that it comprises a combined alluvium and fractured-bedrock system in which solvent and dissolved mass are present directly in the bedrock. The relative contributions of the source zones versus the plumes to total CMD were determined. Constrained contaminant mass removal was observed to influence the plumes for all three sites, and was attributed to a combination of uncontrolled (or imperfectly controlled) sources, back diffusion, and well-field hydraulics. The results presented herein illustrate that detailed analysis of operational pump-and-treat data can be a cost-effective method for providing value-added characterization of contaminated sites.

Upscaling of Regional Scale Transport Under Transient Conditions: Evaluation of the Multirate Mass Transfer Model

“…Regional groundwater quality management models must upscale the transport equation sufficiently to represent the effects of heterogeneity, including preferential flow in well‐connected high‐conductivity networks and mass transfer between the high‐ and low‐conductivity media. The early arrivals of contaminants caused by the preferential flow and the asymptotical tails resulting from transport through low permeability materials have been frequently observed, which significantly challenges long‐term management (e.g., Bianchi et al, ; Brusseau & Guo, ; Brusseau et al, , ; Dearden et al, ; Fogg et al, ; Fogg & LaBolle, ; Fogg & Zhang, ; Guo & Brusseau, ; LaBolle & Fogg, ; Matthieu et al, ; Seyedabbasi et al, ). Although the effects of heterogeneity can be simulated by explicitly including the spatial variability of hydraulic properties in the model, this approach is not feasible for most plume‐scale contamination sites and is definitely not feasible for regional‐scale models covering 10s to 100s of kilometer.…”

Determining the long-term operational performance of pump and treat and the possibility of closure for a large TCE plume

“…Examples of compounds of concern include chlorinated solvents (e.g., trichloroethene, tetrachloroethene, carbon tetrachloride), 1,4-dioxane, methyl tertiary-butyl ether (MTBE), and perchlorate. The transport processes of these contaminants are highly impacted by the heterogeneity and anisotropy of alluvial subsurface environments [e.g., [1][2][3][4][5][6][7][8][9][10][11][12][13]. Extensive dissolved-phase plumes typically form at sites contaminated by these constituents, which has been reported in many previous studies [e.g., 6,7,[14][15][16][17].…”

The impact of well-field configuration and permeability heterogeneity on contaminant mass removal and plume persistence

“…The results are evaluated using a recently-developed metric that examines the relationship between reductions in contaminant mass discharge (CMDR) and reductions in contaminant mass (MR). This metric has been demonstrated to be an effective method to examine remediation efficiency [e.g., 4, 6, 29–35]. The previous research efforts that have applied the CMDR-MR metric were conducted for source-zone systems or for systems with combined sources and plumes.…”