Mass discharge assessment at a brominated DNAPL site: Effects of known DNAPL source mass removal

Management and closure of contaminated sites is increasingly being proposed on the basis of mass flux of dissolved contaminants in groundwater. Better understanding of the links between source mass removal and contaminant mass fluxes in groundwater would allow greater acceptance of this metric in dealing with contaminated sites. Our objectives here were to show how measurements of the distribution of contaminant mass flux and the overall mass discharge emanating from the source under undisturbed groundwater conditions could be related to the processes and extent of source mass depletion. In addition, these estimates of mass discharge were sought in the application of agreed remediation targets set in terms of pumped groundwater quality from offsite wells. Results are reported from field studies conducted over a 5-year period at a brominated DNAPL (tetrabromoethane, TBA; and tribromoethene, TriBE) site located in suburban Perth, Western Australia. Groundwater fluxes (qw; L(3)/L(2)/T) and mass fluxes (Jc; M/L(2)/T) of dissolved brominated compounds were simultaneously estimated by deploying Passive Flux Meters (PFMs) in wells in a heterogeneous layered aquifer. PFMs were deployed in control plane (CP) wells immediately down-gradient of the source zone, before (2006) and after (2011) 69-85% of the source mass was removed, mainly by groundwater pumping from the source zone. The high-resolution (26-cm depth interval) measures of qw and Jc along the source CP allowed investigation of the DNAPL source-zone architecture and impacts of source mass removal. Comparable estimates of total mass discharge (MD; M/T) across the source zone CP reduced from 104gday(-1) to 24-31gday(-1) (70-77% reductions). Importantly, this mass discharge reduction was consistent with the estimated proportion of source mass remaining at the site (15-31%). That is, a linear relationship between mass discharge and source mass is suggested. The spatial detail of groundwater and mass flux distributions also provided further evidence of the source zone architecture and DNAPL mass depletion processes. This was especially apparent in different mass-depletion rates from distinct parts of the CP. High mass fluxes and groundwater fluxes located near the base of the aquifer dominated in terms of the dissolved mass flux in the profile, although not in terms of concentrations. Reductions observed in Jc and MD were used to better target future remedial efforts. Integration of the observations from the PFM deployments and the source mass depletion provided a basis for establishing flux-based management criteria for the site.