Streamtube project overview : longitudinal transect assessment of the SABRE site DNAPL source zone

Industrial use of DNAPL (dense non-aqueous phase liquid) chlorinated solvents began in the 1930s and hence groundwater contamination arising from DNAPL releases at some sites may date back decades.Widespread occurrence of plumes of solvents such as trichloroethene (TCE) is often attributed to their generation from DNAPL residing below the water table. Sources typically comprise a complex architecture of DNAPL pooled on low permeability strata with residual ganglia spread throughout the post-spill DNAPL migration pathway. Combined complexities of DNAPL architecture and groundwater flow may lead to heterogeneous dissolution with DNAPL contained in low permeability strata or pools taking decades to dissolve. DNAPL sources pose a significant remediation challenge internationally. The SABRE (Source Area BioRemediation) site located at a UK industrial site has afforded an opportunity to undertake high resolution assessment of a DNAPL source zone arising from industrial TCE releases some 19 - 45 years ago. In January 2007, the SABRE Project installed a 3-sided sheet-pile cell that enclosed 720 m3 of soil and subsoil. The cell was estimated to contain 0.3 - 2.9 (median 0.9) tonnes of DNAPL. The cell was 4 m wide and had the long 30-m axis oriented parallel to groundwater flow that was drawn towards an extraction well at the closed cell end (Fig. 1). The 6 m deep cell was keyed into a mudstone aquitard and enclosed made ground, alluvium and a 3 - 4 m thick river terrace gravel (RTG) aquifer of primary interest. This bulletin presents an overview of research conducted by the Streamtube Project (also CL:AIRE Research Project RP14) under the umbrella of the SABRE project that had a primary goal of bioremediation performance assessment (CL:AIRE SABRE Bulletin #1). Streamtube contributed to this goal through its specific aim to evaluate the use of detailed longitudinal monitoring transects in assessing DNAPL source zone architecture, dissolution and remediation at the field-scale. An array of multilevel samplers (MLSs) was installed within the SABRE cell to generate detailed 2-D longitudinal transect snapshots of dissolved concentrations along a flowline, or 'streamtube', through the DNAPL source zone (Fig. 1). Data obtained represent the most detailed field-scale longitudinal transect through a real site DNAPL source observed to date.