A controlled field experiment on groundwater contamination by a multicomponent DNAPL: creation of the emplaced-source and overview of dissolved plumedevelopment

A unique field experiment has been undertaken at the CFB Borden research si te to investigate the development of dissolved chlorinated solvent plumes f rom a residual dense non-aqueous phase liquid (DNAPL) source. The "emplaced -source" tracer test methodology involved a controlled emplacement of a blo ck-shaped source of sand containing chlorinated solvents below the water ta ble. The gradual dissolution of this residual DNAPL solvent source under na tural aquifer conditions caused dissolved solvent plumes of trichloromethan e (TCM), trichloroethene (TCE) and perchloroethene (PCE) to continuously de velop down gradient. Source dissolution and 3-D plume development were succ essfully monitored via 173 multilevel samplers over a 475-day tracer test p eriod prior to site remediation research being initiated. Detailed groundwa ter level and hydraulic conductivity data were collected. Development of pl umes with concentrations spanning 1-700,000 mug/l is described and key proc esses controlling their migration identified. Plumes were observed to be na rrow due to the weakness of transverse dispersion processes and long due to advection and significant longitudinal dispersion, very limited sorptive r etardation and negligible, if any, attenuation due to biodegradation or abi otic reaction. TCM was shown to be essentially conservative, TCE very nearl y conservative and PCE, consistent with its greater hydrophobicity, more re tarded yet having a greater mobility than observed in previous Borden field tests. The absence of biodegradation was ascribed to the prevailing aerobi c conditions and lack of any additional biodegradable carbon substrates. Th e transient groundwater flow regime caused significant transverse lateral p lume movement, plume asymmetry and was likely responsible for most of the, albeit limited, transverse horizontal plume spreading. In agreement with th e widespread incidence of extensive TCE and PCE plumes throughout the indus trialized world, the experiment indicates such solvent plumes are likely to be highly mobile and persistent, at least in aquifers that are aerobic and have low sorption potential (low f(oc) content). (C) 2001 Elsevier Science B.V. All rights reserved.