Multiphase modeling of DNAPL seepage history as possible site investigation tool : Influence of geological material, groundwater flow and subsurface morphology on the pathways of DNAPLs
Modeling of the seepage history of DNAPLs is investigated as a new, non-invasive site investigation tool in order to elucidate the possible position of still unknown DNAPL source zone at many investigated industrial sites. Therefore, the spatio-temporal spreading behavior of the DNAPL TCE is studied with the multiphase modeling software TMVOC in small and large scale 2D multiphase scenarios with varying parameter sets concerning groundwater flow, composition of aquifers and aquitards and subsurface morphologies, as depressions and trenches. The small scale models were calibrated by laboratory experiments conducted at La Sapienza University, Rome. They exhibited that even groundwater pore velocities of vw = 0.05 m/d have a strong impact on the spreading behavior and the position of a DNAPL body. Downstream inclined percolation path ways, enhanced dissolution rates and lateral transportation in downstream direction are the most dominant impacts. Small scale layering of the subsoil with horizontal lenses of impermeable materials affects the distribution pattern only slightly at vw > 5 m/d, which are common flow velocities in many gravelly aquifers in Europe. Upscaling of the models to field scale problems exhibited potential transportation length in downstream direction of several hundreds of meters, assuming a moderate spill rate of ca. 3 kg/day over an area of several square meters. Investigating real subsurface morphology including real material parameters provided by the ModelPROBE reference site Chimica di Bianchi in Rho, Italy, revealed that the DNAPL TCE will be transported out of moderate depressions (slope of 2.5°) even at groundwater flow velocities of vw ≤ 1 m/d, which is in the range of documented groundwater flow velocities at the reference site. Moreover, the documented material classes, which comprise the aquitard at the site, are not in general impermeable for percolating DNAPLs. Only pure clays with a hydraulic conductivity of kf ≤ 10-9 m/s are long-term barriers for vertical DNAPL percolation. The conducted investigations deliver a reasonable explanation for the often unknown position of DNAPL source zones at former industrial sites and are, as far as it is known, the first large scale scenarios of DNAPL spreading behavior in real subsurface morphology. Based on the conducted research it can be concluded that at the reference site Chimica di Bianchi the main mass of DNAPLs was not at the assumed hot spot, which was encapsulated in the 1980s, but probably migrated considerable distances in downstream direction, passing through or following partly the topography of the aquitard. But the applicability of multiphase modeling as additional non-invasive site investigation tool is still challenging due to software restriction concerning size and resolution of the models and handling of heterogeneous permeability fields.
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