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Séminaire R J. Lenhard - 12 juin

29 mai

Predicting subsurface LNAPL specific volumes and mobility from observational well

Séminaire R J. Lenhard - (physicien des sols, spécialiste dans les écoulements multiphasiques)

Mardi 12 juin à 14 h 00 - Amphithéâtre Nougaro

Abstract :

Contamination of groundwater by petroleum fuels (i.e., petrol, diesel, heating oil) can harm human health and the environment. To help reduce the potential harmful impacts, the petroleum fuels, which are light nonaqueous phase liquids (LNAPL), are pumped from wells to the surface for disposal or reuse. This is commonly referred to as petroleum fuel or LNAPL recovery. Initially, LNAPL recovery from wells is likely to be effective. With time, the rate of LNAPL recovery will decrease and may become impractical, reaching an endpoint for the technology, even though there may be some LNAPL remaining in wells. The design of cost-effective LNAPLl recovery activities depends on assessing the subsurface LNAPL volume and the LNAPL migration rate toward recovery wells. Additionally, assessing the immobile LNAPL volume entrapped by water and the LNAPL volume that is relatively immobile above the water table (i.e., residual) are important for evaluating risk assessment. To help bring science to practice for petroleum fuel remediation, a model was developed to account for elevation-dependent free, residual, and entrapped LNAPL above and below the water-saturated zone when predicting subsurface LNAPL specific volume (fluid volume per unit area) and transmissivity from current and historic fluid levels in wells. Physically-based free, residual, and entrapped LNAPL saturation distributions and LNAPL relative permeabilities are integrated over a vertical slice of the subsurface to yield the LNAPL specific volumes and transmissivity, respectively. The model accounts for effects of fluctuating water tables. The presentation includes a theoretical basis for the model and hypothetical predictions for different porous media, fluid levels in wells, and historic water-table fluctuations. It is shown the elevation range from the LNAPL-water interface in a well to the upper elevation in the subsurface where the free LNAPL saturation approaches zero is the same for a given LNAPL thickness in a well regardless of porous media type. Further, the LNAPL transmissivity is largely dependent on current fluid levels in wells and not historic levels. Results from the model can aid developing successful LNAPL remediation strategies and improving the design and operation of remedial activities. Results of the model also can aid in accessing the LNAPL recovery technology endpoint, based on the predicted transmissivity.