A SEMIANALYTICAL MODEL TO PREDICT RECOVERY OF LIGHT, NONAQUEOUS PHASELIQUIDS FROM UNCONFINED AQUIFERS

Contamination of soil and ground water due to spills of light nonaqueo us phase liquids (LNAPL or simply oil) is a widespread environmental p roblem. This paper describes the development and testing of a semianal ytical model that may be used to design LNAPL containment and recovery systems at spill sites. The objective of this study was to derive an enhanced semianalytical algorithm for calculating recovery and trappin g of free phase oil. The enhancements were derived and evaluated by co mparison with an established numerical model that describes transient flow of oil and water. The semianalytical model employs an analytical solution for steady-state drawdown in an unconfined aquifer due to wat er pumping. When pumping rates are sufficient to contain the separate phase plume, the model calculates recoverable and residual oil volumes based on the initial free oil distribution. Refinements were implemen ted to calculate the water-table drawdown and the maximum unsaturated zone residual saturation (So,) as functions of soil type. Also the inf luence of hysteresis on the oil-water capillary fringe was incorporate d into the calculation of oil trapping below a rising oil-water interf ace. A method was derived to reduce saturated zone trapping to account for oil recovery that occurs while pumping proceeds. The above enhanc ements yielded close agreement between the semianalytical model and th e transient model predictions of recoverable oil and residual oil in t he unsaturated and saturated zones. The models were compared for hypot hetical gasoline spills in a sandy and a silt loam soil, using a range of pumping rates and regional water-table fluctuations. Field data fr om a pipeline leak were evaluated by the semianalytical model for hypo thetical scenarios involving oil recovery from three wells and a falli ng regional water table. Results suggest that the semianalytical model captures many of the trends of transient oil recovery. Recovery is le ss accurately predicted when irregular water-table fluctuations occur.