Characteristics of light nonaqueous phase liquid recovery in the presence of fine-scale soil layering

Abstract

The effects of fine-scale layering of soil properties on light nonaqueous phase liquid (LNAPL) recovery using pumped groundwater drawdown were investigated using a Monte Carlo approach. Three-dimensional simulations were performed to study the characteristics of a variety of measures describing LNAPL recovery in heterogeneous sandy aquifers. Heterogeneity models included both uncorrected and correlated vertical permeability distributions, with both permeability-scaled and unsealed retention characteristics. Lognormal permeability variances of 0.594 and 1.188 were considered. The Monte Carlo ensemble mean LNAPL recovery rates and cumulative recovery volumes differ significantly from homogeneous soil models. The ensemble variances are large, and confidence intervals on recovery measures are wide. Generally, lognormality is not preserved; recovery rates and volumes display asymmetric densities with standard deviations of the order of the means. Overall recovery efficiency depends critically on the layer permeability at specific elevations. Practical outcomes for field practitioners are discussed including the degree to which substantially increased times may be required for LNAPL recovery and the increased occurrence of situations where larger than expected volumes will remain unrecovered. Guidance is also given for the key measurements of layer permeabilities and their expected influence on recovery performance. It is concluded that moderate vertical layering of soil properties is sufficient to render homogeneous recovery models inappropriate. Copyright 2009 by the American Geophysical Union.