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

The effects of fine-scale layering of soil properties on light nonaqueous phase liquid(LNAPL) recovery using pumped groundwater drawdown were investigated using aMonte Carlo approach. Three-dimensional simulations were performed to study thecharacteristics of a variety of measures describing LNAPL recovery in heterogeneoussandy aquifers. Heterogeneity models included both uncorrelated and correlated verticalpermeability distributions, with both permeability-scaled and unscaled retentioncharacteristics. Lognormal permeability variances of 0.594 and 1.188 were considered.The Monte Carlo ensemble mean LNAPL recovery rates and cumulative recoveryvolumes differ significantly from homogeneous soil models. The ensemble variances arelarge, and confidence intervals on recovery measures are wide. Generally, lognormality isnot preserved; recovery rates and volumes display asymmetric densities with standarddeviations of the order of the means. Overall recovery efficiency depends critically on thelayer permeability at specific elevations. Practical outcomes for field practitioners arediscussed including the degree to which substantially increased times may be required forLNAPL recovery and the increased occurrence of situations where larger than expectedvolumes will remain unrecovered. Guidance is also given for the key measurements oflayer permeabilities and their expected influence on recovery performance. It is concludedthat moderate vertical layering of soil properties is sufficient to render homogeneousrecovery models inappropriate