The influence of waterflood design on the recovery of mobile DNAPLs

This study examines the effectiveness of various waterflooding strategies to recover pooled dense nonaqueous phase liquid (DNAPL) from the subsurface at an industrial facility. The relative influence of horizontal injection/recovery well configuration, established hydraulic gradient, and fluid properties is investigated for a site characterized by a homogeneous silty sand underlain by an impermeable clay layer. The top of the clay layer is located 5 m below the water table and supports a laterally extensive 2 m deep DNAPL pool. The sensitivity study employs a two-phase flow numerical model that simulates both DNAPL infiltration and redistribution, including the formation of immobilized DNAPL residual. This is accomplished with constitutive relations featuring hysteretic capillary pressure-saturation pathways in which the local amount of residual formed is a function of the maximum nonwetting saturation attained during infiltration. Sixteen simulations, performed in two-dimensional vertical cross-section, demonstrate that strategies effecting increased wetting phase gradients, namely increasing drawdown at the recovery drain, adding injection wells, and reducing their distance to the recovery drain, result in an increased DNAPL volume recovered with time at the expense of increased volumes of ground water removed per unit volume of DNAPL recovered. Strategies which do not increase wetting phase gradients result in DNAPL recovery with a minimum volume of produced contaminated ground water. Three pulsed pumping simulations indicate that increasing the length of pump shut-down time decreases the recovery of DNAPL with time but increases efficiency with respect to ground water pumped. Decreased nonwetting density and increased interfacial tension result in poorer DNAPL recovery with respect to both time and volume of ground water removed, while reduced nonwetting viscosity corresponds to dramatically increased efficiency in both respects.