INFLUENCE OF DECIMETER-SCALE VARIABILITY ON DNAPL INFILTRATION AND MIGRATION IN A GLACIAL SAND AQUIFER

Groundwater contamination poses a serious problem for communities that depend on it as a source for drinking water supplies, industry, or agriculture. At the Bachman Road site in Oscoda, Michigan, USA, tetrachloroethene (PCE) was released into the ground beneath a former dry cleaning business, causing contamination of the underlying aquifer. PCE is a Dense Non-Aqueous Phase Liquid (DNAPL) that persists in the ground for long periods of time. At the Bachman Road site, a PCE plume emanates from the affected aquifer and subsequently discharges into Lake Huron to the east. The sandy unconfined aquifer consists of glacial outwash sands with a thick, confining clay layer at its base. Previous studies conducted at the site modeled variability within the aquifer assuming uniform or completely random properties throughout. The goal of this project was to quantify stratification within the aquifer at a finer, decimeter-scale, and to determine if such variability could influence DNAPL infiltration and entrapment.

An uncontaminated, 7.3 m soil boring was taken from the affected aquifer, 50 m up-gradient from the contaminated area. The core was segregated into sixty-five 10 cm intervals, which were sieved individually. The variance of the natural log of hydraulic conductivity (σ² ln(K)), estimated using the Kozeny-Carmen equation, was 0.65, suggesting that the aquifer is relatively homogeneous. However, upon direct observation, the sample core showed evidence of finer scale stratification. To determine the significance of the observed stratification, a hypothetical PCE spill was modeled in the aquifer using UTCHEM-9.82, a multiphase flow simulator. Porosity, permeability, and capillary pressure-saturation parameters for the Bachman Road Site, were adopted from previous work. The aquifer was simulated with a two-dimensional 0.3 x 0.1 m grid. PCE was released for 30 days at the water table in an initially saturated unconfined aquifer. Results indicate that decimeter-scale variability can influence the infiltration and spreading of PCE DNAPL, even in relatively homogenous sand aquifers.