THE TRANSPORT OF A BTEX PLUME IN FRACTURED LIMESTONE

The numerical model FRAC3DVS was used to analyze groundwater flow and contaminant migration at a BTEX release site in eastern Ontario Canada. The local geology surrounding the site consists of orthogonally fractured Ordovician limestone bedrock underlying an intermittent layer of thin surficial soils. The water table is found between 10 to 15 m below ground level. FRAC3DVS, which simulates flow and contaminant migration in variably-saturated media, was chosen to model the site because of its effectiveness for modeling discretely-fractured media, as typified by the fractured limestone. The regional three-dimensional spatial domain was discretized using prismatic elements with faces at the scale of the BTEX plume coinciding with the orientation of the orthogonal fractures at the site. The top of the model corresponds to surface topography. The logs for regional water supply wells and site monitoring wells were used to define the overburden thickness and the bottom of the spatial domain which was set at the top of the Precambrian bedrock that underlies the limestone.

To model the site, three different characterizations of the limestone were investigated: one representing the fractured limestone as isotropic equivalent porous media, a second assuming anisotropic equivalent porous media with the principal directions of the hydraulic conductivity tensor oriented with the orthogonal fractures, and a third with anisotropic equivalent porous media for the rock matrix as well as discrete fractures. The fracture apertures were varied with direction and depth. The purpose of the variation of the parameters was to determine what effect, if any, the discrete fracturing and their representation in a model would have upon the groundwater flow and BTEX migration at the site. The model results for the case with discrete fractures provided the best fit to field data for both flow and benzene migration. The results clearly show that the fracture flow dominates the site. An alternate conceptual model with a reduced discrete fracture zone width was also investigated. The benzene migration is sensitive to the fracture permeability, dip, strike and areal extent. The EPM realizations provided unsatisfactory representations of the groundwater system and benzene migration.