Modeling of strategies for performance monitoring of groundwater contamination at sites underlain by fractured bedrock

A three dimensional flow and transport modeling using FRAC3DVS was undertaken to examine factors which influence plume detection in a performance monitoring network for a site where an unconfined aquifer composed of uniform unconsolidated sediments overlies fractured bedrock. The bedrock is assumed to contain a fracture system with three orthogonal fracture sets embedded in a low permeable homogeneous rock matrix. A dissolved phase, non-reactive contaminant is released from a source zone located at the ground surface. The processes which influence plume geometry, and probabilities of plume detection for a performance monitoring network located between the contaminant source and a downstream compliance boundary, are evaluated. Factors considered include the hydraulic conductivity of the unconfined aquifer, the geometric properties of the fracture network and the matrix permeability of the bedrock, and the contaminant detection threshold concentration. The simulations demonstrate that the character of the fracture network not only controls contaminant transport and plume detection in the bedrock but also influences plume detection in the overlying unconfined aquifer. The ratio of the hydraulic conductivity of the unconfined aquifer to the effective hydraulic conductivity of the fractured bedrock, and the contaminant detection threshold concentration, are principal factors influencing detection probability in the performance monitoring network. Results suggest that in many instances encountered in field practice, the unconfined aquifer and fractured bedrock should be viewed as an integrated hydrogeologic system from a monitoring perspective.