Developing Effective Conceptual Site Models for Contaminated Bedrock Aquifers

Evaluating the nature and extent of groundwater contamination within a bedrock aquifer is complicated and expensive. Use of a conceptual site model (CSM) during preparation of a work plan ensures that data gaps are minimized and investigation objectives are met. The problem may be limited to the source area, or it may be much larger. CSMs are scale dependent, so understanding the potential breadth of the problem is important. Identifying the site-related flow field, both laterally and vertically helps frame the potential extent of the problem and determines the investigation approach. Groundwater flow in bedrock is controlled by hydraulic gradient and, where bedrock has limited primary porosity, the interconnected network of fractures. Published data are usually readily available for an area to begin to formulate a preliminary CSM before any borings have been installed. Understanding the contaminant chemistry, contaminant discharge mechanisms and duration of discharge, type, orientation and frequency of fractures, location of faults, location of potential groundwater discharge areas, vertical gradients, and hydraulic stresses within the flow field will enable development of a defensible CSM that can be used to develop a cost-effective bedrock investigation and later be used as a springboard for effective remedial designs.