PREPARING A PRELIMINARY FRACTURED BEDROCK CONCEPTUAL MODEL TO ADDRESS WATER QUALITY CONCERNS USING EXISTING SITE AND REGIONAL SCALE INFORMATION: APPROACHES AND CASE STUDIES

Groundwater flow in crystalline bedrock in New England is complex. A preliminary conceptual model combining regional- and site-scale information may provide considerable understanding of the system and guide additional investigations. Fracture patterns form a heterogeneous aquifer resulting in groundwater flow that varies with location, depth, and rock type. Fracture transmissivity has a range of several orders of magnitude. Low transmissivity fractures can function as reservoirs retaining and slowly releasing legacy contaminants. The travel time is also important to consider as groundwater chemistry evolves over time affecting the fate and transport of natural and anthropogenic contaminants. For example, nitrate can become degraded over long paths and travel times as oxygen is consumed in rock-water reactions but relatively persistent where oxygen is available. We advocate a coupled examination of aqueous chemistry and traditional aquifer and geologic mapping data to help identify low- and high-transmissivity fracture zones.

Information from regional aquifer studies and geologic maps can help guide conceptual model development in areas without data and can constrain the boundaries of the hydrologic system. For example, overburden aquifer studies may have information describing the bedrock surface that can serve as criteria to help identify fracture zones. In addition, water table or potentiometric surfaces can be used to identify regional points of discharge or spatial patterns in pumping drawdown that may indicate where an underlying anisotropy has influenced flow directions.

Case studies documenting the identification of potential high-transmissivity zones in the aquifer (preferential flowpaths) as well as low-permeability zones that may result in lingering contamination through back diffusion will be presented. A workflow involving an iterative process of interpretations based on regional understanding, data processing, and integration will be outlined. The relation between geologic features, contaminants, and geophysical methods will also be highlighted. Rigorous retrospective data analysis as the first step in a preliminary assessment, can save time, costs, and serve to focus subsequent efforts that will minimize duplication and maximize understanding.