LINEAMENTS AS RECHARGE BARRIERS IN A FRACTURED BEDROCK AQUIFER

Bedrock lineaments are typically considered zones of higher fracture density and thus potential recharge features. Detailed stable isotope and water level data from a fractured rock aquifer approximately 350 km northeast of Toronto, ON indicate that recharge is highly localized. However, predicting the location of recharge features remains elusive using traditional hydrogeological tools. The goal of this research is to evaluate the recharge potential of bedrock lineaments by integrating geomatic, geological and hydrogeological data and numerical modeling. Lineaments were identified using multispectral Landsat imagery and a digital elevation model. Rock type, fracture patterns, and water well records were compiled using a GIS for areas near lineaments. A lineament was drilled and hydraulically characterized to evaluate site-scale conditions. Finally, a synthetic lineament was generated from field data for 2-D numerical modeling of the coupled groundwater-surface water system using HydroGeoSphere. In the watershed under study, minimal unconsolidated sediments overlie low gradient, Paleozoic and Precambrian bedrock aquifers. Two principal lineament sets are consistently oriented 041 and 305 degrees, indicating they are significant textural and topographic features. All lineaments are linear hydrologic features: lakeshores, stream reaches or wetlands. Anomalously high gradients between these hydrologic features indicate low subsurface connectivity. Steep to vertical fracture sets, which are expected to control recharge, have mean strikes of 039 and 306 degrees. The specific capacity of water wells near lineaments is lower than the watershed average. The drilled lineament is a significant topographic break with an adjacent elevated wetland complex. The well does not have significant water-bearing fractures until over 30 m depth. Fracture data suggests lineaments are structurally controlled. However, the low specific capacity, lack of connectivity and water-bearing fractures, and numerical modeling results paradoxically indicate lineaments cannot be significant recharge features. Contrary to previous conceptual models of recharge in fractured rock aquifers, we suggest that in certain physiographic settings, lineaments are barriers to recharge.