MULTISCALE FRACTURE ANALYSIS OF THE IDAHO BATHOLITH, DRY CREEK EXPERIMENTAL WATERSHED, BOISE, IDAHO

Infiltration of groundwater into impermeable bedrock requires fractures which act as conduits through which infiltration may occur. The intersection of fractures with the bedrock-soil interface is important to describing and quantifying infiltration to impermeable bedrock. Detailed field mapping and lineament analysis of the fracture network in the biotite-quartz monzonite Idaho Batholith has been undertaken to quantify the potential infiltration rate and estimate the recharge to aquifers in the Boise valley in such an impermeable bedrock environment. Previous geochemical and residence time studies have estimated that the contribution to recharge from the Idaho Batholith occurring to the north and northeast of Boise is important to the recharge of deep Boise valley aquifers however the quantity of recharge is unclear. The field mapping of joints has identified a mean strike direction of joints is N15W with a mean length of 1.26m and a mean aperture of 52mm. A mean strike direction of S80W has been calculated for the region within 100km of Dry Creek Experimental Watershed through spatial analysis of fault data sets available for the Idaho Batholith. Future results of this study will quantify the hydrogeologic parameters of permeability and porosity for the fractured rock mass. These parameters will improve estimates of the future sustainability of Boise groundwater supply.