Saturated hydraulic conductivities of granitic materials of the Idaho batholith

Abstract

Saturated hydraulic conductivity ( K sat) of granitic bedrock in the Idaho batholith was determined using a borehole pressure testing technique. Tests were conducted at approximately 1.6 m depth increments ranging from about 1.6 m to an average maximum depth of 7.8 m. A total of 58 valid tests were obtained in nine holes located at five sites in a 145 km long transect line running north-south in the southwest quarter of the batholith. Sites represent a wide range in rock fracturing and weathering properties. Seismograph profiles were also run at each test hole. A conditional probability analysis showed that K sat values were lognormally distributed with a lower bound at zero. Values for K sat were unrelated to depth, rock matrix porosity, seismic velocity, or rock fracture density. However, K sat did vary with rock weathering characteristics. Conductivity was lowest in unweathered rock probably because of restricted fracture apertures. The second least weathered rock class had the highest average K sat. There was a general decrease in K sat with increased weathering through the remaining five weathering classes even though rock porosity and fracture density increased with rock weathering. We believe this inverse trend in K sat with increased weathering is the result of progressively increasing clay formation and mineral expansion that restricts flow in both fractures and the rock matrix. K sat for bedrock averages an order of magnitude less than K sat for soil cores and two orders of magnitude less than K sat obtained by tracer tests during subsurface flow making shallow, sub-surface flow a major hydrologic process at many locations on the steep, mountain slopes.