QUANTIFYING BEDROCK RECHARGE IN TWO PAIRED MOUNTAINOUS WATERSHEDS

This study focuses on 1) estimating the magnitude of bedrock recharge and 2) mean transit time as a function of bedrock permeability in two paired catchments underlain by different lithology. Networks of bedrock wells and stream gauges were installed in two adjacent catchments of approximately the same size in west central Montana. Fracture network properties for both lithologies were characterized from borehole core logging and scan lines on exposed outcrop. Recharge was evaluated using the Water Table Fluctuation (WTF) method on bedrock well hydrographs installed in both catchments. Stable water isotope samples, collected approximately monthly, from precipitation and stream water were used to evaluate stream water mean age in each catchment from isotopic signal damping. Fracture intensity in CW is 12.7 m while NFEC granite has an intensity of 1.53. Fracture spacing (characteristic distance between fractures) in CW is 0.088, while in NFEC it is 0.65. The specific yield in CW and NFEC is 0.11 and 0.007, respectively. Average annual bedrock recharge in the watershed with more permeable bedrock is 665 mm and 253 mm for the upper and lower elevation wells, respectively, while in the catchment with less permeable bedrock, annual average bedrock recharge is 26.3 mm and 11.0 mm for the upper and lower wells. The proportion of precipitation that becomes bedrock recharge ranges from 50% - 140% in the more permeable bedrock, while in the less permeable bedrock that percentage was \u3c 3.5%. The mean transit time in the more permeable bedrock is 4.74 years while the mean transit time in the less permeable bedrock is 2.10 years. These results indicate that bedrock permeability, determined by fracture network characteristics such as mean fracture spacing, imparts a strong influence on subsurface partitioning and transmission of water within mountain blocks in locations with similar climate, soil and ecology