Groundwater Flow Through a Fractured Rock Aquifer in the Sierra Nevada Foothills of California

The Sierra Nevada foothill areas along the east side of California's Central Valley have experienced a continuous increase in population and land development activities. This has in turn put pressure on the local groundwater supply which is strongly controlled by the complex fracture hydrology of the region. Therefore, understanding how the ground water flows is essential for land use planning and water supply management. Two different approaches were employed in this study. Long-duration pumping tests were conducted to characterize parameters including transmissivity and storativity of the fractured granitic aquifer. δ 18O and δ D isotope ratios were used to trace the history and evolution of the surface and ground waters in the watershed. Results were examined in relation to the distributions and orientations of the fracture systems as observed on the surface by outcrop mapping and lineament studies using satellite and aerial photographs. Because of limited well capacities, constant-head pumping test methods have been found to be more practical than constant-discharge or step-drawdown methods. The results of different pumping tests suggest that both the flow patterns (radial or linear) and the aquifer parameters (transmissivity and storativity) are scale-dependent, and the scale effect is related to the anisotropy controlled by the fracture orientation and connectivity independent of the test methods. To characterize a large area of the fractured aquifer, a pumping test of at least 15 days is required to get a realistic trend line of drawdown versus time. Isotope data collected from a small watershed adjacent to the pumping test area show that the δ 18O and δ D isotope ratios are correlated significantly with the fracture distribution and orientation. Enrichment anomalies of isotopic ratios indicate that the fracture system imposes a strong control on the groundwater flow. Combining the use of pumping tests and isotopic survey can help assess the sustainability of groundwater supply for various locations in the fractured terrain.