USING ISOTOPE HYDROLOGY, FRACTURE MAPPING, AND PUMP TESTS TO CHARACTERIZE GROUNDWATER FLOW THROUGH THE FRACTURED ROCK TERRANE OF THE SIERRA NEVADA FOOTHILLS

The hydrogeology of the foothills of the Sierra Nevada is mainly dominated by the complex geometry and distribution of fractures in the granitic rocks. Although the role of fracture parameters, such as orientations, density, and continuity, in controlling groundwater flow can be demonstrated based on conceptual and numerical models, the actual effects on groundwater supply availability in field or regional scale are seldom fully understood. The goal of our field studies is to characterize these effects by using long duration pump tests up to 34 days and stable isotope ratios in conjunction with fracture mapping and satellite/aerial photo interpretation.

Isotopic data of 121 surface and groundwater samples collected from a small watershed of 82 km² show that the δ¹⁸O and δD isotope ratios are correlated significantly with the fracture distribution and orientations obtained by outcrop mapping and satellite photos. Enrichment anomalies of isotopic ratios confirm that the fracture system imposes a strong control on the general direction and velocity of groundwater flow.

Pump tests were performed in an adjacent area with similar hydrogeological characteristics. Pump test results based on two test wells and 17 observation wells 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, but they are independent of the test methods used. These results also reveal that the radius of influence may reach up to a distance of 4,000 feet, dependent on the extent of linear fracture zones intersecting the pumping wells, and that a duration of at least 15 days is required to get a reasonable curve of drawdown versus time.

These preliminary studies demonstrate that stable isotope and pump test data used in conjunction with satellite photo and outcrop fracture mapping can be applied to characterize fracture systems and help assess the sustainability of groundwater supply for fractured rock terranes.