GEOPHYSICAL EXPLORATION OF A POTENTIAL HARD ROCK AQUIFER IN AN EXPOSURE OF SALINIAN BLOCK GRANITE, PASO ROBLES, CA

Location of groundwater resources is critical due to high water demand and the reliance of many communities on it as their water supply. On lands lacking alluvial aquifers, hard rock aquifers may provide an adequate water supply for local water needs. In the complex geologic settings of hard rock aquifer exploration, geophysical data can often improve well siting decisions.

Decomposed Salinian Block granite exposed west of Paso Robles, California, provides a novel case study of a non-sedimentary hard rock aquifer system. To our knowledge, this is the first geophysical study of Salinian Block granite as a hard rock aquifer resource in central California. The geology of the field site is complicated by its immediate proximity to the Rinconada Fault, which separates marine shale and sandstone units from the Salinian Block granite.

Two-dimensional Electrical Resistivity Tomography (ERT) is used to investigate depths to saprolitic, fractured, and competent granite. Resistivity varies from 3 to 357 Ω∙m, both laterally and with depth, indicating significant spatial variation in the degree of weathering and fracturing of the granite in the subsurface. The saprolite is interpreted to extend approximately 100-meters where a change from approximately 60 Ω∙m to greater than 300 Ω∙m is observed, suggesting a transition to fractured granite. The ERT also images two sub-parallel fault strands to the Rinconada Fault. This suggests that not only weathering, but also past tectonic processes influence the fracture network that could be water bearing, making this location a high-potential water resource for a granitic hard rock aquifer.

The total thickness of the fractured granite and depth to competent bedrock have not been observed in the 120-meters depth currently imaged in the ERT data. Further ERT and time-domain electromagnetic surveys are planned to determine competent bedrock depth and thus thickness of the potentially water bearing fractured granite. The results of this study will inform potential use of other exposed Salinian Block granite sites in central California as a potential local groundwater resource.