College of Agricultural, Food, and Environmental Sciences
Natural Resources Management and Environmental Sciences Department
BS in Environmental Earth & Soil Sciences
The United States Geological Survey is developing an integrated hydrologic model of the San Antonio Creek Groundwater Basin to better understand and simulate the integrated surface water and groundwater system. An abrupt 60 meter offset in groundwater depth over a distance of less than one kilometer is observed in well readings within the Cañada De Las Flores region of the groundwater basin. Abrupt changes in groundwater levels are often explained by the presence of a fault in the subsurface vertically offsetting sedimentary units. However, observations of the structural geology of this region indicates that faulting is unlikely and suggests an alternative hypothesis: subsurface folds of the sedimentary units may provide distinct groundwater pathways to separated groundwater sub-basins. To test the two hypotheses Electrical Resistivity Tomography profiles were collected to image the subsurface and constrain the geologic structure responsible for the offset in groundwater levels. The subsurface imaging results do not support the fault model, but instead image a layer interpreted as impermeable clay that may be extensive throughout the folded structure of the region. The folded clay structure provides a plausible geologic model for diverting the subsurface flow of water producing the abrupt offset in groundwater well levels. The correct identification of the geologic structure in Cañada De Las Flores is important because each model significantly changes the characteristics and predictions of a groundwater model with respect to predicted subsurface flow, groundwater distribution, and recharge.