Field Investigation of Stream-Aquifer Interactions: A Case Study in Coastal California

This study is a detailed investigation of the dynamic interaction between a stream and an alluvial aquifer at Swanton Pacific Ranch in the Scotts Creek watershed in Santa Cruz County, California. The aquifer is an important source of groundwater for cropland irrigation and for aquatic ecosystem support. The potential for groundwater pumping to deplete Scotts Creek stream flows is a source of serious concern for land managers, fisheries biologists, and regulatory agencies due to the presence of federally protected steelhead trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch). An understanding of the interaction between the stream and pumped aquifer will allow for assessment of the impacts of groundwater extraction on stream flows and is essential to establishing minimum instream flow requirements. This will aid in the development of sustainable groundwater pumping practices that meet agricultural and ecological needs. Based on findings from studies conducted in similar hydrogeologic settings and Scotts Creek stream survey reports filed by the California Department of Fish and Wildlife, we hypothesized that the stream is directly connected to the aquifer and that abstraction of groundwater from agricultural wells has a measurable impact on Scotts Creek stream flows, particularly during the summer low-flow period. We tested this hypothesis by employing a multi-scale approach combining multiple measurement techniques. Results of field investigations, including extensive direct push subsurface sampling, constant rate pumping tests, dye tracer tests, electrical resistivity interrogation of the subsurface, and long-term passive monitoring of aquifer hydraulic heads and stream stage, are reported. Additionally, results of laboratory falling-head permeameter tests and particle size analyses of aquifer sediments, and numerical groundwater flow modeling (MODFLOW) are presented. Findings indicate that the permeable subsurface formation tapped by irrigation wells is a leaky semi-confined aquifer, overlain by a thin and laterally discontinuous very low-permeability aquitard of silt and clay above which lies Scotts Creek. The aquitard reduces the hydraulic connection between the stream and pumped aquifer resulting in a low and steady streamflow depletion rate over the short term. These results are particularly useful to land managers responsible for sustainable groundwater abstraction from wells that tap into the aquifer. Calculations of stream depletion rate based on aquifer hydraulic parameters and well pumping rates are included to allow land managers to conveniently modify groundwater abstraction practices, minimizing concerns of streamflow depletion. The conclusions presented herein are based on the results of a two year study. Stresses imposed by changes in climate and