Analysis of Low-Impact Development groundwater recharge in the East Bay Plain using Stream Isotope Data Across Elevation Gradients

Low Impact Development (LID) structures (e.g. bioretention cells, bioswales, permeable pavement, etc.) are increasingly incorporated in urban areas to reduce peak runoff, decrease runoff and/or improve water quality during storm events. LID structures increase water infiltration likely without a corresponding increase in evapotranspiration increasing the potential for groundwater recharge. As classified under the 2014 California Sustainable Groundwater Management Act (SGMA), the East Bay (EB) Plain, located east of San Francisco, CA, is a medium priority basin. The Mediterranean climate of the EB Plain brings a wet season and dry season, allowing punctuated recharge events to dominate the alluvial basin. As part of SGMA policies, our research goal is to quantify groundwater movement, groundwater recharge, and groundwater-surface water interaction in the EB Plain. Radon and stable isotopes of tributary surface water samples were analyzed to identify groundwater influx to the creeks, and to conduct hydrograph separation during runoff events. We compared two creek sites across the northern and southern part of the plain, which differ in their depth to bedrock. We examine water quality at LID structures near the stream to interpret efficiency of recharge. Additionally, we use historical data to analyze pre-colonization conditions and patterns to aid in understanding future behavior of water in the EB Plain and other basins. Results show increasing groundwater discharge to the stream with distance down the elevation gradient. A thorough understanding and quantification of gaining and losing conditions across the EB Plain basin will provide key data for water management decisions under SGMA.