EXPLORING SURFACE WATER-GROUNDWATER INTERACTION IN URBAN WATERSHEDS USING GEOCHEMICAL AND ISOTOPIC TRACERS

One of the ‘undesirable results’ that must be avoided under the Sustainable Groundwater Management Act, being implemented in California, is depletion of connected surface water due to groundwater development. In particular, groundwater dependent ecosystems (GDEs) should be identified and protected under the Act. Delineating stream reaches that are connected to shallow aquifers and identifying GDEs can be challenging in urban watersheds where stream courses are modified and infrastructure intended for flood control alters the connections between surface water and groundwater. However, several streams in the urbanized East San Francisco Bay area of California host GDEs and provide recharge to shallow aquifers. Most of the streams are not gaged, and little data exist to assess how changes to land use (including planned green infrastructure) and pumping might affect surface water-groundwater interaction.

We are using dissolved radon gas, stable isotopes of water, and other tracers to study stream dynamics in several stream reaches in the East Bay, where perennial and intermittent streams are sustained by groundwater inflow in summer and fall. Radon is an effective marker of locations of groundwater influx, and stable isotopes allow hydrograph separation during storm events. Stable isotopes and dissolved ions are useful for quantification of an imported water component in these watersheds, where water from distant Sierra Nevada watersheds has been the main water supply for several decades. Stable isotopes also provide a distinct tracer of evaporated water released from reservoirs. Mixtures of locally-derived water and imported water sustain perennial streams and GDEs, even where runoff ratios are high due to the predominance of impervious land cover. Geochemical and isotopic data reveal key characteristics like water source(s) and locations of groundwater discharge to streams where physical data from shallow wells and stream gages are lacking.