EXAMINING SURFACE WATER – GROUNDWATER INTERACTION IN CALIFORNIA COASTAL STREAMS USING TIME-SERIES ANALYSIS OF STREAMBED THERMAL RECORDS

Many coastal groundwater aquifers are stressed by increasing extractions and decreasing recharge. Coastal streams are particularly impacted during summer and fall, when baseflow discharge is most important. Seepage to ground water through streambeds comprises a major source of recharge in many basins, but also represents a loss of streamflow. Exchange of water between streams and aquifers also influences water chemistry. Determining time series of seepage rates (rather than a single value) allow us to better understand these dynamic processes.

Established methods for estimating seepage from streambed thermal data can be time consuming, are applicable to relatively short periods, and may require independent determination of hydraulic properties and sensor depths. We apply a new method for interpretation of streambed thermal data, using long-duration temperature records from multiple depths. We use analytical calculations to define relations between seepage and thermal response, and apply these relationships to thermal data to generate time-series records of streambed seepage. We then compare these estimates to rates derived from other methods including forward modeling, seepage meters and differential discharge gaging.

Time series of temperature were collected and analyzed from three streams that are heavily impacted by ground water pumping and agricultural development in Coastal California from 2002-05: Russian River, Pajaro River, and Corralitos Creek. Stream seepage (specific flux) estimates for the Russian River site using the time series thermal method (q=-0.27 m/day, down) correspond well to estimates from seepage meters (q=-0.31 m/day, down), slug tests (q=-0.40 m/day, down) and forward models (q=-0.35 m/day, down), (uncertainty ~ 0.1 to 0.5 m/day). Corralitos Creek and Pajaro River time series estimates also correspond well to estimates from differential discharge gaging.

Benefits of this approach include generation of long records of seepage rates and directions, low cost, use of a signal present in nature that can be easily and accurately measured, and insensitivity to sedimentation and scour. Particularly when applied in conjunction with other methods, the time series thermal approach provides useful indications of surface water - groundwater interaction at multiple scales.