Overview

The purpose of the project was to demonstrate the value of age-dating and isotopic tracers in characterizing the flow dynamics and water quality changes in a complex groundwater domain that includes high capacity municipal pumping wells, a geologic fault, and artificial recharge facilities with deep lake-like recharge ponds. Characterizing water quality changes during recharge and transport in groundwater was also an objective of this investigation. Below (west of) the Hayward Fault (BHF), water ages correlated well with aquifer layer sequence. BHF tracers did not reach the BHF wellfield within the time frame of the experiment. Above (east of) the fault, (AHF) tracers reached the targeted wellfield in only 60 days, indicating substantial heterogeneity and a fast travel time along preferential pathways compared to the average travel time of 2+ years indicated by age-dating and more classical estimating methods. A reconnaissance of water quality, conducted concurrently with the tracer studies, suggested certain water quality improvements occurring in either the pond sediment or the near-pond aquifer media. Variations in groundwater age depended on location and depth. A survey of natural isotopes indicated mixing of young and older water in wells, increasing age with depth of aquifer layer, and possible dissolution of carbonate minerals. AHF tracer experiments, along with other analysis, suggested that tracers probably percolated preferentially at shallow depths in the pond near the shoreline. Much of the tracer remained in deep pond water over time, increasing residence time in down-gradient wells. The tracer studies provided evidence of preferential pathways and heterogeneity in the AHF aquifer and a fast minimum travel time to the AHF wellfield. The tracer added to BHF ponds was detected in just two monitoring wells, but not at the BHF wellfield over the 10-month period. An AHF tracer from a small pond spanning the fault did, however, appear at the BHF wellfield. BHF pond water flows mainly to distal portions of the groundwater basin, or reaches the wellfield over a slow, circuitous route. Originally published by AwwaRF for its subscribers in 2003

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