Deep Groundwater Signatures in a Shallow Aquifer System, Sonoma Valley, California

A shallow aquifer system near the city of El Verano in Sonoma Valley, California, supplies urban and rural water users. In this study, nine wells ranging in depth from 82 to 674 feet were sampled for tritium and 3-helium to estimate groundwater age. Seven wells 90 to 312 feet deep yielded a non-zero fraction of modern (< 50 year old) groundwater. However, the modern fraction ranged from only 2 to 7% for five of these seven wells, with apparent ages of 24-29 years. For two relatively shallow wells (~100 feet deep) closer to a stream, modern fractions increased to 22-25%, and apparent age decreased to 16-18 years, indicating closer proximity to a recharge sources.

Stable isotopes of the water molecule in local precipitation, streams, and groundwater fall within a narrow range, indicating locally derived water as the source of recharge. In deeper wells, colder noble gas recharge temperatures suggest a component of recharge dating back thousands of years. Radiogenic 4-helium concentrations up to 4.0e-6 cc(STP)/g indicate contribution from deep groundwater sources in both deep and shallow wells.

These groundwater age and isotopic data indicate groundwater supplies in the El Verano area mostly originate from pre-development recharge and deep sources. Even the shallowest wells are remotely connected to local recharge processes. Water level data show declining trends with depth. Simulation of gas-liquid phase flow and transport of water, air, tritium, and 3-helium components is used to reconcile age data with groundwater level trends. Hydrostratigraphy of thin aquifers and thick aquitards having depth-decreasing permeability yields simulation results consistent with the data. In terms of groundwater age and isotopic signature, this shallow aquifer system behaves like a relatively deep basin.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. DE-AC52-07NA27344.