DELINEATION OF FAST FLOW PATHS IN POROUS MEDIA USING NOBLE GAS TRACERS

摘要

In some areas of the arid western USA, water purveyors use artificial recharge to augment natural recharge to groundwater basins where water demand is high. In coastal regions of California, USA, artificial recharge replenishes basins that were subject to overdraft, mitigates seawater intrusion, and provides benefits to water quality. Artificial recharge facilities usually consist of abandoned quarries or injection wells, with high capacity, public water supply wells located down-gradient. Source water for artificial recharge may be water captured from the watershed, imported water, or recycled wastewa-ter. For the latter case, the proposed regulations for the potability of water of wastewater origin stipulate a minimum subsurface residence time of one year and dilution of ≥50% with a non-wastewater source. In order to demonstrate compliance to the proposed regulations, and to improve the understanding of groundwater flow in the affected basins, a tracer may be added to the artificial recharge source water. Movement of the tracer is observed at down-gradient points in the groundwater basin. In practice, the period of observation is on the time scale of several months to a couple of years; shorter than for natural (or bomb pulse) isotopic tracers such as tritium, 14Carbon, or stable isotopes of oxygen, but longer than the time scale for pump tests. The data acquired from such a tracer experiment can be used to calibrate groundwater flow models and to identify fast flow paths.