Abstract
Subsurface residence time of groundwater is an important criterion for understanding and monitoring water quality and in situ biogeochemical reactions. In California, travel time is also used to regulate sites that artificially recharge reclaimed wastewater. Hydrologic calculations, transient tracers, and deliberate tracer experiments are established methods of determining groundwater travel times. A two-year long deliberate tracer experiment using sulfur hexafluoride was conducted at the Montebello Forebay (LA County, CA) artificial recharge site to determine travel time to wells within 150 m of spreading ponds. The tracer was detected at seven monitoring wells, all screened within 50 m of the ground surface and at nine of the eighteen production wells. Travel time was best correlated with screen depth. At four of the nine wells with sulfur hexafluoride detections, hydrologic travel times were less than 0.3 years (16 weeks) and are in basic agreement with the sulfur hexafluoride results. However, at the other five wells, the hydrologic travel times were estimated to be more than 4 years, significantly longer than indicated by the tracer data. Transport through low permeability layers due to gaps or leakage must be occurring. At all wells where sulfur hexafluoride was not detected, the hydrologic travel times were greater than 3 years. At the production wells, tritium/He-3 apparent ages were greater than 7 years, indicative of mixing between young (<2 yr) and old (> 10 yr) ground water caused by the long well screens.