Groundwater Travel Times near Spreading Ponds: Inferences from Geochemical and Physical Approaches

Groundwater travel time is an important criterion for regulating managed aquifer recharge (MAR) operations because of its relationship to water quality. Here, three complementary methods for determining travel times are examined. Sulfur hexafluoride $\left(\mathrm{S}{\mathrm{F}}_6\right)$ , a gas tracer, was injected into 23 spreading basins at the Montebello Forebay MAR operation (Los Angeles County, United States) and monitored at ten monitoring and 18 production wells within $150\mathrm{m}$ . Over $2\text{years}$ , $\mathrm{S}{\mathrm{F}}_6$ was detected at nine monitoring and 11 production wells. Travel times showed a significant relationship with depth, but not with horizontal distance or pumping rate. A pumping influence was apparent as the tracer arrived sooner at production wells then at monitoring wells of similar depth. In the unconfined aquifer, estimated hydrogeologic travel times were $<0.2\text{years}(<10\text{weeks})$ and agree with the $\mathrm{S}{\mathrm{F}}_6$ data. However, in the confined aquifers, estimated travel times were $>4\text{years}$ and the agreement with the $\mathrm{S}{\mathrm{F}}_6$ travel times was poor. At the seven production wells with $\mathrm{S}{\mathrm{F}}_6$ detections, leakage through low permeability layers leading to earlier tracer arrival provides a likely explanation. All tritium/ ${}^3\mathrm{He}$ ages at production wells are greater than $10\text{years}$ ; this data combined with the $\mathrm{S}{\mathrm{F}}_6$ results indicate the wells produce a mixture of young and old groundwater.