GAS TRANSPORT BELOW ARTIFICIAL RECHARGE PONDS: INSIGHTS FROM DISSOLVED NOBLE GASES AND A DUAL GAS (SF6 AND 3HE) TRACER EXPERIMENT

A dual gas tracer experiment using sulfur hexafluoride (SF6) and an isotope of helium (He-3) and measurements of dissolved noble gases were preformed at the El Rio Spreading Grounds to examine gas transport and trapped air below an artificial recharge pond. At the time of the experiment, the artificial recharge rate was ~4 m/day and the water table was ~12 m below the ground surface. Breakthrough curves of SF6 and He-3 at two nearby production wells were very similar. At one well screened between 50 and 90 m below ground, both tracers were detected after 5 days and reached a maximum at ~24 days. Despite potential dilution caused by mixing within the production well, the maximum concentration observed at this well was ~25% of the mean pond concentration. Noble gas concentrations in the groundwater were much higher than in the pond due to the dissolution of trapped air. However, significant retardation (absolute and relative) of the gas tracers was not detected suggesting that the amount of trapped air was small. The experiment demonstrates that at artificial recharge sites with high infiltration rates and moderately deep water tables, transport times between recharge locations and wells determined with gas tracer experiments are reliable.