Nature of Flow and Gas Dynamics Below Spreading Ponds
The El Rio Spreading Grounds, an area of artificial recharge, lies within a regional groundwater depression. It consists of ten small ponds surrounded by eight production wells. A deliberate tracer experiment using sulfur hexafluoride (SF6) and an isotope of helium (3He) was conducted to determine groundwater flow. The tracer was continuously injected into one pond for one week prior to a ~50 day period of no recharge. At the time of the experiment, the artificial recharge rate through the highly permeable alluvial sediments which underlie the ponds was ~4 m day-1 and the water table was ~12 m below the ground surface. Tracer breakthrough curves from the production wells indicate that the plume remained near (within 500 m) the injection site during the 18 month experiment. Its movement was influenced by a number of factors. First, variable groundwater production in the production wells cause frequent changes in the water level, and thus, rates and direction of the plume flow respond to local groundwater cones of depression. Second, additional recharge events, which were free of tracer, create groundwater mounds that block the transport of the tracer to specific production wells. 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. Breakthrough curves of SF6 and 3He at two nearby production wells (within ~10 m of the pond) were very similar and suggest that nonequilibrium gas transfer was occurring between the percolating water and trapped air. 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 ~33% of the mean pond concentration. More than 50% of the SF6 recharged was recovered by the production wells during this 18 month experiment. 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.