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 (3He) and measurements of dissolved noble gases was performed at the El Rio spreading grounds to examine gas transport and trapped air below an artificial recharge pond with a very high recharge rate (approximately 4 m day(-1)). Noble gas concentrations in the groundwater were greater than in surface water due to excess air formation showing that trapped air exists below the pond. Breakthrough curves of SF6 and 3He at two nearby production wells were very similar and suggest that nonequilibrium gas transfer was occurring between the percolating water and the 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 approximately 24 days. Despite the potential dilution caused by mixing within the production well, the maximum concentration was approximately 25% of the mean pond concentration. More than 50% of the SF6 recharged was recovered by the production wells during the 18 month long experiment. Our results demonstrate 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.
Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA. jfclark@geol.ucsb.edu
Jordan F
JF
Clark
G Bryant
GB
Hudson
Dror
D
Avisar
D004784:Environmental Monitoring / Q000379:methods; D006371:Helium / Q000032:analysis*; D007553:Isotope Labeling; D005741:Noble Gases / Q000032:analysis*; D013459:Sulfur Hexafluoride / Q000032:analysis*; D013997:Time Factors; D014872:Water Movements; D014881:Water Supply
D005741:Noble Gases; D006371:Helium; D013459:Sulfur Hexafluoride