EXCESS AIR DURING AQUIFER STORAGE AND RECOVERY IN AN ARID BASIN

Aquifer storage and recovery (ASR) has become an important component of water supply systems in arid and semi-arid environments. The Las Vegas Valley Water District (LVVWD) has operated an ASR program since 1989 in which water is recovered from wells in the summer (generally June through September) and artificially recharged (AR) through wells in the remaining months. In summer 2001, observations of gas exsolving from tap water prompted a more extensive examination that indicated excess gas in a number of wells. Subsequent investigations revealed total dissolved gas pressures (TDGP) approaching 2 atm. Analysis of this gas has shown that it is predominantly air (i.e. N2, O2, and Ar) ruling out in-situ generation processes. Noble gas measurements show that excess neon (delta Ne) is highly correlated with TDGP making probe-based measurements of TDGP a cost-effective alternative to specific gas analyses. Measurements of TDGP at well heads and in the distribution system indicates two potential sources for excess air; (1) air entrainment during AR operations, and (2) temperature changes between the winter recharge season and the summer withdrawal season. Air entrainment during pumping was investigated by intentionally allowing the forebay of a large pumping station to drawdown to the point of vortex formation. This resulted in up to a 0.7 atm increase in TDG pressure. The solubility of gases in water decreases as the temperature increases. Water that acquires a modest amount of dissolved gas during artificial recharge operations will experience an increase in dissolved gas pressure as the water warms. Water temperatures during the artificial recharge season range from about 11 to 15 ˚C whereas the in-situ groundwater temperature ranges from about 24 to 27 ˚C. Solubility calculations indicate that the change in TDGP with temperature can be as much as 0.04 atm/˚C. Thus, a combination of air entrainment during AR operations that is amplified by temperature increases after recharge can account for the observed amounts of excess gas and point to the importance of controlling air entrainment during AR operations.