TRAPPED GASES BENEATH A RECENTLY COMPLETED RESERVOIR: USING ARTIFICIAL RECHARGE AS AN ANALOGUE TO NATURAL PROCESSES

A time series of total dissolved gas pressures (TDGP) and noble gas concentrations have been measured in the Navajo Sandstone beneath the recently completed Sand Hollow Reservoir in Southwestern Utah. As the reservoir filled, a systematic increase in TDGP and excess Ne were observed as the water table rose more than 20 m. TDGP rose from about 0.9 atm (similar to the barometric pressure at the site) before the reservoir was filled to more than 3 atm afterwards. Noble gas measurements indicate excess air values near zero before reservoir filling to more than 0.020 cc/g afterwards and dissolved oxygen increased from about 3 mg/L (40% of saturation) to over 25 mg/L (260% of saturation.) The dissolved gas content of the reservoir water itself remained near equilibrium solubility with the atmosphere.

The increases in TDGP and excess air were coincident with changes in water chemistry suggesting that near surface fluids were transported to wells during the monitoring period. We attribute the rise in dissolved gas content to trapped air even though the Navajo Sandstone consists of well-sorted eolian sand that is relatively homogeneous. The occurrence of trapped gas is consistent with a previously conducted tracer test in the saturated zone beneath an infiltration basin at Sand Hollow in which dissolved He was highly attenuated relative to a non-volatile tracer (Br) due to partitioning into trapped air. The volume of trapped gas estimated from the tracer test and relative permeability curves suggest that prior to dissolution the trapped air could have resulted in an order of magnitude decrease in permeability. A comparison of noble gas recharge temperatures with reservoir temperatures along with a comparison of models that describe the formation of excess air is currently underway. The data from this study provide direct evidence that excess air in groundwater can result from rising water tables and indicates the potential for using excess air to evaluate paleo water-table rises.