USE OF A DISSOLVED GAS TRACER TO EVALUATE PERMEABILITY REDUCTION CAUSED BY TRAPPED GAS BENEATH AN ARTIFICIAL RECHARGE POND

Dissolved helium and bromide tracers were used to evaluate permeability reduction caused by trapped gas beneath a 60-m-diameter infiltration pond during a 260-day artificial recharge experiment at Sand Hollow Basin in southern Utah. Dissolved helium preferentially partitioned to a gas phase when it encountered either trapped gas bubbles or unsaturated conditions because of its low solubility in water. This produced a net retardation of helium relative to the bromide up to a factor of 12. Simulations of helium breakthrough using both (1) the equilibrium advection/dispersion/retardation equation and (2) the non-equilibrium advection/dispersion/retardation equation with a kinetic mass transfer coefficient between the trapped gas and water were not able to closely approximate observed helium concentrations. However, breakthrough curves were reasonably simulated by including a decay term to represent the diffusive loss of helium through interconnected gas-filled pores. Because saturated conditions were measured with piezometers and tensiometers to a depth of at least 2 m, the observed mass loss of helium indicates regions of both connected gas and fluid phases in the same porous media. Simulations using both a first-order decay term and a kinetic mass transfer coefficient indicate that about 7 to 13 percent of the pore space in the first few meters beneath the pond contained trapped gas. Analysis of laboratory hydraulic properties of core samples indicates that a 10 percent gas-filled porosity would reduce hydraulic conductivity by at least one order of magnitude in the well-sorted Navajo Sandstone, but less in the overlying soils. This is consistent with in situ hydraulic-gradient measurements made during the experiment, which show steeper hydraulic gradients between the pond and the sandstone than between the pond and the shallow soils. Managers of artificial recharge basins may want to consider minimizing the amount of trapped gas in order to optimize infiltration rates, particularly in well-sorted porous media where trapped gas may cause substantial reductions in permeability. One alternative to the conventional drying and scraping techniques often used to break up biofilms and silts that accumulate along the bottoms of recharge basins is wet tilling.