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Permeability Reduction Caused by Trapped Gas: Field-Scale Observations and Quantification With a Dissolved-Gas Tracer During a Ponded Infiltration Experiment

By Victor M. Heilweil1,and D. Kip Solomon2
1U.S. Geological Survey, 2329 W. Orton Circle, Salt Lake City, Utah 84119
2University of Utah, School of Mines

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Laboratory studies and field-scale artificial recharge experiments have demonstrated reduced permeability caused by trapped gases. Initially, infiltration rates drop due to air entrapment, causing decreases in vertical hydraulic conductivity. Longer-term permeability reduction caused by bacterially generated gases also has been observed, which has implications for both ponded and injection-well artificial recharge projects.

A ponded infiltration experiment was monitored during a 10-month period to evaluate potential artificial ground-water recharge beneath the Sand Hollow Reservoir near St. George, Utah. As part of this experiment, dissolved helium was introduced to the pond, along with dissolved bromide, to act as a partitioning tracer. Because of its low-solubility in water, helium preferentially partitions to a gas phase when it encounters trapped gas bubbles.  This produces a net retardation of helium relative to a non-partitioning (non-volatile) tracer such as bromide. Previously, such gas-phase partitioning tracers have only been used in laboratory-scale experiments. Because of the large degree of retardation, it was necessary to measure helium at very low concentrations (at C/Co down to 1E-5).  Fortunately, this is possible by using mass spectrometric methods that measure helium isotope ratios. Also, the biological source of the trapped gases was confirmed by high concentrations of dissolved carbon dioxide, methane, and hydrogen measured in water samples from shallow depths beneath the pond.

The helium tracer breakthrough occurred as much as 5 months after the bromide tracer (fig. 1). The retardation of the dissolved-helium tracer in comparison to the bromide tracer at various depths beneath the pond indicates that as much as 10 percent of the porosity of material beneath the pond was filled with gas. Theoretically, this gas would exist mostly in the larger pore-throats. Laboratory measurements of hydraulic conductivity at varying degrees of saturation indicate that a 10-percent air-filled porosity may reduce vertical hydraulic conductivity by as much as two orders of magnitude. Therefore, trapped gases may be the primary rate limiting parameter for artificial recharge at the site.

 [Graph: Figure 1 - Retardation of breakthrough of dissolved helium compared to dissolved bromide.]

Figure 1. Retardation of breakthrough of dissolved helium compared to dissolved bromide at a depth of 5.5 feet below land surface during the infiltration pond experiment at Sand Hollow near St. George, Utah.

Net infiltration rates quickly dropped off after the first few days from about 0.4 ft/d to less than 0.2 ft/d (fig. 2). Infiltration rates varied over a narrow range of 0.13 to 0.18 ft/d from August through February, but increased during the last 2 months of the experiment (March and April) to more than 0.2 ft/d. Much of the decrease in infiltration rates in the fall and increase in the spring was likely caused by changes in hydraulic conductivity caused by changes in the dynamic viscosity of water at different temperatures.

 [Graph: Figure 2 - Infiltration rates from January 200 to May 2001.]

Figure 2. Infiltration rates during the 10-month infiltration pond experiment at Sand Hollow near St. George, Utah.

In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89

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