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A conceptual model, based on field observations and assumed physics of a perennial firn aquifer near Helheim Glacier (southeast Greenland), is evaluated via steady-state 2-D simulation of liquid water flow and energy transport with phase change. The simulation approach allows natural representation of flow and energy advection and conduction that occur in vertical meltwater recharge through the unsaturated zone and in lateral flow within the saturated aquifer. Agreement between measured and simulated aquifer geometry, temperature, and recharge and discharge rates confirms that the conceptual field-data-based description of the aquifer is consistent with the primary physical processes of groundwater flow, energy transport and phase change. Factors that are found to control simulated aquifer configuration include surface temperature, meltwater recharge rate, residual total-water saturation and capillary fringe thickness. Simulation analyses indicate that the size of perennial firn aquifers depends primarily on recharge rates from surface snowmelt. Results also imply that the recent aquifer expansion, likely due to a warming climate, may eventually produce lakes on the ice-sheet surface that would affect the surface energy balance. [ABSTRACT FROM AUTHOR]

Regulatory agencies are becoming increasingly interested in using young-ground water dating techniques, such as the [.sup.3]H/[.sup.3]He method, in assessing the susceptibility of public supply wells (PSWs) to contamination. However, recent studies emphasize that ground water samples of mixed age may be the norm, particularly from long-screened PSWs, and tracer-based 'apparent' ages can differ substantially from actual mean ages for mixed-age samples. We present age and contaminant data from PSWs in Salt Lake Valley, Utah, that demonstrate the utility of [.sup.3]H and [.sup.3]He measurements in evaluating well susceptibility, despite potential age mixing. Initial [.sup.3]H concentrations (measured [.sup.3]H + measured tritiogenic [.sup.3]He) are compared to those expected based on the apparent [.sup.3]H/[.sup.3]He age and the local precipitation [.sup.3]H record. This comparison is used to determine the amount of modern water (recharged after ~1950) vs. prebomb water (recharged before ~1950) samples might contain. Concentrations of common contaminants were also measured using detection limits generally lower than those used for regulatory purposes. A clear correlation exists between the potential magnitude of the modern water fraction and both the occurrence and concentration of contaminants. For samples containing dominantly modern water based on their initial [.sup.3]H concentrations, potential discrepancies between apparent [.sup.3]H/[.sup.3]He ages and mean ages are explored using synthetic samples that are random mixtures of different modern waters. Apparent ages can exceed mean ages by up to 13 years for these samples, with an exponential age distribution resulting in the greatest discrepancies., Introduction There is growing interest among regulatory agencies in using ground water age data in wellhead protection planning. Tracers such as tritium ([.sup.3]H) and radiogenic carbon ([.sup.14]C) have been widely [...]

Dissolved helium and bromide tracers were used to evaluate trapped gas during an infiltration pond experiment. Dissolved helium preferentially partitioned into trapped gas bubbles, or other pore air, because of its low solubility in water. This produced observed helium retardation factors of as much as 12 relative to bromide. Numerical simulations of helium breakthrough with both equilibrium and kinetically limited advection/dispersion/retardation did not match observed helium concentrations. However, better fits were obtained by including a decay term representing the diffusive loss of helium through interconnected, gas-filled pores. Calculations indicate that 7% to more than 26% of the porosity beneath the pond was filled with gas. Measurements of laboratory hydraulic properties indicate that a 10% decrease in saturation would reduce the hydraulic conductivity by at least one order of magnitude in the well-sorted sandstone, but less in the overlying soils. This is consistent with in situ measurements during the experiment, which show steeper hydraulic gradients in sandstone than in soil. Intrinsic permeability of the soil doubled during the first six months of the experiment, likely caused by a combination of dissolution and thermal contraction of trapped gas. Managers of artificial recharge basins may consider minimizing the amount of trapped gas by using wet, rather than dry, tilling to optimize infiltration rates, particularly in well-sorted porous media in which reintroduced trapped gas may cause substantial reductions in permeability. Trapped gas may also inhibit the amount of focused infiltration that occurs naturally during ephemeral flood events along washes and playas., Introduction Artificial recharge by the spreading of water in surface basins often occurs under an effective field hydraulic conductivity that is lower than the fully saturated hydraulic conductivity. This lower [...]

Measurements of dissolved gases have numerous applications in ground water hydrology, and it is now possible to measure the total dissolved gas pressure in situ using a probe. Dissolved gas pressure is measured by submerging a headspace volume with a gas-permeable membrane, allowing dissolved gases in the water to equilibrate with gases in the headspace, then measuring the pressure in the headspace with a pressure transducer. Total dissolved gas pressure (TGP) probes have many potential uses in ground water studies employing dissolved gases, including: (1) determining approximate excess air levels, which may provide information about the time and location of recharge; (2) screening wells for air contamination, which can compromise the accuracy of dissolved gas tracer techniques: (3) detecting a trapped gas phase, which can significantly reduce hydraulic conductivity and impede the transport of dissolved solutes and gases; (4) enabling the use of gas-filled passive diffusion samplers for determining accurate dissolved gas concentrations; and (5) determining relative concentrations of C[H.sub.4] and C[O.sub.2] when they are known to be highly abundant. Although TGP probes designed for surface water have been available for several years, TGP probes suitable for ground water applications have only recently become available. Herein we present what are, to our knowledge, the first reported ground water dissolved gas data collected using a TGP probe. We also explain the basic operating principles of these probes and discuss the potential applications listed., Introduction The measurement of dissolved gas concentrations has become widespread in ground water hydrology. Dissolved gases are used as environmental tracers in several ground water dating techniques ([.sup.3]H/[.sup.3]He, chlorofluorocarbons, noble [...]

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A passive in sire headspace sampler has been developed for the collection of dissolved gases in ground water during tracer tests using dissolved, inert gases. The sampler is constructed of a semipermeable membrane and copper tubing equipped with a Schrader valve. Because the headspace in the sampler equilibrates with the dissolved gases in the ground water, no water is collected, which can be a great advantage in use at contaminated sites. The injection of the dissolved gases into the ground water is accomplished by diffusion through a semipermeable membrane. Oak Ridge National Laboratory routinely uses this technology for site characterization. The passive-diffusive technology also has potential as a low-cost, low-maintenance, and long-term method for containment assessment.