TOTAL DISSOLVED GAS PRESSURE IN GROUNDWATER: AN OVERVIEW AND SELECTED CASE STUDIES

The pressure that exists in a gas phase that is in equilibrium with dissolved gases is known as the total dissolved gas pressure (TDGP). This quantity can be measured using a simple probe and is extremely useful for both physical and chemical studies of groundwater. It is numerically equal to the sum of all dissolved gas partial pressures, but is typically dominated by the partial pressures of N₂, O₂, and Ar as these are the most abundant gases in the atmosphere. Once groundwater is isolated from the atmosphere, the TDGP is affected by temperature and the production or consumption of subsurface gases, but is not changed by the increasing fluid pressure as groundwater descends below the water table. When the TDGP approaches the pressure of groundwater, gas exsolution and bubble formation is possible; thus TDGP can be used to assess the state of gas saturation.

Selected cases include: (1) TDGP used to trace a plume of artificial recharge beneath a newly constructed reservoir in southwestern Utah. Our analysis also indicates the presence of bubbles beneath the reservoir that initially led to a reduction in the permeability and infiltration rates, but after 10 years the bubbles appear to have been dissolved with an associated increase in infiltration rates. (2) In the Las Vegas Basin, excess gas in municipal supply wells led to customer complaints of “milky” water, and TDGP measurements helped identify the source of this gas as an artifact of injecting cool surface water into warm groundwater. (3) In studies of tufa formation in streams draining the Wasatch Mountains, TDGP measurements have shown transient periods of gas exsolution related to in-stream microbial activity. (4) Denitrification can result in the stripping of atmospheric gases used in age dating and noble gas thermometry, and TDGP measurements can be used to assess the depth and timing of gas stripping.