A conceptual model of deep unsaturated zones with negligible recharge

Abstract

When net recharge is less than about 0.03 mm/yr, moisture movement in deep unsaturated zones at steady state will be dominated by upward movement of vapor driven by the geothermal gradient. With zero net recharge there will be a downward flow of liquid water equal to the upward vapor flux. This will produce a profile of suction potential versus depth qualitatively similar to that expected if recharge is not negligible. Consequently, the existence of a region of uniform potential is not in itself evidence of recharge.

Numerical Modeling of Isothermal and Nonisothermal Flow in Unsaturated Fractured Rock: A Review

“…Certainly in the modeling of ambient conditions, other effects from multiphase flow and mild geothermal gradients should be taken into account. Ross [1984] and Montazer and Wilson [1984] discussed moisture movement driven by vapor diffusion under an ambient geothermal gradient in low-infiltration systems. Weeks [1986] reported observations of strong seasonal air flow in boreholes at Yucca Mountain, driven by, PRUESS AND WANG 15 thermal buoyancy effects.…”

Two-phase unsaturated flow at Yucca Mountain, Nevada - A Report on Current Understanding

“…Both effects limit the ability to make inferences about water seepage from temperature data. Natural geothermal gradients are associated with vapor pressure gradients and an upward diffusive flux of vapor which has been estimated at a water-equivalent rate of .03 -.04 mn-dyr (Ross, 1984).…”

Status of understanding of the saturated-zone ground-water flow system at Yucca Mountain, Nevada, as of 1995

“…A conceptual model of the unsaturated zone (Montazer and Wilson, 1984) indicated a very small downward flux of water deep in the unsaturated zone. Ross (1984) studied vapor movement in the unsaturated zone where little recharge occurred and concluded that vapor movement could dominate the system, and upward flow could occur where net recharge was less than 0.03 mm/yr. Whatever the interaction between the saturated zone and the unsaturated zone at Yucca Mountain, the volume probably is small compared to downgradient outflow.…”