On the thermal effects of groundwater flow: 1. Regional scale systems

Abstract

Numerical solutions of the equations of fluid flow and heat transport are used to quantify the effects of groundwater flow on the subsurface thermal regime. Simulations are carried out for a vertical section through a basin with a distance of 40 km separating the regional topographic high and low. Emphasis is placed on understanding the conditions under which advective effects significantly perturb the thermal field. The transition from conduction-dominated to advection-dominated thermal regimes is sharp and depends primarily on the topographic configuration of the water table, the magnitude and spatial distribution of permeability, hydraulic anisotropy, and the depth of active flow. Deviationsof surface heat flow from the background heat flux are a measurable effect of groundwater flow and depend on the same factors. Our results show that from 0% to almost 100% of the section may have surface heat flow significantly different from background heat flow, depending upon the nature of the hydrogeologic environment. A limited spatial variability in a distributed set of heat flow measurements and/or linear temperature-depth profiles does not ensure that surface heat flow measurements are not disturbed. The results of our simulations suggest that knowledge of the complete environment of a site, including the water table configuration and subsurface flow system, combined with more closely spaced heat flow measurements may be necessary to unravel the true background heat flux in active flow regions. 0148-0227/83/002B-1616505.00 593 Bredehoeft and Papadopu!os, 1965; Parsons, 1970; Cartwright, 1971; Domenico and Palciauskas, 1973; Kilty and Chapman, 1980], to utilize this dependence as an aid in delineating the flow field [Bredehoeft and Papadopulos, 1965; Cartwright, 1970; Donaldson, 1962; Sorey, 1971; Keys and Brown, 1978], to determine hydraulic properties [Bair and Parizek, 1978], or to prospect for shallow aquifers [Cartwright, 1968; Kappelmeyer, 1957; Birman, 1969].Both heat flow investigators and hydrologists who have studied thermal properties of flow systems recognize that neither conductive nor advective heat transfer effects totally dominate the other for all surface geologic environments. Some regions, principally those stable tectonic areas with low topographic relief and relatively impermeable crystalline basement rocks at the surface appear to have a thermal regime dominated by heat conduction. In contrast, both more permeable sedimentary basins and active tectonic zones of high topographic relief appear to contain regions of significant area in which the thermal regime is controlled by advection. Hydraulic and thermal properties of basins and earth materials are such that conductive and advective terms in the heat transport equation can in some cases be of the same order of magnitude, and in other cases either term can dominate the other. An important corollary to this observation is that thermal fields are sufficiently sensitive to changes in measurable hydraulic and thermal parameters to make mathematical...

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