Groundwater induced advective cooling of an ash-flow sheet

Groundwater induced advective cooling of an ash-flow sheet: Valley of Ten Thousand Smokes, Alaska

Hogeweg, N.

United States Geological Survey, Menlo Park, California, USA
Faculty of Earth Sciences, Utrecht University

Molengraaff Fonds

2002

The Valley of Ten Thousand Smokes (VTTS) in Alaska was filled by a voluminous ash-flow tuff in 1912. In 1987 the chloride rich mid-valley thermal springs (MVTS) were discovered, indicating a persistent heat source at depth. A previous study proposed that the heat source is a cooling welded tuff unit (Keith et al., 1992). Discharge, temperature, and chloride-concentration data, collected during fieldtrips in 1993 and 2001 reveal spring temperatures of approximately 21°C, a linear relationship between the water discharge and chloride concentration in the Ukak River which drains the VTTS, and a heat discharge from the VTTS < 285 MW. An analytical model of conductive cooling of a 13x3 km layer with a thickness of 60 meters (a rough approximation of the thickest part of the actual sheet) with an initial temperature of 700°C results in a heat flux of 176 MW after 90 years. To describe heat transport to the MVTS by conduction and advection, the finite difference numerical code USGS-HYDROTHERM was used to solve the coupled, non-linear equations of multi-phase groundwater flow and heat transport in a heterogeneous domain. Because little is known about the sub-surface parameters and structures in this ash-flow sheet, a very simple three-layer model is invoked. Input parameters were obtained from field measurements carried out in this study and in earlier work in the VTTS, from geophysical surveys in the VTTS, and from studies of tuff units in other locations. Simulation goals were to quantify and characterize the transient cooling of the welded tuff layer and the associated heat discharge. The sensitivity of simulation results to permeability, anisotropy of the permeability, initial temperature and slope was examined. Permeability has the largest effect on the maximum temperature in the domain and at the location representing the MVTS. The dominant heat transport mechanism shifts from advection to conduction at permeabilities of -10-(15) m2.

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(c) Noor Hogeweg