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Chemical geothermometry* and the chemical composition of fluids from thermal springs and shallow drill holes document a linear relationship between the chloride concentration and fluid enthalpy. On this basis nearly all of the sampled thermal waters in Yellowstone can be related to a deep water source containing about 400 ppm chloride at an enthalpy of 1,570 J/g (about 335-340°C) (Fournier, 1989).

The chloride-enthalpy relationship suggests that advective heat flux can be estimated from total chloride discharge from the park. In Yellowstone, nearly all of the chloride discharged from springs eventually enters one of the four major rivers draining Yellowstone National Park: the Yellowstone and Madison Rivers east of the continental divide, and the Snake and Fall Rivers west of the divide (map). Using the chloride-inventory method**, which corrects for background chloride (~1-2 ppm) contributed by precipitation and low-temperature rock weathering, long-term river discharge and chloride concentrations in these major rivers (Norton and Friedman, 1991; Friedman and Norton, 2000) provide a quantitative estimate of the total heat discharge from Yellowstone and can also be examined for temporal trends associated with magmatic and tectonic activity.

The advective heat flux from the thermal areas northeast of the continental divide in Yellowstone, based on thousands of measurements from 1966-2000, is approximately 4.5 GW; the heat flux from the entire Yellowstone system is approximately 6.1 GW (Ingebritsen et al., 2001).

Norris Geyser Basin hosts the greatest variety of thermal features in Yellowstone National Park. More than 97 percent of the outflow from the basin exits via Tantalus Creek to the Gibbon River, a tributary of the Madison River. Discharge from Tantalus Creek is monitored by a weir located about 100 meters above the confluence of Tantalus Creek with the Gibbon River (map). The discharge is measured every 10 minutes, and telemetered via GOES satellite.

*Chemical geothermometry is a method of estimating the "reservoir" temperature of a fluid at depth, prior to cooling en route to the surface. There are two general types of geothermometers: 1) those based on absolute concentrations of a constituent in solution, and 2) those based on ratios of two or more constituents in solution. Some chemical geothermometers have been calibrated by laboratory experiments, whereas others are empirical relations based on many high-temperature fluid samples.

**Using the chloride-inventory method (Ellis and Wilson, 1955), the discharge rate of a hot spring group (Qt) is calculated from the chloride concentration upstream (Clu) and downstream (Clu) of the hot springs, the chloride concentration in the thermal water (Clt), and the discharge rate of the stream (Qs): Qt=[Qs(Cld-Clu)]/[Clt-Clbkgd], where Clbkgd is the "background chloride concentration upstream of any thermal source and assuming that Qt<<Qs and Clt>>Clu.

References

Fournier, R.O., 1989, Geochemistry and dynamics of the Yellowstone National Park hydrothermal system: Annual Reviews of Earth and Planetary Science, v. 17, p. 13-53.

Friedman, I. and Norton, D.R., Data used for calculating chloride flux out of Yellowstone National Park for the water years 1983-1999, Open-File Report - U. S. Geological Survey, Report: OF 00-0194, 48 pp., 2000.

Ingebritsen, S.E., Galloway, D.L., Colvard, E.M., Sorey, M.L. and Mariner, R.H., Time-variation of hydrothermal discharge at selected sites in the western United States: implications for monitoring, Journal of Volcanology and Geothermal Research, 111, 1-23, 2001.

Norton, D.R. and Friedman, I., Chloride flux and surface water discharge out of Yellowstone National Park, 1982-1989, U. S. Geological Survey Bulletin, Report: B 1959, 42 pp., 1991.