Page 128 - Geothermal Energy Renewable Energy and The Environment

114 Geothermal Energy: Renewable Energy and the Environment

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FIGUre 6.17 Contour map of soil mercury concentrations (in parts per billion) in the Fallon, Nevada region.
Temperature gradient wells are indicated by the solid dots. The deep test well discussed in the text is indicated.
(Modified from Combs, J., Monastero, F. C., Bonin, Sr., K. R., and Meade, D. M., Geothermal Exploration,
Drilling, and Reservoir Assessment for a 30 MW Power Project at the Naval Air Station, Fallon, Nevada,
U.S.A., Proceedings of the World Geothermal Conference, v. 2, 1317–1378, 1995.)

Soil samples were collected and analyzed for mercury (Hg; Katzenstein and Danti 1982). As
noted in Chapters 3 and 12, Hg is a common trace metal present in geothermal fluids. The results of
this soil survey showed elevated Hg concentrations in the vicinity of the temperature gradient holes
(Figure 6.17). It was suggested that these elevated concentrations were the result of geothermal flu-
ids escaping from the reservoir along faults.
Gravity and magnetic surveys were conducted in the area. A magnetic high was determined to
be present approximately in the area where the Hg concentrations were elevated. A gravity anomaly
was observed, but did not correlate with these other anomalies and remains enigmatic.
In 1986, a temperature gradient hole was drilled to a depth of 1367 m. It was found that hydro-
thermal alteration had affected the rocks, resulting in the development of pyrite and other alteration
of primary minerals. Hydrogen sulfide was also detected. The bottom hole temperature for this hole
was found to be 155°C, and the temperature gradient was 104°C/km. However, there was no fluid
flowing into the well.