Page 111 - Geothermal Energy Systems Exploration, Development, and Utilization

2.5 Geochemistry 87
proportions of water in hydrothermal discharges are of local meteoric origin. On
the basis of these findings, the formation of hydrothermal solutions was explained
largely in terms of the interaction of meteoric waters with crustal rocks at elevated
temperatures, with magmatic contributions limited to the supply of heat (Ellis and
Mahon, 1964).
Thermal waters can be described comprehensively by collecting samples from
a reasonable number of thermal and nonthermal waters, distributed all over the
investigated area, to be analyzed, for the following constituents: Na, K, Mg, Ca,
alkalinity, SO 4 ,Cl,Li,F, B,SiO 2 ,and NH 3 . Additional constituents useful to
investigate specific problems are Al, H 2 S, Rb, Cs, Br, As, and Hg. The analysis
of H 2 S is generally performed for environmental purposes and is not part of
the standard analytical routine. If possible, the hydrogeochemical survey should
be carried out at the end of the dry season to get water samples least affected
by mixing with surface water. The field measurements to be carried out are
temperature, pH, Eh, conductivity, and alkalinity. Sample size depends on the
number of constituents to be determined and on laboratory requirements. Large
water amounts are generally needed for tritium determination and when trace
elements are analyzed. It is advisable to repeat the analysis of a given sample
(stored in sufficient amount in the laboratory), although the concentration of
some solutes may change with time. Acidification is needed to preserve cation
contents of high temperature waters, which become supersaturated upon cooling,
and to prevent precipitation of trace metals from both high- and low temperature
waters. Dilution of filtered or filtered–acidified samples is advisable for silica
determination. Unfortunately, Al concentrations are rarely measured in geothermal
liquids and sometimes poor representative values are obtained, because finely
dispersed aluminum oxyhydroxides pass through the membrane filters. The quality
of water analysis is usually checked computing the ionic balance; however, possible
errors for minor constituents (e.g., Li and F, but also Mg and SO 4 in high
temperature geothermal fluids) or neutral species (e.g., SiO 2 and NH 3 ) cannot be
detected in this way. At best, ionic balance gives an indication on the analytical
accuracy of major constituents.
The dissolved constituents of geothermal waters may be subdivided into two
groups according to their behavior:

• Mobile or conservative constituents are those whose activity is not limited by
saturation with respect to a solid or a gas phase; comparatively mobile constituents
in geothermal waters (and in most natural waters as well) are Cl, Br, B, and, to a
minor extent, Li, Rb, and Cs; once they have been added to a geothermal water
through a complex history, their contents along the upflow path are changed only
by mixing and steam loss.
• compatible or reactive constituents are those whose activity is controlled by
saturation with respect to a solid or a gas phase; they equilibrate under reservoir
conditions and may respond to thermochemical changes along the upflow path
of the geothermal water; Ca, Mg, Na, K, HCO 3 ,SO 4 ,F, SiO 2 , and so on, usually
have compatible behavior in geothermal environments.