Calculations of fluid-mineral equilibria in the Aspo Hard Rock Laboratory Page: 4 of 13
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-270 mV. In KAS06, all values were assumed to equal -300 mV. The spread of all mea-
sured Eh values was only 70 mV. These Eh values sometimes result in calculated HS- con-
centrations greater than SO4 concentrations. The analyzed total organic carbon content of
the waters was not considered in the simulation input.
Total Al concentration was assumed to equal 0.04 mg/kg in all waters. This value repre-
sents an approximate average of waters from all boreholes with measured Al obtained
from the GEOTAB data base. No Al analyses were reported in Table 6.3 from Smellie and
Laaksoharju (1992) from which all other water properties were obtained. Computed
charge balances for these waters were excellent, as noted by Smellie and Laaksoharju
Temperature in all EQ3 and EQ6 simulations was assumed to equal 15*C.
Calculation of saturation indices: comparison between
PHREEQE and EQ3 results
Saturation indices (log Q/K) calculated with EQ3 and version comR16 of the GEMBO-
CHS data base were compared to those computed using PHREEQE and plotted as histo-
grams in Smellie and Laaksoharju (1992) for boreholes KASO2, 3, 4 and 6. Exact
comparisons were not possible because of uncertainties in how Smellie and Laaksoharju
(1992) set the temperature, Al concentration and the redox state of the system, as well the
unavailability of tabular data in their report. Differences between thermodynamic data in
GEMBOCHS and the PHREEQE data base may also account for discrepancies in com-
puted saturation indices. The contents of the two data bases were not compared in this
report. Nonetheless, results from borehole KASO2 will be used to focus on the similarities
and differences between the EQ3 and PHREEQE simulations.
Quartz is close to saturation or supersaturated in all waters from KASO2 whether calcu-
lated using PHREEQE or EQ3. Quartz solubility data from both Walther and Helgeson
(1977) and Fournier (1983) are used in EQ3. Smellie and Laaksoharju (1992) predicted
supersaturation with respect to fluorite at all depths, whereas EQ3 calculates undersatura-
tion in the samples from 200 to 500 m, and near-equilibrium in the 800 m samples. Calcite
is predicted to be supersaturated by both codes; calculated saturation indices are similar.
Results for dolomite are roughly similar. Gypsum indices can be very different, although
undersaturated or close to saturation in all cases. Tremolite is variably over- and undersat-
urated in both codes. PHREEQE predicts large supersaturations with respect to low albite,
muscovite and laumontite (see also GEOTAB data base; Laaksoharju, 1993) as does EQ3.
However, their calculated indices may differ by 1 or 2 orders of magnitude.
The GEOTAB data base (Laaksoharju, 1993) shows that in KASO2, hematite and goethite
saturation indices tend to be low when pyrite's are high, and vice versa. Saturation indices
from EQ3 suggest that Fe-bearing minerals are undersaturated in the 202 and 308 m sam-
ples, owing largely to the lower pH of the waters. At 802 m and deeper, Fe-bearing phases
are undersaturated owing to lower measured Fe concentrations. It appears that disequilib-
rium exists between the fluids and the observed fracture mineral hematite which may be a
remnant of an earlier hydrothermal event (Smellie and Laaksoharju, 1992). However, EQ3
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Bruton, C.J. Calculations of fluid-mineral equilibria in the Aspo Hard Rock Laboratory, report, May 1, 1995; California. (digital.library.unt.edu/ark:/67531/metadc793421/m1/4/: accessed February 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.