Jökull - 01.12.1977, Side 55

Fig. 3. Variation in the quartz equilibrium
temperature (tqtz), the NaKCa-temperature
(t\aKCa) (see Fournier ancl Truesdell, 1973), and
the Na and K concentrations of the discharges
of wells 5 and 7 during the period 1970—1976.
Mynd 3. lireytingar á kisilhita, NaKCa-hita,
og styrk natríums og kalis i holum 5 og 7 á
timabilinu 1970—1976.
quartz on one hand and the relevant NaKCa-
silicate on the other, is similar for the tempera-
tures and water compositions in question. The
erratic and low NaKCa-temperatures for well 7
result from the variable calcium content of the
discharge. The calcium content of all the wells
is somewhat erratic but without any observable
trends witli time. This may be due to dissolu-
tion of fine calcite crystals upon storage of the
samples prior to analysis. The flashing in the
aquifers and in the wells leads to calcite pre-
cipitation and a fraction of the precipitate
could well be brought to the surface with the
rapidly ascending steam-water mixture.
At the beginning of the exploitation period
■well 7 contained the lowest sodium concentra-
tions or about 105 ppm as compared with 140—
150 ppm in the other wells. It is observed that
the initial sodium content of individual well
discharges increases with distance from well 7.
Well 7 has the highest temperature and the
liighest content of volatiles. For these reasons
this well is assumed to be fed by the least
evolved reservoir water, while the other wells
are fed by water which has undergone more
flashing and degassing. The difference in the
sodium concentrations o£ these well discharges
with that of well 7 cannot, however, be account-
ed for by steam loss. To increase the sodium
concentration from 105 to 140 ppm the re-
servoir water must loose about 25% of steam.
For flashing by one step adiabatic cooling this
steam loss corresponds to a temperature drop of
the order of 100° C. The temperature differ-
ence of the aquifer feeding well 7 and the
aquifers feeding the other wells is in the range
of 15—40° C. Such cooling by one step adia-
batic flashinff corresponds to a steam loss of
6-11%.
It is thought that the relatively higher sodium
concentrations in the more elvolved waters has
resulted from leaching of sodium from the rock
by the flashed water. This leaching is favourcd
by a reaction towards an ionic exchange equi-
librium between the major dissolved cations
(Na+, K+ and Ca++) and protons, and the
relevant hydrothermal minerals. Arnórsson et
al. (1978) present data on water compositions
for many geothermal areas in Iceland which
support the existence of such an equilibrium.
As water temperatures decrease the ratios of in-
dividual cation/proton ratios increase. Re-equi-
libration upon cooling would therefore involve
a combination of loss of protons from solu-
tion and leaching of the major cations from the
rock.
With the exception of well 7 the sodiurn
content of all the well discharges has decreased
during the exploitation period. This decrease
amounts to 13—30% over a six year period
(Table 4, Fig. 3). The potassium concentrations
liave decreased to conform with the decrease in
the concentrations of sodium and the lowering
of the water temperatures in the aquifers. This
decrease amounts to 7—51% for different wells
(Table 4).
The sodium and potassium values shown in
Table 4 have been obtained by using the well
JÖKULL 27. ÁR 53