Jökull - 01.12.1977, Page 56

TABLE 4 - TAFLA 4.
The variation in the sodium and potassium concentrations of well discharges at Námafjall during
the period 1970—1975. The reported values have been read from the best fit lines through the
analytical points and refer to a reservoir water enthalpy, which corresponds to the initial quartz
equilibrium temperature of indiviclual well discharges.
Breytingar á styrk natriums og kalis i holum við Námafjall. Gildin, sem sýnd eru, iiafa verið lesin
út úr beinni tinu, sem fellur best i gegnum mcelipunkta og miða við entalpiu, sem svarar til
upphaflegs kísilhita einstakra borhola.
Well Reservoir water enthalpy Kcal/kg Sodiurn ppm Potassium ppm
01.1970-12.1975 % decrease 01.1970-12.1975 % decrease
4 267 154-117 24 25.5 - 20.3 20
5 262 139- 97 30 27.6-13.6 51
6 276 142-124 13 22.9-21.2 7
7 298 104 -110 -5 22.0-18.8 15
8 256 146-115 21 28.3-18.8 34
9 265 152-115 24 30.2-19.2 36
discharge enthalpies reported in that table.
Flashing in the reservoir and some separation
of the water and steam phases would cause in-
dividual well discharges to become depleted or
enriched in steam. An error in the value of the
discharge enthalpies will affect the sodium and
potassium results. If the actual discharge en-
thalpies were lower than reported in Table 4,
higher concentration values would have been
obtained for both elements. If individual well
discharge enthalpies had decreased during the
observation period the corresponding lowering
of the sodium and potassium concentrations
would be less. In the extreme case for well 5
the decrease for sodium would be 17% as com-
parecl to 30%. This refers to an enthalpy value
of 203 kcal/kg (200° C) as compared to 262
kcal/kg and a sampling pressure of 10 bars
absolute. If individual well discharges had in-
creased with time, the decrease in the sodium
and potassium concentration would be some-
what greater than reported, but in every case
only to an insignificant extent.
The decrease in the sodium and potassium
concentrations of the well discharges cannot be
explained by dilution of the geothermal water
with cold water from above. If dilution was
taking place there should exist a correlation be-
54 JÖKULL 27. ÁR
tween changes in silica and sodium concentra-
tion. This is not the case. For example well 4
has retained constant silica content but sodium
and potassium have decreased by 24% and 20%
respectively. It is thought that less effective
leaching of sodium and potassium from the
rock by the flashed geothermal water is the
cause of the progressive decrease in the con-
centrations of these elements in the well dis-
charges. The exploitation is expected to have
accelerated the replacement of the shallow
flashed water by less reacted water from below.
In this way less time has become available for
the ílashed water to leacli sodium and potassium
from the rock. Precipitation of quartz ancl cal-
cite may also have decreased the effective leach-
ing by reducing the contact area between water
and rock. If the water feeding well 7 represents
the unevolved reservoir water, one would expect
the sodium concentrations in the other well dis-
charges to approach gradually the sodium level
in well 7 and stabilize there, or at a higher level
depending on the steam loss in aquifers where
flashing occurs.
Major gas constituents
The gas phase of the well discharges at Náma-
fjall consists 80—90% of hydrogen, hydrogen