Jökull - 01.12.1984, Page 40

river emerges from the glacier. The data show
large scattering during the year but there may be
seasonal variations with the lowest 6D values
towards the end of the year. During the jökul-
hlaup in 1972 low and high values were both
recorded. Preliminary results of ö lxO values
indicate a similar scattering (Sigfús J. Johnsen,
pers. comm.). Samples taken at various times
during the years 1982- 1983 varied from 6 lsO =
-13.5 to -11 %o. The variation seems to be irregu-
lar and neither seasonal nor showing a systematic
change from normal flow to jökulhlaups. During
the jökulhlaup in 1982 samples collected on the
same day during the maximum discharge of the
flood showed -13.5 to -12 %o. The scattering of
the isotopic ratios may to some extent be due to
inhomogeneity of the water mixture in the rivers
during jökulhlaups. In 1972 estimates of dis-
charge by the method of dilution of radioactive
materials proved to be unsuccessful. The radioac-
tive material did not mix completely in the river
course (Páll Theodórsson pers. comm.).
CHEMISTRY OF WATER IN THE
GRÍMSVÖTN LAKE.
The water that drains the Grímsvötn lake has
obtained its original chemical composition from
four components (with concentrations (C) and
the flow of mass per unit time (M) given in
parenthesis): geothermal water (Cgw, Mgw),
geothermal vapour (Cgv, Mgv), ablation from the
surface of the glacier (Ca, Ma), and ice melted
inside the geothermal area (Q, Mj). Later this
composition may have been modified by chemi-
cal reactions in the lake and on the way to
Skeidarársandur. The river water in jökulhlaups
on Skeidarársandur is a mixture of water from
the lake and river water from the normal drain-
age basin outside the lake (with Cr and Mr). The
concentration in jökulhlaups of substances that
do not undergo chemical reaction in the lake can
be expressed as
Cj = k(CgWMgW+CgvMgv)/(Mgw+Mgv) +
(l-k)(Ca Ma+Q Mi+Cr Mr)/(Ma+M|+Mr), (1)
where k is a geothermal mass fraction defined as
the ratio
k=(Mgw+Mgv)/(Mgw+Mgv+Ma+Mi+Mr). (2)
The geothermaí fraction k could be calculated
if we knew the original chemical concentrations
and the masses of the components. Not all of this
information is available but the model is
nevertheless useful as a guide in the discussion of
the chemical analyses. For some substances we
are able to conclude that the model does not
apply and chemical changes must have occurred
in the lake. For others the model is applicable
and for one of them, silica, reasonable estimates
can be made and k evaluated.
Substances whose concentrations are
changed by reactions in the
Grímsvötn lake
The observed concentrations of sodium, potas-
sium, calcium and magnesium in jökulhlaups are
not consistent with a mixture of meltwater and
high-temperature geothermal water. They
demonstrate that chemical changes have taken
place within the lake. The model described by
equation (1) does not apply for those substances.
That such is the case is clear from comparison
with known concentrations in deep water from
other geothermal areas like Námafjall, Krafla
and Nesjavellir (see Table 3). In those areas the
concentration of sodium is much too low to
account for the concentration in the jökulhlaups
from Grímsvötn. The same applies for potassium
although not as conclusively as for sodium.
Sodium and potassium are the most soluble of the
cations and their rates of leaching out of basaltic
glass will be enhanced by the low pH in the lake.
The pH of the lake water must be considerably
lower than that of the melted ice. This is due to
dissolution of acid gases from the geothermal
fluid.
Calcium does not exceed about 3 ppm in deep
water at non-saline geothermal areas in Iceland
(see Table 3) and that is considerably lower than
in the normal discharge of the glacial rivers.
Dissolution of Ca must have occurred in the
highly carbonated water of the Grímsvötn lake.
The magnesium concentration of water from
the Grímsvötn jökulhlaups, (10-15 mg/kg), is
similar to those of cold groundwater (3-12 mg/kg,
NEA data). Experimental data (see Mottl and
Seyfried 1980) and field data (Kristmannsdóttir
1980, Tómasson et al. 1977) show that Mg is
precipitated at temperatures above 40 °C. The
magnesium concentration of non-saline geoth-
ermal water is generally in the range of 0.001-0.1
38 JÖKULL 34. ÁR