Jökull - 01.12.1965, Qupperneq 20

TABLE 1.
Chemical analysis of glacier burst water from Skeiðará.
mg/1 4/9 ’65 6/9 ’65 8/9 '65 9/9 ’65 10/9 ’65 11/9 ’65
Si02 56 56 56 50 47 43
Na 42.5 63.5 47.5 39.5 30.7 32.8
K 2.8 19.0 3.5 2.5 2.0 2.4
Ca 45.4 59.5 54.5 42.2 43.9 40.0
Mg 5.8 10.4 0.7 5.0 9.9 9.0
Ci 12.5 42.7 12.4 11.0 10.0 8.7
F 0.05 0.05 0.05 0.15 0.05 0.05
S04 25.1 38.7 20.6 27.6 24.3 16.9
HCO3 303.8 944.9 330.6 313.5 255.0 240.0
Diss. solids*) 292.0 416.0 296.0 224.0 204.0 254.0
PH 6.35 7.0 6.1 6.7 6.9 6.1
*) Dissolved solids determined as residue on evaporation, heated to 105° C.
but more likely 1:3. To this might be added,
that thermal areas in Iceland situated at high
levels such as tiie Grímsvötn, usually do not
have discharging hot springs but display fuma-
rolic activity only. The Grimsvötn caldera can
therefore be looked upon as a thermal system
with a closed water circulation. The main com-
ponents of a geothermal system are the heat
source and circulating groundwater as a medi-
um for the heat transport.
Frequent volcanic eruptions in the Gríms-
vötn caldera suggest a magmatic body as a heat
source. The high lieat flow is probably not
compatible with any other forrn of heat source
than a cooling magmatic body at relatively shal-
low depth.
The Grímsvötn are a subsidence caldera prob-
ably with ring fractures along the edges, which
provide open channels for downpercolating
water. Because of the steep geothermal gradient
the water is heated and density differences pro-
vide for effective convection in the system. This
description could fit to a number of geothermal
systems, but two additional factors characterize
the Grímsvötn geothermal area and provide a
possible explanation of the chemical composi-
tion of the water.
1. From Thorarinsson’s estimate on the equal
126 JÖKULL
amount of precipitation in the period between
jökulhlaups and the total amount of water clis-
charged during tlie jökulhlaup one may con-
clude, that the meteoric water circulates with-
in a closed system with no escape or addition
of water from en external source.
2. The thermal system is closed by a confin-
ing icecap, which is continuously being melted
at a similar rate as the accumulation of ice.
The thermal energy is thus used up at tlie
same rate as it is transferred to the surface by
convective currents ancl contained within the
system. This effective cooling and the pressure
of the confining icecap prevent boiling in the
system and considerable buildup in the con-
centration of volcanic gases such as CO2 and
SO2 is possible in the water phase. Chemical
components, which are absent in other high
temperature thermal waters clue to precipita-
tion upon boiling and loss of CO2 such as Ca
and Mg are found in consiclerable amounts as
bicarbonates.
In Table 2 analysis of a few types of Ice-
landic thermal waters are listed along with an
analysis of water from a glacier river. Compared
to analysis of the jökulhlaup water a close ana-
logy is found to low temperature thermal water
with regarcl to sodium sulfate and chloricle, but