Jökull - 01.12.1962, Síða 51

The Use of Isotopes of Hydrogen and Oxygen
for Hydrological Purposes in Iceland
BY
GUNNAR BODVARSSON
STATE ELECTRICITY AUTHORITY, REYKJAVIK, ICELAND
INTRODUCTION
The stable isotopes D and O18 as well as the
radioactive H3, or tritium, are now becoming
important tools in the science of hydrology. In
the following a brief discussion will be given
of their possible applications in hydrological
work in Iceland.
Hydrology has two important applications
in Iceland. Firstly, the study of the general
pattern of flow of near-surface ground water
which is of importance for the planning ot'
water-power projects. Secondly, the study of
the origin and the pattern of flow of subsur-
face thermal water. Natural heat has become
a prolific natural resource in Iceland and the
hydrology of the thermal water is therefore of
great importance. The latter type of applica-
tion is in many ways quite unique ancl will
consequently in the following receive the main
attention.
GENERAL DATA
Before entering the discussion of specific pro-
blems it appears useful to state some general
data which will appear later in the present
report.
Thermal activity in Iceland. A discussion
of some general aspects of the geothermal ac-
tivity in Iceland is given in a report by the
present writer (Bodvarsson, 1961). The reader
is referred to this report for general informa-
tion. Only a few facts that are relevant to the
present report will be given in the following.
It is believed that mainly two conditions are
responsible for the abundant thermal activity
in Iceland.
Firstly, the main formation of the country,
the flood basalts, forms a plateau of a thickness
of not less than 2 km. This formation is per-
meable along the contacts of individual Iava
flows and up through dikes and faults. The
permeable structures are scattered throughout
the formation and ground water can, therefore,
percolate through the entire formation, that is,
down to a depth of 2 km, at least.
Secondly, the outward flow of terrestrial heat
in Iceland appears to be two or three times
the average for normal areas. Therefore, a
temperatui'e of 100°C to 150°C may in general
be encountered at the bottom of the flood
basalts. Also, there are in places clear signs of
the presence of volcanic intrusives which may
lead to considerably higher local temperatures.
The combination of the permeability at deptli
and the relatively high temperature in the flood
basalts appears to be the natural explanation
for the unique geothermal activity.
At this juncture it is not known how far the
subsurface water can percolate. In places there
are definite indications of distances of 50 km
and possibly more. An important problem to
be solvecl is whether the water flows mainly
along or transverse to tectonic lines.
Near-surface ground water. The main aqui-
fers in Iceland are lava-flows and other vol-
canic structures of postglacial age. Ordinary
sediments are practically absent. The lavas and
the volcanic formations are very irregular
formations and the pattern of ground-water
flow is generally quite obscure. A ground-water
inventory is, therefore, often a difficult task.
Moreover, the application of dams for water
storage on the volcanic formations involves a
considerable hazard of leaks. A study of leaks
JÖKULL 49