Jökull - 01.12.1990, Síða 131

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Figure 9. Increase of chloride in groundwater through
evapotranspiriation. —Aukning klóríðs í grunnvatni
vegna raungufunar.
regular, correspond to hydrological and hydrogeolog-
ical conditions and do not show any recognizable con-
nection to the numerical value of the chloride content.
The test is then positive, and this way of correcting the
component values must be regarded as satisfactory at
the present state of information.
CHEMISTRY OF COLD GROUNDWATER
Some changes in the chemical contents take al-
ready place at the surface. The actual evapotranspira-
tion causes a concentration, as can be seen from the
comparison of precipitation and the chemistry of local
springs.
Another factor is the differentiation due to
snowmelt, i.e. partial melting of snow (Gíslason,
1985; Gíslason and Eugster, 1987). The meltwater
Part of the snow is more concentrated than the residual,
purified part. Where the snowmelt occurs suddenly
on frozen or impermeable ground, much of the solutes
could be expected to be carried off on the surface and
thus be detracted from the groundwater. This could
also explain the low content of chloride in rivers in
Borgarfjörður in westem Iceland in late winter (Rist,
1986), when the chemical content of the snowfields
has already been reduced by partial melting. Ground-
water in the Tertiary to Early Quatemary regions could
then be relatively impoverished in chemistry. The
same applies to the highland regions.
The production of meltwater is strongest on the
glaciers in late summer, but the infiltration into the
ground at the base of the glaciers could be more evenly
distributed over the year, due to the delaying effects
of the glacier body, as a reservoir, and the possibly
limited infiltation capacity of the ground. Some pro-
duction of meltwater takes place in the other seasons,
although much less. In the case of a more evenly
distribution the purified phases (snow and ice) would
supply more than their share of the infiltrated water
and the glacial groundwater would be relatively low
in dissolved solids.
The regional distribution of the concentration of
various chemical components has been mapped, based
on more than 330 selected samples, which are indeed
unevenly distributed over the country, leaving wide
areas sparsely or not represented. These samples were
taken by NEA’s staff in 1970-1988 (see Halldórsdótt-
ir, 1982, for sampling 1970-1982. Some later anal-
yses are to be found in various of NEA’s reports, but
the majority of them are still unpublished). Samples
from Gíslason (1985) and analyses given by Ólafsson
(1979) for Lake Mývatn are also included (Fig. 10).
Most samples are from springs, some of which
may be ”local“, with marked seasonal variations in
chemistry. Some are from water works in operation
and the chemistry may be affected by overdraught
or other disturbing causes (especially in the Húnaflói
region).
DISTRIB UTION OF CARBON AND ANIONS
The vegetation cover is a potential source of or-
ganic compounds (Fig. 11). This would be reflected
in a high content of carbon dioxide (total), which is
indeed obvious in the peatbog covered southem low-
lands. High-temperature geothermal water has as a
rule a high content of carbon dioxide and increases the
JÖKULL, No. 40, 1990 127