Jökull - 01.12.1984, Side 37

autumn of 1983 increased concentrations of dis-
solved solids showed geothermal influence in the
rivers. This raised the question of leakage out of
Grímsvötn two months before the jökulhlaup in
December. At the beginning and the end of the
period the normal variations were obscured by
the influence of jökulhlaups.
Data for the concentration of total dissolved
solids (dry residue) exist for Skeidará over a
period of twenty years (Tómasson et al. 1982).
The two years 1982/83 show a seasonal variation
that appears to be present for the whole period.
However, we should note that sampling was less
frequent in the years before 1980 than later.
At the time of the Grímsvötn eruption in 1983
almost no analyses existed for other rivers that
drain Vatnajökull. Since then several samples
have been collected from the rivers Sandgígju-
kvísl, Súla, Djúpá, Hverfisfljót and Skaftá (see
Fig. 6-8). The concentration of dissolved solids
shows considerable variation but the time is too
short to give seasonal variations. In September
1983 a jökulhlaup occurred in the river Skaftá.
The water in the rivers has a pH value of 7 to
7.7 but drops down to 6 during jökulhlaups in the
rivers Skeidará, Sandgígjukvísl, Súla and Skaftá
(Fig. 6, Table 2).
The normal concentration of silica (Si02) in
Skeidará is about 10-20 mg/kg and during the
jökulhlaups it rises up to 50-60 mg/kg (Fig.7,
Table 2). In the jökulhlaup in 1972 a lower
concentration, 44 mg/kg, was reported by Stein-
thórsson and Óskarsson (1983). The normal con-
centration of silica in the river Skaftá is similar to
that of Skeidará but appears to be somewhat
lower, 5-10 mg/kg, for the other rivers. The mean
concentration of Si02 for Icelandic rivers is 5-15
mg/kg (NEA data).
The concentration of carbonate (as C02) is
shown in Fig. 8. The normal concentration is
around 30 mg/kg in Skeidará but up to 20 times
higher during jökulhlaups. Concentrations
between 10 and 30 mg/kg are observed in the
other rivers.
The sodium (Na) concentration varies norm-
ally from 4 to 15 mg/kg in Skeidará but increases
to around 60 mg/kg in the jökulhlaups (Fig. 8). In
the other rivers the concentration of sodium is
2-15 mg/kg and the seasonal variation appears to
be large.
The potassium (K) concentration in Skeidará is
normally 0.3-1.2 mg/kg and exceeds 4 mg/kg in
water samples from jökulhlaups (Fig. 7). The
concentration of potassium in the five other riv-
ers is similar to that of normal water in Skeidará.
The concentration of calcium (Ca) varies
between 6 and 19 mg/kg in Skeidará and other
rivers have somewhat lower concentrations
(Fig.7). During the jökulhlaups the calcium con-
centration has reached 50 mg/kg in water from
Skeidará.
The magnesium (Mg) concentration varies
normally between 1.5 and 6 mg/kg and reaches 12
mg/kg in the jökulhlaups in Skeidará. The mag-
nesium concentration is generally Iower in the
other rivers.
The concentration of sulphate (S04) varies
normally from 3.5 to 9 mg/kg in Skeidará but
reached about 20 mg/kg in the jökulhlaups in
1976 and 1982, around 40 mg/kg in the jökul-
hlaup in 1965 and nearly 50 mg/kg in 1983. Nor-
mal concentrations in the other rivers were
observed as 1.5-12 mg/kg during the period June
to December of 1983.
During jökulhlaups the river Skeidará reeks of
sulphur. However, the concentration of hyd-
rogen sulphide (H2S) was below the detection
limit (0.04 mg/kg) in all water samples taken
more than 100 m from the glacier snout.
The concentration of chloride (Cl) in Skeidará
is normally 1 to 8 mg/kg and has been observed to
increase up to about 13.5 mg/kg during jökul-
hlaups. Sigvaldason (1965) reported maximum
values of 43 mg/kg. The normal concentration of
chloride in the five other rivers mentioned above
varies from 2 to 15 mg/kg.
The concentration of fluoride (F) in Skeidará is
normally 0.05 to 0.15 mg/kg and increases up to
0.3 mg/kg during jökulhlaups. The other rivers
have a similar normal concentration of fluoride as
Skeidará.
Dissolved iron , in concentrations up to 5 mg/
kg , was measured in samples of water from the
jökulhlaup in Skeidará in Dec. 1983. The samples
were filtered one to three days after collection
and a part of each acidified. In the fraction of the
samples that was not acidified a rust red precipi-
tate of iron oxide appeared the day after filtra-
tion. The iron concentration was measured in
both parts of the samples. The concentration in
the untreated samples was in all cases below
detection limit, but 2-5 mg/kg in the acidified
samples. In February 1982 samples from the
jökulhlaup were filtered a few days after collec-
JÖKULL 34. ÁR 35