Jökull - 01.12.1984, Qupperneq 35

TABLE 2. Concentration of dissolved solids in water from Skeidará sampled at the peak of
jökulhlaups.
TAFLA 2. Styrkur uppleystra efna í hámarki jökulhlaupa.
Jökul- hlaups pH / °C sío2 Na K Ca Mg co2 tot so4 H2S C1 F Tot. dis.
1954” 57 60.9 15.6 18.1 0.0 8.7 0.3 388
1965+ 7.0/ 56 63.5 19.0 59.5 10.4 680 38.7 42.7 0.5 416
1972* * 7.5 / 44 89.0 3.0 28.0 10.0 480 13.0 11.0
1976# 50.5 43.0 3.8 45.6 9.9 23.5 13.5
1982# 6.02 / 22 60.0 53.1 4.2 50.4 10.8 595 19.2 0.3 13.2 0.17 369
1983# 6.45 / 21 56.5 50.3 4.8 38.9 11.8 343 48.8 0.0 7.6 0 0.31 359
” Rist (1955)
# NEA data
+ Sigvaldason (1965)
★ Steinthórsson and Óskarsson (1983)
and the measured maximum ground temperature
is 72°C (in 1981).
Marked changes have not been observed in the
the geothermal activity on the mountain. How-
ever, one of the ice cauldrons on the E-Svíahnúk-
ur collapsed in 1983-84 and a 500 m long, 200 m
wide and 50 m deep depression was formed just
west of Hithóll (Fig.2). This may have been
caused by increased thermal activity. Increased
thermal activity was observed at W- Svíahnúkur
after the eruption in Grímsvötn in May-June
1983. Ice caves were formed on the slopes similar
to those that are usually observed at the eastern
nunatak.
INFLUENCE OF JÖKULHLAUPS ON
RIVER WATER CHEMISTRY
During jökulhlaups the chemical composition
of water in the rivers on Skeidarársandur is influ-
enced by geothermal water from Grímsvötn.
Results of chemical analyses of river water have
been reported by Rist (1955), Sigvaldason (1965),
Steinthórsson and Óskarsson (1983). The present
paper adds further data from the jökulhlaups in
1976, 1982 and 1983, together with data for the
rivers Skeidará, Sandgígjukvísl and Súla when
they are not influenced by jökuihlaups. In addi-
tion, data are presented for other glacier rivers
(Djúpá, Hverfisfljót and Skaftá) which drain
Vatnajökull (Fig.l). Jökulhlaups are not dis-
charged regularly into the rivers Djúpá and
Hverfisfljót but periodic jökulhlaups occur in
Skaftá from ice cauldrons 10 km northwest of
Grímsvötn (Fig. 2, Thorarinsson and Rist 1955,
Björnsson 1977, 1983).
Analyses of river water
Water from Skeidará has been sampled and
analyzed during the jökulhlaups in 1965 (Sig-
valdason 1965), 1972 (Steinthórsson and Óskars-
son 1983), 1976 (National Energy Authority
(NEA) data), 1982 (NEA data and Steinthórsson
and Óskarsson 1983), and 1983 (NEA data). Up
to the year 1982 very little was known about the
normal seasonal changes in the chemistry of the
water. Since the jökulhlaup in February 1982
water from Skeidará has been sampled and analy-
zed regularly. Results of the analyses are pre-
sented in Figs.5-8 and Table 2. Fig. 5 shows the
concentration of Na, Si02 and S04 during the
whole period. The data are believed to represent
normal concentrations. They may also show sea-
sonal variation. There appears to be a maximum
concentration of most components during the
early spring (1983) and a minimum in the late
summer (1982 and 1983). However, during the
JÖKULL 34. ÁR 33