Jökull - 01.12.1983, Side 86

to be considered for the diíferent types of hlaups, i.e.
involving purely melt water, as in Graenalón
(.Bjömsson 1976), hydrothermally derived melt
water, as in Grímsvötn or Skaftá, and hlauþs
associated with volcanic activity, such as Katla
(Thorarinsson 1975) and in part Grímsvötn:
Type 1 - meteoric. If chemically detectable at all an
initial decrease in dissolved abrasives should be
observed together with an increase of precipitation-
derived ions. If high altitude precipitation is involv-
ed an overall decrease in dissolved load would sup-
posedly be observed.
Type 2 - hydrothermal. An initial increase in
carbonate and sulphate (sulphide) would be
expected, especially for activity of high temperature.
If thermal water is involved, sodium would also be a
feasible monitoring ion (Fig. 3).
Type 3 — volcanic. Only the gaseous species
dissolved in the fluid can be recommended for
monitoring in the purely-volcanic system, since the
degassing of ascending magma would release gases
regardless of geothermal activity near the surface.
Carbonate (pH) is undoubtedly the easiest monitor,
although sulphate(SO|'derived from S02), dissolv-
ed hydrogen (redox monitor), and the noble gases
all might come into consideration.
ACKNOVVLEDGEMENTS
We are indebted to Drs. S. Amórsson, H. Bjömsson, Sv.
Björnsson, E. Gunnlaugsson, and G.E. Sigvaldason for
reading an earlier version of the paper and contributing
valuable crilicisms and imþrovements. The 1972-series of
samples was collected by Mr. Ragnar Stefánsson of Skafta-
fell; the other samples of Skaftá and Skeidará wereprocured
by Mr. Sigurjón Rist, hydrologist at the State Energy
Authority.
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84 JÖKULL 33. ÁR