Jökull - 01.12.1990, Side 101

The chemistry of precipitation on the Vatnajökull glacier and
chemical fractionation caused by the partial melting of snow
Sigurður R. Gíslason
Science Institute, University oflceland
Dunhagi 3, IS -107 Reykjavík Iceland
abstract
The chemistry of Icelandic precipitation is dom-
mated by the marine aerosol contribution with the
exception of sulfate and calcium where some ” excess
eoncentration “ is present. The ” excess concentra-
tion“ of sulfate may be partly attributed to anthro-
Pogenic activities. The average pH oflcelandic pre-
apitation is 5.4. The snow on the Vatnajökull glacier
ls for away from any anthropogenic aerosol source,
thus its chemical constituents are primarily of ma-
rine origin. The concentration of chloride in snow
froni the Grímsvötn area on the Vatnajökull glacier
equals one drop of seawater mixed with about 19000
drops of pure water. The concentration of salts in
the snow, collected in June 1988 on the glacier, in-
creases with increased elevation, contrary to what
has been documented for precipitation in Iceland and
other parts of the world. The average salt concen-
tration ofthe 1987-1988 snow layer in the Grímsvötn
area is two to three times greater than the average
salt concentration ofthe 1986-1987 snow layer below
and there, the average pH ofthe 1987-1988 layer is
0-28 to 0.14 pH units lower than the pH ofthe 1986-
1987 layer below. The increase in salt concentration
with increased elevation and the downcore changes
ln chemistry is attributed to chemical fractionation
caused by the partial melting ofsnow. During partial
melting the chemical constituents are preferentially
leached from the snow into the meltwater leaving be-
hind purified snow. However, some ions are more
fcadily released than others. The order of prefer-
ential release of ionsfrom the partially melted snow
is H+ >Mg2+ >Cl~ >Na+ >S042~ > K+ >Ca2+. The
most readily released cations, with the same charge,
are the ones with the greatest ejfective hydrated di-
ameter. The smaller the hydrated cation at a given
charge, the more strongly it adheres to the snow. The
preferential release of protons from the snow causes
the pH ofthe meltwater to be lower than the pH ofthe
residual snow. This is reflected in the relative high pH
of residual snow compared to pristine snow. The pH
ofmeltwaters can be derivedfrom the measured pH of
snow and residual snow samples. Theoretical melting
of about 10% of the original mass of a snow, using
constraints from the Vatnajökull glacier, produces a
meltwater with a pH that is more than 1 pH unit lower
than that ofthe original snow. The lowering ofthe pH
ofmeltwaters can happen by partial melting ofunpol-
luted snow. That is to say, snow that consists ofpure
water and salts but no strong acids. This is caused by
the relative mobility of anions and cations balanced
by the release ofprotons. The higher the concentra-
tion ofthe immobile relative to the mobile cations the
greater the lowering ofthe pH ofthe meltwater:
INTRODU CTION
Episodic acidification of streams and lakes can
occur during hydrological events associated with
snowmelt, rainstorms, or rain on snow events. The
accumulation of snow in the winter time and the sub-
sequent spring melt produce the largest acidification
episodes in the northem part of North America, Scan-
dinavia and upland Britain (Wigington, 1989; Tranter,
1989; Davies, 1989). A lowering of pH by 1 to 1.5
JÖKULL, No. 40, 1990 97