Jökull - 01.12.1990, Síða 113

charge balance
bution from H+ and HC03-, %
Figure 7. A frequency diagram showing the charge balance according to equations 1 (Fig. 7a) and 2 (Fig. 7b).
'—Súlurit sem sýnir reiknað hleðslujafnvœgi í sýnumfrá Vatnajökli samkvœmt jöfnu 1 (Mynd 7a) og samkvæmt
jöfnu2 (Mynd 7b).
([JVa+] + [K+] + 2[Ca2+] + 2[Mg2+]) - ([C/-] + 2[S042-]) ]
---------------------------------------------------------- * IUU
([Na+] + [K+] + 2[Ca2+] + 2 [M52+] + [Cl~] + 2[502“])/2) J
([H+] + [Na+] + [K+] + 2[Ca2+] + 2[Mg2+] - ([HCOf] + [C/~] + 2[5Q42~]))'
([H+] + [tVa+] + [K+] + 2[Ca2+] + 2[M52+] + [HCOf] + [Cl~] + 2[S02~])/2
melting during the summer 1977. The depth scale is
shaded for the 1986-1987 precipitated layer in Fig. 9.
If a given ion is released more rapidly than chloride
during partial melting the ion to chloride ratio in the
snow decreases. Judging from Fig. 9, magnesium ion
is released faster than chloride since the Mg2+/Cl-
ratio is lower in the partially melted snow than in
the unmelted one. The data for the Na+/Cl" ratio
ís inconclusive because of scatter, but if samples 2.2,
2.3 and 2.20 are excluded (charge balance greater than
30%, see discussion above) the ratio does not change
much upon partial melting, indicating similar rate of
release. Potassium and calcium ions are released at
slower rate than chloride as reflected in high ion to
chloride ratios for the partially melted snow and the
same is true for sulfate and bicarbonate, but some of
Ihe bicarbonate is produced upon melting the snow in
the laboratory as discussed below.
There is an overall preferential loss of anions com-
pared to cations in the partially melted snow. This
is reflected in a positive charge balance not includ-
mg protons and bicarbonate (equation 1). This ex-
cess loss of anions over cations, is balanced by a loss
of equivalent amount of protons, from the snow that
is melting. Thus, the greater the concentration of
potassium and calcium (immobile cations) relative to
sodium and magnesium (mobile cations), the more
protons are needed to balance the preferential loss of
anions from the snow and the lower is the pH of the
melt water. In the presence of air this loss of H+ will
call for a production of protons and bicarbonate in the
subsequent generation of meltwater according to the
reaction H2CO3 —> H+ + HCO3-, diminishing the
effect of proton loss from the snow to the previous
melt. This process is shown in more detail in Figs. 10,
a and b, where the average percentages of equivalent
charges are shown forthe 1987-1988 and 1986-1987
layer in core 1 (Fig. 4 and Table II). The 1987-1988
layer is probably close to the original composition of
the precipitation, although the first 190 cm have un-
dergone partial melting (Fig. 4). Sodium and chloride
are the most important ions in the 1987-1988 snow
(Fig. lOa). After partial melting of the 1986-1987
layer(Fig. 10b,samples l.lOto 1.17inTableII),bicar-
JÖKULL, No. 40, 1990 109