Jökull - 01.12.1990, Side 117

m
+
H
mc
mHCO~ mHCO~
(7)
The bicarbonate concentration is higher in the
residual snow than in the original snow, thus m//rc
takes a negative value. However, bicarbonate concen-
tration is lower in the meltwater than in the original
snow, thus mjjmc takes a positive value. The bicarbon-
ate concentrations in equations 6 and 7 can be written
m terms of proton concentration using the equilibrium
constants of Plummer and Busenberg (1982) assuming
air saturation at the temperature of the pH measure-
ments, the partial pressure of CO2 to be 10-3 5 bars
and assuming activity to be equal to molality. Rewrit-
mg equations 6 and 7 at 22° C and pluggingthe results
into equation 5, yields
mH+ =
( mH+ +
/ÍQ-"'30
V mHt
/ 1Q-1130
V mHt
1Q-H.30 \ \
mHt ))
ÍQ-11 30\\
(1 - Xm) +
(8)
Equation 8 can be solved for example for the pro-
ton concentration of meltwater ni//rc which can be
Iranslated to pH of meltwater assuming activity to be
equal to molality. This tums out to be a second degree
cquation with one real solution
ZH±
-b+y/b2 + 4XilO-H'3
2X„
where b is equal to
(9)
lO-n.3 jo-n.3
b = m„+ ---------------------Xmm +
m mnt mH+
lO-n.3
+ Xm----------m + (10)
mH+
The solution of equation 9 is shown in Fig. 11 as
a function of the mass fraction of the original snow
melted, where the pH of the original snow is 5.45 and
•+00 and the final pH of the partially melted snow
that is left is in both cases 5.85. The upper curve in
Fi§- 11 could represent core 1 (Fig. 5). The average
PH of the frozen part of the 1987-1988 layer in core
1 is 5.45 but the average pH of the 1986-1987 layer
Figure 11. The calculated pH (at 22 °C) of water
produced by partial melting of snow is plotted versus
the mass fraction of the original snow that has melted
in the case where the pH of the original snow is
5.00 (the lower curve) and 5.45 (the upper curve) and
the final pH of the remaining residual snow in both
cases is 5.85. —Reiknað pH vatns, sem myndast við
það að snjór bráðnar að hluta, er sýnt sem fall af
massahluta bráðvatnsins. Þegar massahlutinn er 0
er ekkert bráðið en ef hann er 1 þá er allur snjórinn
bráðinn. Neðri ferillinn lýsir sýrustigi bráðvatnsins
þegar pH snœvar fyrir bráðnun er jafnt og 5,00 en sá
efri þegar pH er jafnt og 5,45. Sýrustig snævar, eftir
að hafa bráðnað að hluta er 5,85 í báðum tilfellum.
is 5.85. Constraints on the mass fraction of the winter
precipitation that melts in the summer can be found in
Table 4.2 in Björnsson (1988). Bjömsson measured
the mass balance of the ice drainage basin of Tungná
in 1985-86. Below 1150 m.a.s.l. all of the winter
precipitation melts, but above that there is a continuous
decrease in summer melt with increased altitude. At
1300-1350 m.a.s.l. about 36% has melted, 18% at
1400-1450, 9% at 1500-1550, and 7% at 1600-1650
m.a.s.l. Core 1 was collected at 1680 m.a.s.l. (Fig. 2,
Table II). Thus if the mass balance is similar to the one
measured by Bjömsson (1988) one would expect the
melt fraction for core 1 to be somewhat less than 10%.
According to the upper curve in Fig. 11, the weighted
average pH of the meltwater whose mass fraction of
the original snow is less than 0.1, is below 4.4. In
JÖKULL, No. 40, 1990 113