Jökull - 01.12.1990, Síða 116

According to the discussion above and if the
release of protons to the meltwater is equal to or
greater than the increase in bicarbonate in the re-
maining snow, the order of preferential release of
ions from the partially melted snow is H+ >Mg2+>
Cl_ >Na+ >S042- >K+ >Ca2+ (Figs. 8, 9, and 10).
The most readily released cations, with the same
charge, are the ones with the greatest effective hy-
drated diameter. The smaller the cation at a given
charge, the stronger it is ”attached“ to the snow.
CALCULATED pH OF MELTWATERS DE-
RIVED FROM THE PARTIAL MELTING
OF SNOW
The accumulation of a seasonal snowpack and
the subsequent spring melt produces the largest acid
episodes in streams and lakes in the northem part of
North America, Scandinavia and upland Great Britain
( e.g. Wigington, 1989). It is therefore of interest
to analyse the effect of the degree of partial melting
of snow upon the pH of meltwater. This pH can be
derived from the measured pH of snow and residual
snow samples as discussed below.
In the following discussion snow that is undergo-
ing a partial melting is termed residual snow but the
original snow is simply referred to as snow or the orig-
inal snow. The pH of snow and residual snow samples
is obtained by total melting of the samples in the pres-
ence of air. In this study, melting of snow occurs upon
storage for up to a week in the laboratory at room tem-
perature, prior to pH measurement. Equilibrium with
the atmospheric CO2 is probably closely approached
during that time.
The moisture from which the snow precipitation
is derived is likely to be close to equilibrium with the
C02 of the atmosphere just after its formation over
the ocean. Upon freezing of the moisture higher in
the air some of this CO2 is forced out of solution
although some might be adsorbed on the surface of
the ice and some could be trapped as air bubbles in
the ice. Upon melting in the laboratory, equilibrium
with respect to the CO2 of the atmosphere tends to be
restored, but perhaps at another temperature. As is
discussed in the previous sections the residual snow
is poorer in H+ than the original snow. This tends
to cause more dissolution of atmospheric CO2 in the
water sample when the residual snow melts in the
laboratory, compared to the melting of the original
snow, prior to pH measurements.
Dissolution of CO2 and subsequent dissociation
into bicarbonate produces protons. The concentration
of H+ is higher in the melt water than in the original
snow. This causes less production of protons by disso-
lution of CO2 and subsequent dissociation into bicar-
bonate than if the original snow were melted. Thus,
when using pH measurements of snow and residual
snow to estimate the pH of meltwater, generation or
consumption of protons by C02 dissolution and disso-
ciation or bicarbonate association, must be taken into
account as shown below.
Assuming the conservation of mass, one can write
Ms = Mr + Mm (3)
where Ms is the mass of the original snow in kg, Mr is
residual snow and Mm the mass of meltwater derived
from partial melting of snow. Equation 3 can be cast
in terms of the mass fraction of the original snow
l = Xr+Xm (4)
where, Xr and Xm are the mass fractions of residual
snow and meltwater, respectively. A mass balance for
protons in a unit mass of snow undergoing melting
can be written in terms of the proton concentration
in snow, n\Hs, residual snow, m[Ir, and meltwater,
mHm, taking into account the production of protons
by C02 dissolution and dissociation in the residual
snow, mrc, and the meltwater, mmc, as is discussed
above.
mHs = (mH+ + mH+ ) ö “ -V") +
(mH+ + mH+c) Xm (5)
The proton production by dissociation of C02 into
bicarbonate is equal to the difference between the bi-
carbonate concentration in the original snow,mHCO- »
and the bicarbonate concentration in the residual snow,
mffCO- , and meltwater mHCO~ .
3 r 3 m
mHrc = mHCO~ - mHCO- (6)
112 JÖKULL, No. 40, 1990