Chemical Processes in Seasonal Snowpacks
Martyn Tranter1 and H. Gerry Jones2
We review the processes that may modify the chemical composition of both dry and wet snow
cover, covering processes at the snow/air and snow/ground interfaces and within-pack processes.
The chemical composition of dry snowpacks may be modified by dry deposition, photochemical
processes, sublimation of organic material, and by wind pumping, which may exacerbate the
effects of dry deposition and sublimation. In addition, gases may be added to the snowpack by
diffusion from the underlying soil. Wind transport both adds or removes chemicals from surface
snow, depending on whether or not chemicals are lost from or scavenged by airborne snow crystals.
Whether or not solute is gained or lost from ice crystals during dry snow metamorphism is currently
an area of contention. Finally, rain falling on cold snow adds as a solute source if total freezing
occurs within the pack.
Ripe snowpacks contain liquid water, which enhances the rate of dry deposition at the surface and
allows microbial activity and dissolution/ion exchange reactions to modify the chemical composi-
tion of wet snow. These latter reactions serve to partially neutralize acidic snows in some locales.
Wet snow metamorphism is believed to relocate solute into the adjacent liquid water, so allowing
percolating melt to efficiently scavenge solute from the pack. The micro- and macro-distribution of
solute throughout the pack, the flowpaths in operation, the depth of snow, and the rate of melting all
have an influence on the scavenging efficiency.
To date, attempts to model the variable chemical content of snow cover and snowmelt have been
rather limited, due to the physical and chemical variability in snowcover at both local and
catchment-wide scales. Efforts are now underway to incorporate chemical leaching parameters into
distributed snowmelt models, to improve on the prediction of snowmelt quality at catchment scales.
Traditionally, the chemical composition of snowcover has been the domain of environmental pollu-
tion, since dry snow cover potentially acts as a passive record of atmospheric deposition and serves
as a store of overwinter atmospheric pollutants. Increasingly, the modern trend is to challenge the
notion of snowcover as a passive reservoir of chemical signatures. This being the case, there is a
note of caution for the uncritical acceptance of the chemical signatures frozen into ice cores.
Finally, the chemical composition of snow and snowmelt on glaciers and ice sheets is receiving
attention, since these may have an impact on the amount of chemical weathering that may occur in
Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK
INRS-Eau, 2800 rue Einstein, Ste. Foy, Quebec, G1V 4C7, Canada