Modelling the fate of non-polar organic chemicals in an ageing snow pack

Falling snow scavenges organic contaminants from the atmosphere, which are thus contained in snow packs accumulating on the ground. During snow pack ageing and metamorphosis, a chemical can be transferred with the melt water to the terrestrial or aquatic environment underlying the snow pack or it may volatilise back into the atmosphere. A model is presented which can provide a quantitative understanding of these processes. After specifying how the physical characteristics of a hypothetical snow pack (such as depth, density, volume fractions, surface area etc.) change during three typical metamorphic phases (snow settling, snow melting, and snow firnification), an organic chemical's behaviour in the snow pack is calculated using an equilibrium partitioning approach. A chemical can partition into the pore space, the liquid water and the organic material contained in the snow pack, or it can adsorb to the air-ice interface. It can be lost from the snow by volatilisation or by drainage with the melt water. Illustrative calculations with 1,4-dichlorobenzene, hexachlorobenzene, gamma-hexachlorocyclohexane, and p,p'-DDT reveal that even though all four selected chemicals partition on to the air-ice interface shortly after snow fall, they show a very different calculated fate during the course of the snow pack's metamorphosis. While the volatile 1,4-DCB is already lost to the atmosphere during snow pack settling, HCB evaporates during the melting phase. The relatively water soluble gamma-HCH is lost with the draining melt water, while DDT is retained by the organic matter. During the formation of glacier ice, substantial fractions of HCB and gamma-HCH contained in the firn can be lost to the atmosphere. The modelling calculations suggest that for a better quantitative understanding of these processes a better knowledge is required of the specific surface area of naturally occurring snow and ice, and of the interfacial partitioning properties of environmentally relevant chemicals. (C) 1997 Elsevier Science Ltd.