Snowpack Storage of Pollutants, Release during Melting, and Impact on Receiving Waters

Abstract

Information on the snowpack content of major ions, trace metals, and organic contaminants (pesticides and total PCBs) has been reviewed and discussed. Although several limitations exist, regional snowpack surveys have been successfully used to delineate spatial trends in acidic deposition. In contrast to the annual anionic predominance of SO ^2-₄ in atmospheric deposition, NO ^-₃ is often of a similar magnitude or even greater than SO ^2-₄ in the snowpack in locations affected by acidic deposition. Trace metal concentrations are generally greater than tenfold higher at European and North American regional locations than in Arctic or Antarctic “background” sites. The dry deposited component of the total snowpack pollutant load is generally less significant (1% to 45%) than the wet-deposited component, although there is much variability among chemical parameters and locations. There is conflicting evidence on the premelt stability of snowpacks; stability is clearly governed by many factors, and the occurrence of unfrozen underlying soils may be very important. The net radiative energy flux is primarily responsible for melting. Delivery of meltwater is often greatly complicated by the snowpack mesostructure; ice layers and the development of within-pack pipe flow can make modeling of this process very difficult. Rain-on-snow events can be very important both hydrologically and chemically. Fractionation of the pollutants during normal snowpack metamorphosis gives early meltwater ion and metal concentrations that are five- to ten-fold greater than those in the parent snow. Major ions are lost from the snowpack at differing rates during melting, a process known as preferential elution. Springtime reductions in pH, acid-neutralizing capacity, and base cations observed in surface waters occur due to both simple dilution and the differential release of snowpack pollutants. In contrast, lake and stream concentrations of Al and NO ^-₃ typically increase, although the behavior of the latter is variable from location to location. Concentrations of SO ^2-₄ remain comparatively constant, an observation attributed to rapid excahnge of this ion in the soil environment. In lakes, the effect of snowmelt is generally limited to a near surface layer, 1 to 3 m thick. The spring melt event may hold grave consequences for several species of aquatic biota; most reported fishkill events have occurred in Scandinavia.