ABSTRACT
Soil surfaces receive large quantities of organic contaminants, including combustion products, pesticides, and naturally occurring substances. The heterogeneity of surfaces, however, has not allowed successful modeling of photolysis processes as compared to water or air, which offer greater homogeneity. Several factors must be examined and understood before an assessment of the importance of photochemical processes on soil surfaces can be made. The three primary factors which affect photolysis include the following: depth of photolysis, photochemical quenching-sensitization reactions, and transport processes. Photolysis depths are unlikely to be uniform. Because the depth of photolysis on soil surfaces is shallow, any process that moves compounds into or out of the sunlight-exposed zone will affect the rate of photolysis. For water-soluble, photolabile compounds, photolysis in soils is therefore likely to be a significant transformation pathway. Transport of soluble organic compounds to the soil surface in evaporating water or in soil films may provide a substantial increase in the rate of photolysis at soil surfaces.
