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Soil treatment systems are usually installed in soils with deep water tables at arbitrary depths with little regard to soil properties other than permeability. Variability of soil properties related to onsite wastewater treatment with depth was investigated. Soil forming processes produce differences in properties with depth. Permeability of medium- and finer-textured soils tends to decrease with depth. Distribution of free Fe tends to parallel the distribution of clay in many soils. Translocation of organic C, Al and Fe occurs in some sandy soils with the maximum accumulation usually above 45 cm. Iron and Al react with P to form insoluble compounds and therefore are important for removing P from effluent. If systems are installed below 45 cm in these sandy soils, little P will be removed from percolating effluent. Frost is a major reason for installing soil treatment systems deep in cold regions. In cold climates frost is common near the soil surface but rare at depth unless snow is removed or compacted. Temperatures below 20 cm in natural soils increased with depth and were above 0 o C with one exception on one date and at one site. Soil temperatures measured in and above three soil treatment systems in winter months were all above 1 o C.