Soil texture is a description of the particle size of a given soil. Terms like clay, loam, and sand refer to soil texture. Clays are very small particles, sands are the largest, and silts are in the middle. Found in different portions, these ingredients make up more than a dozen textural classes. Soil texture is an important factor in the sizing of septic drainfields because different textures absorb water or effluent at different rates. Sandy soil may allow the rapid passage of water while clay soils may be most impermeable.

The second important factor in septic system siting is the depth to seasonal saturation or water table. This saturation is the depth at which soil pores become full of water and is where water would be found standing in the hole. The depth varies with the seasons and can rise considerably during wet conditions. Since septic systems must function properly year around, identification of wet season (winter) saturation is necessary.

Saturation depth is important because only unsaturated soils can effectively purify sewage effluent. Saturated soils, on the other hand, allow dangerous bacteria to be carried long distances within the soil, endangering drinking water supplies, shellfish beds, and other valuable resources.

The depth to the zone of saturation is measured directly during the wet months and indirectly during dry summer months. Determination of the depth to saturation can be made by observing subtle color variations in the soil profile. Extremes of color change are seen in gray hardpan, blue clay, or rusty colored sands. More subtle coloration called mottling appears as faint spots of orange or gray. The depths at which mottles are found identify the wet season water table. The soil texture and depth to saturation are but two pieces of information gathered in a site inspection. The lay of the land, soil variation, historic drainage patterns, vegetation, and a multitude of other characteristics are also assessed. These factors, along with the soil analysis, are used to design septic systems which protect the health of the residents and the environment at the lowest cost to the owner.