Soils are highly variable across the landscape. An individual site and its soil profile is only part of the story of water movement in a landscape. The processes at broader scales are very important in deciding on the suitability of an on-site wastewater system and adequately siting and designing it. Subtle changes in slope and environmental conditions frequently provide a variety of soil forming conditions, and are associated with soils with contrasting properties. Site-specific observations made by a qualified, experienced practitioner are necessary for understanding the fine-scale variability of soil properties and local site conditions.
Topography is an important feature of the site and its surroundings. Topography is itself an important soil-forming factor, and has a strong influence on surface and subsurface water flow (hydrology). Sequences of contrasting soils exist along slopes (geomorphic gradients), largely related to the movement of soil, water and dissolved materials (solutes). On a sloping site, rapid changes in soil air and water properties can occur, reflecting past and current drainage and soil transport conditions, parent material, and vegetation differences. Slope grade itself may influence suitability and design. Assessments of a site should include observations of slope gradient, shape, aspect and slope position, particularly as they affect drainage.
Water not only moves over and into the soil, but on sloping sites, water moves through the soil. It is important to understand the dynamics of water movement at the slope scale to understand where and how wastewater will move, and how the seasonal water table and ground water table, as well as surface water, will interact with it.
Observation of the topography around the site will help provide an understanding of the locations of water accumulation and dispersion zones in and on the landscape. Water collecting areas (often with concave shapes) may have more poorly drained soils, while water-spreading areas (convex) may have better drained soils.
The best soils for wastewater treatment are deep, and topography can provide information to help locate them. More soil development may occur where water and soil accumulate. Hence deeper soils may be present in lower slope positions where erosion moves material removed from higher positions and where weathering may have extended to greater depths. However, these sites may have poor drainage and shallow seasonal or permanent water tables.
High, convex sites tend to shed water and to be better drained, but may have been subject to more erosion and less soil development, and hence have limiting conditions such as bedrock at shallow depths.