Hydrological Evaluation of Septic Disposal Field Design in Sloping Terrains

Abstract

The most common form of onsite domestic wastewater treatment in the United States is the septic system. Although the design of these systems has been well established, no systematic evaluation of septic system performance exists for sloping hardpan soils. In this paper, we develop a simple hydrologic model for assessing the probability of failure for a set of hydrologic conditions, septic loading rates, and soil and landscape parameters that are readily available for sloping soils. To demonstrate the model capabilities, input data for a septic field of a two-person residence in the New York City drinking water basin in the Catskills was utilized. Our analysis showed that the saturated hydraulic conductivity, depth to the impermeable layer, and slope of the drain field are critical parameters to assess in the design and siting of these systems. We concluded that septic systems perform poorly in undulating landscapes where the hydraulic conductivity is low and the impermeable layer is close to the surface. Under prolonged rainfall conditions on these soils, the septic field and downslope field saturate, causing hydraulic failure of the septic system and saturation in the downslope field; as a result, effluent may be routed directly to streams via overland runoff.