Interpretation of Saturation Hydraulic Conductivity for Designing Septic Systems
Soil Science Department, NCSU
the law describing water flow in the soil is called Darcy’s Law. Darcy’s law relates the amount of water moving through a unit cross sectional area of the soil (commonly referred to a flux or Darcian velocity) to soil hydraulic conductivity and the hydraulic gradient along the flow path of water. Soil hydraulic conductivity can be defined as a measure of the ability of soil to transmit water. When the soil is saturated, almost all the pores in the soil are filled with water and the matric potential is zero. Under saturated conditions (e.g., below a water table), hydraulic conductivity, referred to as saturated hydraulic conductivity and generally denoted by Ks or Ksat, is a constant value for any given time and location within the soil body. The numerical value of the hydraulic conductivity under unsaturated conditions, however, depends on the amount of water in the soil (i.e., soil water content) or how strongly water is held by the soil (i.e., soil water pressure head, h). In general, unsaturated hydraulic conductivity is denoted as Ksat and expressed as K(soil water content) of K(h). Since soil water flow is directly related to the soil hydraulic conductivity, we may be able to use soil hydraulic conductivity for designing the drainfield of septic systems.
In septic systems, wastewater from the septic tank is generally applied to the soil through a series of trenches or drip lines buried below the soil surface. In order for a septic system to function properly, wastewater applied daily to the trenches of the system must infiltrate the soil and move away from the drainfield area within each 24-hour period. Otherwise, the trenches of the system will fill up, resulting in the surfacing of effluent in the drainfield area, and/or the soil under the drainfield become saturated to within the required unsaturated zone below the bottom of the trenches (currently 30cm or 12 inches for single family homes under North Carolina regulations). Regardless of the methodology by which wastewater is applied to the soil (conventional, low-pressure pipe, drip system), water movement through the soil in the drainfield area of septic systems is complex. As a result, there is no easy way of relating soil hydraulic conductivity to the long-term acceptance rate (LTAR) for designing septic systems. for designing septic system, however, one can use the saturated hydraulic conductivity to assess the infiltration of wastewater from the trenches into the soil, to determine if wastewater can infiltrate vertically through the least permeable layer below the bottom of the trenches, and to evaluate lateral transfer of water from the drainfield area. In areas with seasonally high water table, a ground water mounding analysis can be performed to assess the maximum height of water table that can be reached after wastewater application to the drainfield of the system. For areas with deep water table, lateral flow analysis above an impermeable or slowly permeable layer under the drainfield area can demonstrate the potential for hydraulic failure of the system. By completing hydrological analysis of the system, modifications in the design loading rate and/or configuration of the proposed system can be made to confirm that the allowable loading rate under the prevailing regulations can be used for the system under consideration.