Modeling trench sidewall and bottom flow in on-site wastewater systems

Little is known about how much wastewater infiltrates the soil via the trench sidewall versus the trench bottom in onsite wastewater systems. Our objectives were to develop a method of simulating trench bottom and sidewall flow using a two-dimensional numerical computer model that would include the trench within the model space and determine how much sidewall flow would occur under steady-state conditions. We used HYDRUS-2D to simulate water flow in a two-dimensional cross section of a conventional gravel-filled trench and the surrounding drainfield. Hydraulic properties of the gravel were assumed (saturated hydraulic conductivity K-s=1,000 cm d(-1)) and simulations were run for drainfield soils consisting of a clay loam (K-s=3.2 cm d(-1)) and a sand (K-s=41 cm d(-1)). Biomats were simulated at the bottom of the trench and part way up the trench sidewall (K-s=0.2 to 2.8 cm d(-1) depending on the simulation). Typical wastewater loading rates for Georgia of 2 and 4 cm d(-1) for the clay loam and sand, respectively, were simulated in three daily doses of 1.4 min in length. Simulations were run until the water level in the trench reached a dynamic equilibrium, as indicated by a repeating pattern of water level daily changes. The method we developed predicted the water level in the trench instead of specifying it as a boundary condition, as has been done in previous modeling studies. In the clay loam soil, we found that the water level in the trench at steady conditions averaged a depth of 9.5-10.9 cm and that 29-31% of the total flow occurred through the sidewall. In the sand soil, we found that water did not pond in the trench and there was no sidewall flow. Much of the sidewall flow in the clay loam soil appeared to be in the "lip" area just above the maximum height of the sidewall biomat. Our results show that sidewall flow is important, but not as high as others have estimated.