Modeling Trench Sidewall and Bottom Flow in On-Site Wastewater Systems

Abstract

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 Ks =1,000 cm d−1 ) and simulations were run for drainfield soils consisting of a clay loam ( Ks =3.2 cm d−1 ) and a sand ( Ks =41 cm d−1 ) . Biomats were simulated at the bottom of the trench and part way up the trench sidewall ( Ks =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, respecti...