Wastewater Infiltration and Water Movement Around Trenches of Septic Systems

Abstract

In designing septic systems it is often assumed that wastewater infiltration from the trenches into the soil and water movement away from the drainfield occurs uniformly through the soil in all directions. Soils, however, are heterogeneous and vary spatially in all directions. Wastewater infiltration and movement of water through the soil in the drainfield area of a number of actual and simulated septic systems with gravel-filled trenches installed in different soils were assessed. In one study, 56 tensiometers were installed on a 20- by 20-cm grid system perpendicular to the middle trench of three low-pressure pipe septic systems. The distance between the tensiometers in both horizontal and vertical directions was 20 cm. A series of observation wells were also installed in the trenches of these systems to evaluate infiltration rate. In another study, using a tracer dye and bromide, movement of water through the soil around the trenches of four simulated septic systems installed at three sites were assessed. Wastewater infiltration from the trenches into the soil varied along the length of the trenches as well as among different trenches. The equipotential lines generated from the tensiometer readings around the trenches showed that water flow from the trenches through the soil is neither symmetrical nor uniform. Assessment of the distribution patterns of tracer dye and bromide under the trenches of the four simulated drainfields indicated that preferential flow is the main mechanism for water movement when trenches are installed in a structured clayey Bt horizon. When trenches are installed in coarse textured materials above a clayey horizon in sloping areas, most of the water applied to the trenches move away from the area above the clayey Bt layer.