Water Flow from Trenches through Different Soils

It is often assumed that soils are homogeneous when designing septic systems or modeling wastewater flow from their trenches. The main objective of this study was to assess water infiltration and movement from the trenches similar to the ones commonly used for on-site wastewater dispersal by septic systems. Four separate experiments, each using a small drainfield with four parallel trenches, were conducted at three sites with different soils. In two experiments the trenches were in the coarse-textured soil above a clayey Bt horizon. In the other two experiments the trenches were in the Bt horizon. For each experiment, $50\mathrm{L}$ of a solution containing potassium bromide and brilliant blue FCF (as a tracer dye) were applied once a day to each trench for 14 or 15 days. A sampling pit was dug perpendicular to the trenches after the tracer solution application, and the distribution of the tracer dye and ${\mathrm{Br}}^{-}$ around the trenches on the two walls of the pit were assessed. Tracer solution infiltration from the trenches was not uniform in any of the experiments. Water flow in the Bt horizon was mainly through macropores. Further, water containing ${\mathrm{Br}}^{-}$ and dye moved a substantial distance from the trenches through macropores when trenches were installed in the Bt horizon. When trenches were installed in the coarse-textured soil above the Bt horizon, most of the tracer solution moved away laterally from the drainfield through the zone above the Bt horizon. Overall, the results indicate that soil morphological properties and soil horizonations, as well as the nature of water movement from trenches must be considered when modeling water flow from septic system trenches or when designing septic system drainfields.