Soil mineral precipitate responsible for septic system failure

Properly functioning conventional trench septic systems rely on soil hydraulic conductivity to disperse effluent into the soil absorption field. Typically, septic systems installed in coarse-textured soils function very well if managed correctly, but in the last decade 21% of the septic system failures in Elkhart County, IN, occurred on the Tyner loamy sand (mixed, mesic Typic Udipsamment). Upon excavation of a few of these failed septic systems, it was apparent that a mineral precipitate had formed adjacent to the trenches, apparently causing the absorption field to fail. Reducing conditions adjacent to the soil-trench interface resulted in migration of Fe and Mn away from the trench and precipitation under oxidizing conditions about 15 cm from the soil-trench interface. Iron oxides fill pores between the sand grains forming in a continuous band about 2-5 mm thick. Further from the trench, manganese oxides are concentrated in a discontinuous and diffuse zone about 25 mm thick.