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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.