Massachusetts Soil Evaluator Practice Test

When effluent moves downward too fast, there isn’t enough contact time in the upper soil where most treatment happens. The soil and its microbes need that residence time and some oxygen to passively filter, adsorb, and biologically degrade the waste. If flow is rapid, the effluent bypasses much of the treatment zone, and the subsurface environment becomes more anaerobic. Under those reducing conditions, organisms work quickly to break down organics but in a way that can lead to slime production and the reduction of iron oxides to iron sulfide (FeS). The FeS forms can coat pore spaces and clog the soil, further hindering flow and treatment. So the net effect is poorer treatment, potential soil clogging, and byproducts associated with anaerobic conditions, which also tends to increase odors rather than reduce them.