The poliovirus tended to accumulate in the sludge in the bottom of the tank, with periodic eruptions that caused the virus to leave the tank with the supernatant and, in time, be detectable in groundwater down-gradient from the soil absorption field. We also studied methods of disinfection for the material that is pumped out of the septic tank at intervals (Stramer and Cliver 1984) and modeled the mixing of human (septic tank effluent) and animal wastes (manure slurry) for disposal to land (Snowdon et al. 1989a, b) .