Disappearance of microorganisms after sewage infiltration in basins was followed in a plant receiving untreated waste water from 1500 person-equivalents. It had been in continuous use for 13 years, with an estimated infiltration intensity of 50 mm per day. The greatest reduction in bacterial counts occurred within the first meter of the unsaturated zone and was followed by a slight increase in the uppermost part of the saturated zone. From a depth of 2 m the reduction became almost rectilinear. On extrapolation all parameters, except the total counts, approached zero at depths of 16-20 m, corresponding to an elimination time of 107 to 130 days. Following 160 m transport in the horizontal direction, the total number was unreduced at all depths. Refs.
