Abstract: | B ucket-type samplers were installed in the unsaturated zone to intercept septic tank effluent descending through the Chalf at Snowdown, Kent. Better than 90 per cent removal of BOD and COD, and of the order of 99 per cent removal of coiform bacteria, were observed after downward percolation through 2.1 m of Chalk. Nitrification of ammonia had begun at that depth, but not at intermediate depth. The results, though limited, suggest that septic tank effluent is purified in the Chalk in a similar way to settled sewage. |