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Injection of highly treated sewage (reclaimed water) into a sand aquifer on Long Island, N.Y., stimulated microbial growth near the well screen. Chlorination of the injectant to 2.5 milligrams per liter suppressed microbial growth to the extent that it did not contJ,ibute significantly to head buildup during injection. In the absence of chlorine, microbial growth caused extensive well clogging in a zone immediately adjacent to the well screen. During a resting period of several days between injection and well redevelopment, the inhibitory effect of chlorine dissipated and mi,crobial growth ensued. The clogging mat at the welVaquifer interface was loosened during this period, probably as a result of microbial activity. Little microbial activity was noted in the aquifer beyond 20 feet from the well screen; this activity probably resulted from small amounts of biotransformable substances not completely filtered out of the injectant by the aquifer materials. Movement of bacteria from the injection well into the aquifer was not extensive. In one test, in which injected water had substantial totalcoliform, fecal-coliform, and fecal-streptococcal densities, no fecal-coliform or fecal-streptococcal bacteria, and only nominal total-coliform bacteria, were found in water from an observation well 20 feet from the point of injection.

From 1968 to 1973, water from public supply and water reclaimed from sewage were used in a series of 19 artificialrecharge experiments at Bay Park, N.Y. Recharge to the Magothy aquife-r was through a fiberglass-cased well with a 16-inch-diameter stainless-~steel screen set from 418 to 480

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