ABSTRACTThe U.S. Geological Survey, in cooperation with the Nassau County Department of Public Works, recharged tertiary‐treated sewage (reclaimed water) into the Magothy aquifer in 13 recharge experiments between 1968 and 1973. The recharge resulted in a degradation in water quality with respect to iron concentration and pH. Iron concentration increased from the range 0.14 to 0.30 milligrams per litre to as much as 3 milligrams per litre at the 20‐, 100‐, and 200‐foot or 6.1‐, 30‐, and 61‐metre observation wells as the reclaimed water displaced native water. The increase was presumably a result of pyrite dissolution. The pH of the water decreased from the range 5.22 to 5.72 to a low of about 4.50, predominantly as a result of cation‐exc