Short-Term Effect of Injection of Tertiary-Treated Sewage on Concentration of Iron in Water in Magothy Aquifer, Bay Park, New York: ABSTRACT

Abstract

High concentration of iron in groundwater poses problems both in the operation of wells and in the suitability of the water for many uses. As part of an experimental deep-well recharge program at Bay Park, New York, the U.S. Geological Survey, in cooperation with the Nassau County Department of Public Works, has been studying the geochemical effects of injecting tertiary-treated sewage into the Magothy aquifer, Nassau County's primary water-supply source. Of particular interest are changes in the iron concentration that have resulted from the injection. Iron concentrations of the injected treated sewage and the native water are relatively low--a range of 0.1-0.4 mg/l (milligrams per liter) for the former and an average of 0.24 mg/l for the latter. However, the iron concentration of the mixed-water (native and injected water) system has exceeded 3 mg/l. Detailed sampling was made at observation wells 20 and 100 ft from the recharge well. The iron concentration at the 20-ft well began to increase coincidently with the arrival of the injected water front. The iron concentration peaked at 3 mg/l after 3 days and then decreased. After 10 days the iron concentration stabilized at about 0.5 mg/l. The primary source of iron is pyrite, which is native to the Magothy aquifer. On injection, the reducing environment around the injection well is displaced by a progressively more oxidizing one. The initial response to this change is the oxidation of pyrite, which releases Fe+2, SO4-2, and H+ to solution: EQUATION (1) Eventually ferric hydroxide precipitates and the Fe+2 concentration decreases: EQUATION (2) These reactions account for the iron peak observed at the 20-ft well. Although the reactions agree with those predicted from changes in the Eh-pH conditions in the aquifer, the presence of other constituents in the reclaimed water seems necessary for the iron peak because the iron peak did not occur when water from the public supply was injected. The iron concentration at the 100-ft well increased to about 2 mg/l after 7 days of injection and then stabilized at this concentration for at least 3 weeks. As the injectant had not completely displaced the native water at the 100-ft well within this time, it is not known whether the concentration would decrease at this well with prolonged injection as it did at the 20-ft well. Also, application of a pyrite-oxidation model to explain any further pickup of iron by the injected water beyond a 20-ft radius is tenuous, as dissolved oxygen in the injectant is reduced within 20 ft of travel and, therefore, is not available to oxidize pyrite beyond that point. Some information about the increase in iron concentration beyond the 20-ft observation well should be forthcoming from a 6-month inj ction test now being made. End_of_Article - Last_Page 1603------------