Localized sulfate-reducing zones are present in and near lignite deposits, which generally are associated with clay lenses within the Magothy aquifer (Cretaceous) in Suffolk County. Radiolabeled-acetate studies and Most Probable Number counting series of aseptically collected core samples from lignite-rich sediments indicate that microbial sulfate reduction is the dominant terminal-electron-accepting process (TEAP). The sulfate-reducing zones are characterized by abundant iron-sulfide minerals and proximity to clay lenses that contain pore water with relatively high concentrations of sulfate and dissolved organic carbon. The low permeability of these zones and, hence, the long residence time of ground water within them, permit the preservation and(or) formation of iron-sulfide minerals, including pyrite and marcasite. Pyrite is present in or near clays as replacement of lignite, and marcasite forms as interstitial cement in interbedded sand.
Near the beginning of ground-water flow paths, the localized lignitic zones are exclusively sulfate reducing, and may be surrounded by oxic conditions. The low abundance of 0.5 M HCl-extractable Fe(III) (<1 to 87 ug/g) in core samples is consistent with sulfate reduction as the dominant TEAP. At greater depths and distances along flow paths, microbial analysis of core samples indicate the presence of both sulfate- and iron-reducing bacteria; the coexistence of these TEAPs is confirmed by H2 concentrations in ground-water samples. The varied distribution of these lignite deposits and associated sulfate-reducing TEAPs is common in Cretaceous deposits on Long Island and in other Atlantic Coastal Plain aquifer systems and affects the mobility of iron and other metals. Iron biofouling of public-supply wells, which is facilitated by high concentrations of dissolved iron in ground water, is costly for water suppliers on Long Island; in some cases, the sulfate-reducing TEAPs decrease iron mobility by causing precipitation of iron-sulfide minerals.
Citation: Brown, C.J., Coates, J.D., and Schoonen, M.A.A., 1997, Characterization of localized sulfate-reducing zones in the Magothy aquifer, Long Island, New York (abst.), in Biological aspects of ground water, National Ground Water Association, 1997 National Convention and Exposition, Las Vegas, Nevada, September 3-6, 1997, p. 111.