Controls on bacterial sulphate reduction in a dual porosity aquifer system: the Lincolnshire Limestone aquifer, England

Chemical and sulphur isotopic analyses are presented of fissure-waters and pore-waters in the deep confined zone of a dual porosity carbonate aquifer. Some of the fissure-waters show good evidence for bacterial sulphate reduc tion, with low concentrations of sulphide present which is strongly to mode rately depleted in S-34 relative to sulphate. The sulphur geochemistry is b est interpreted as mixing between: (i) a reduced water with sulphide simila r to 60 parts per thousand depleted in S-34 relative to sulphate; and (ii) a sulphate-rich water from up-dip in the aquifer. In addition, sulphide oxi dation occurs where sufficiently oxidizing water is drawn in by abstraction s. The large isotope fractionation factor associated with the sulphidic wat ers is probably the result of redox cycling of sulphur with little net redu ction, rather than a true kinetic fractionation factor. By contrast, pore-w aters in the "sulphate reducing zone" show little or no evidence for the ef fects of sulphate reduction, despite the fact that the pore-waters represen t a significant reservoir of sulphate for reduction. Some pore-waters have been modified recognizably by diffusional exchange with the fissure-waters, but the aquifer matrix has not been colonized by sulphate reducing bacteri a, probably because porethroats in the limestone are too small for bacteria to pass. Physical exclusion of bacteria from the aquifer matrix and limite d diffusional exchange are likely to exert fundamental controls on bacteria l redox processes in dual porosity aquifer systems and other systems with l ow permeability due to small pore interconnections. (C) 2000 Elsevier Scien ce B.V. All rights reserved.