Migration and attenuation of agrochemical pollutants: insights from isotopic analysis of groundwater sulphate

Existing hydrochemical and hydrogeological models of pollution migration an d attenuation in the Lincolnshire Limestone aquifer of eastern England have been examined in the fight of the results of a groundwater sulphate sulphu r isotope investigation. This has allowed the distinction of different sour ces of sulphate and their relative importance in different parts of the aqu ifer. The principal sources are S-34-depleted inputs, derived from the oxid ation of pyrite within both the aquifer matrix material and the overlying m udstone deposits, and S-34-enriched anthropogenic inputs which are derived from acid rain fallout augmented by agrochemicals. Groundwaters sampled over the outcrop zone of the aquifer have sulphate del ta(34)S dominated by contemporary acid rain inputs in the recharge waters. A down-dip decrease in the S-34 content of groundwater sulphate over the un confined and shallow confined areas of the aquifer is indicative of a progr essive increase in the significance of pyrite-derived sulphate in the syste m. The contribution of sulphate from this source is large and indicates tha t pollution front penetration (previously based on total sulphate concentra tions) is more restricted than previously thought. Moreover, the extent of pyrite oxidation is greater than can be accounted for by dissolved O-2 and the additional component corresponds to that which would be expected from r eduction of nitrate using pyrite as the electron donor. We suggest that thi s mechanism is responsible for denitrification in the aquifer, but that it will be ultimately limited by pyrite availability near fissure surfaces whe re the reaction takes place. (C) 2000 Elsevier Science B.V. All rights rese rved.