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

Existing hydrochemical and hydrogeological models of pollution migration and attenuation in the Lincolnshire Limestone aquifer of eastern England have been examined in the light of the results of a groundwater sulphate sulphur isotope investigation. This has allowed the distinction of different sources of sulphate and their relative importance in different parts of the aquifer. The principal sources are 34S-depleted inputs, derived from the oxidation of pyrite within both the aquifer matrix material and the overlying mudstone deposits, and 34S-enriched anthropogenic inputs which are derived from acid rain fallout augmented by agrochemicals. Groundwaters sampled over the outcrop zone of the aquifer have sulphate δ34S dominated by contemporary acid rain inputs in the recharge waters. A down-dip decrease in the 34S content of groundwater sulphate over the unconfined and shallow confined areas of the aquifer is indicative of a progressive increase in the significance of pyrite-derived sulphate in the system. The contribution of sulphate from this source is large and indicates that pollution front penetration (previously based on total sulphate concentrations) is more restricted than previously thought. Moreover, the extent of pyrite oxidation is greater than can be accounted for by dissolved O2 and the additional component corresponds to that which would be expected from reduction of nitrate using pyrite as the electron donor. We suggest that this mechanism is responsible for denitrification in the aquifer, but that it will be ultimately limited by pyrite availability near fissure surfaces where the reaction takes place