CONTROLS ON THE GEOCHEMISTRY OF SULPHUR IN THE EAST MIDLANDS TRIASSIC AQUIFER, UNITED KINGDOM

摘要

Changes in downgradient sulphate concentrations in the East Midlands aquifer fall into three categories: (1) modern inputs; (2) pre-industrial inputs; (3) palaeowater undergoing evaporite dissolution. The relative importance of sulphur derived from external (atmospheric and anthropogenic) inputs and water/rock interaction are investigated using sulphur isotopes and other hydrogeochemical indicators (SO_4, Cl, Br/Cl, NO_3, Sr and B). In the mainly unconfined modern groundwaters the wide spectrum of δ~(34)S_(SO_4) values (ranging from 3.1 to 20.9‰) indicates a variety of input sources. Waters with the lowest values of δ~(34)S_(SO_4) (3 to 5 ‰) are associated with high Cl and high Br/Cl values and must contain a component derived from the oxidation of pyrite; their source is Coal Measures mine drainage which recharges the aquifer via rivers crossing the outcrop. Other likely sources of sulphur include rainfall and dry deposition, inorganic agricultural chemicals and possibly sewage sludge. A zone of waters of pre-industrial age may be recognized in the aquifer by their low chloride (6-10 mg·L~(-1)) and SO_4 concentrations (< 10 mg·L~(-1)). In these waters the δ~(34)S_(SO_4) ratios (around 9‰) are significantly higher than present-day rainfall, which is dominated by heavier industrial emissions. These waters represent the composition of Holocene rainfall. Sulphate increases markedly in the oldest (Late Pleistocene) waters in some of which gypsum saturation is reached. The δ~(34)S_(SO_4) in the sulphate-rich waters is relatively constant at around 20‰ and indicates an evaporite source derived either from the topmost Sherwood Sandstone or the lowest Mercia Mudstone strata.