Isotopic composition of sulfate as a tracer of natural and anthropogenic influences on groundwater geochemistry in an urban sandstone aquifer, Birmingham, UK

Development and management of urban groundwater resources is limited in practice by a lack of knowledge of the nature, distribution and sources of groundwater contamination in an urban aquifer that may have had complex land use and abstraction history. Sulfate is associated with many of the solute sources to urban groundwater and in this study the use of dual isotope (δ18O and δ34S) “fingerprinting” of SO42- in groundwater to assess the contribution of different sources to urban groundwater is investigated. Groundwater (70 locations) and surface water samples from the city of Birmingham on the Triassic Sherwood sandstone aquifer have been analyzed for inorganic chemical and SO42- isotopic composition. Isotopic compositions of SO42- associated with various solute sources have also been determined. Sulfate derived from pyrite oxidation during recharge through Quaternary Drift deposits is characteristically depleted in 18O and 34S compared to other sources and is ubiquitous in unconfined zone groundwater, though the contribution from this source has increased markedly following draw down induced by abstraction from the aquifer. In unpolluted groundwaters other SO42- sources are dissolution of evaporite minerals (confined zone) and rainfall (unconfined zone). Unfortunately, SO42- isotopic composition cannot distinguish between made-ground (i.e. artificial man-made ground, usually demolition waste) and sewage sources of SO42-, which constitute the major contributions to SO42- in polluted urban groundwater at most sites. Contributions of SO42- from spilt industrial acids do have a distinctive isotopic composition seen at many metal-working and former metal-working sites. Estimates of solute contributions derived from these sources derived in this way provide a useful check on the “calibration” of aquifer and pollutant flux recharge models for urban groundwater.