CHEMICAL AND ISOTOPIC EVIDENCE FOR HYDROGEOCHEMICAL PROCESSES OCCURRING IN THE LINCOLNSHIRE LIMESTONE

Over 150 groundwater samples from the Lincolnshire Limestone have been analysed for pH, major ions and delta-13C ratios. Where possible, field E(h) and iodide concentrations were measured and methane concentrations were determined for 12 samples. Stable isotope ratios were determined for soil and rock carbonate samples. A system of zonation allows the division of hydrogeochemical processes occurring in the aquifer. The use of hydrochemical and isotope data in modelling exercises enables the re-evaluation and possible enhancement of the understanding of hydrogeochemical processes. The carbonate chemistry of outcrop groundwaters is explained by calcite saturation being achieved under open-system conditions in the soil zone. delta13C ratios in the range - 15.99 to -10.57% may be generated from a stoichiometric reaction with possible additional partial and/or simultaneous exchange with soil CO2 or carbonate. The isotopic composition of soil carbonate shows the effects of precipitation from soil waters. The incongruent dissolution of primary depositional limestone carbonate results in increasing magnesium and strontium concentrations and increasing delta-13C ratios for the groundwaters with flow down the hydraulic gradient. As a result of incongruent dissolution, secondary calcite may be precipitated onto fissure surfaces. Significant nitrate and sulphate reduction in non-saline groundwaters is not supported by the results of hydrochemical and isotope modelling exercises. However, sulphate reduction and methane fermentation may be affecting the isotopic and chemical compositions of saline groundwaters. Sodium-calcium ion exchange leads to limited calcite dissolution deep in the aquifer, but the evolution of these groundwaers is confused by the uncertain effects of oxidation of organic carbon and mixing with a saline end-member solution.