The karst hydrogeochemistry of the Carboniferous limestone of North Derbyshire
Water samples were collected from 20 representative sites in North Derbyshire and N.E. Cheshire at intervals varying from hourly to monthly over a three-year period. All samples were analysed for all major and some minor components. The results were used to evaluate controls on temporal variability, to examine water classification techniques and to identify aquifer types. Additionally, the results from 62 further sites were used with other published results to evaluate geochemical controls on water composition over the whole Derbyshire karst. A complete spectrum of water chemistry types related to hydrological regime was identified by indicators suggested by other workers and new parameters discovered and developed during the present work. The complete spectrum that previous studies have only partly characterized includes highly variable surface streams, intermediate types and saturated diffuse flow resurgences whose variability approaches the precision of the analytical methods. Particularly successful new indicators were the ionic ratios (Ca + Mg)/(Na + K); Ca/(Cl + SO4); Cl/(Na + K) and relative entropy. The final concentration of dissolved calcium was related to the solubility of calcite, calcite precipitation under open system conditions and ion exchange. The dominant control on temporal variability of conduit resurgences was rainfall during the preceding 48 hours. The dominant geochemical3 process was limestone solution and erosion rates of 55 12 m3/km2/yr were deduced, of which 40% occurs at the surface. Other geochemical controls were contact with or the presence of shale, dolomite or lava in the catchment. Further, pollution by road salt or fertilizer and open or closed system evolution were factors. Thermodynamic studies have shown: the dominant ion-pairs are CaSO4, CaHCO3 and MgSO4 in non-thermal waters; the waters are stable with respect only to kaolinite and saturation levels to calcite and dolomite are influenced by the Ca/Mg ratio. Flood pulse and dye tracing studies have shown the Castleton aquifer to be dominantly of a fractured rock type, with some preferential flow paths and many previously unsuspected crosslinks between known caves.