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Department of Geography, King's College, University of London, London, UK
The chemical composition of groundwater from the East Yorkshire Chalk exhibits a systematic change in chemistry as it moves downdip from the recharge area of the Wolds to the Holderness lowland. Three distinctive hydrogeochemical zones, coincident with the unconfined, semi-confined artesian aquifer and fully confined aquifer of Holderness exist. Chemical composition in the unconfined aquifer is dominated by carbonate equilibria at a closed system PCO2 of –2.09 with calcite and one mole % Mg-calcite. (Ca2+ =2.3 mmol/l; Mg2+ =0.2 mmol/l; HCO3–=4.08 mmol/l; 
CO2=4.48 mmol/l; pH=7.44). In the semi-confined artesian flow aquifer, ingress of soil CO2 and limited reduction of dissolved O2 increase Co2 to approximately 6.4 mmol/l, whilst Ca2+ increases to 3.69 mmol/l at a PCO2 of –1.76, and pH of 7.17. In the fully confined aquifer, reduction of O2, NO3, SO2–4 occurs, with increased CO2 to 6.86 mmol/l, HCO3– to 6.13 mmol/l, and PCO2 to –1.87. Cation exchange of Ca2+ for Na+ also occurs. Mass transfer calculations show that the major reactions in the aquifer take place in the soil zone, and within the semi-confined zone, with low Mg-calcite undergoing solution whilst calcite is precipitated. Throughout the aquifer, groundwaters are saturated with calcite, after allowing for the effects of ion pairing and complex formation.
