A Multistage Model for the Development of Hydrochemical Zonation in Chalk Ground Waters

ABSTRACT Ground waters from the Cretaceous Chalk of east‐central England display hydrochemical zonation comprising four water types. These are, in the general direction of ground‐water flow; Type I – calcium bicarbonate (CaHCO3) waters undersaturated with respect to calcite, Type II – CaHCO3 waters saturated with respect to calcite, Type III – waters showing sulfate (SO4) reduction and softening by ion exchange, and Type IV – saline waters. At first appearance these water types may be interpreted as related members in the classically recognized, long‐term sequence of natural chemical evolution of subsurface waters. This simple model, however, cannot be reconciled fully with all aspects of the hydrochemistry. For example, there is no satisfactory process that could explain an apparent depletion of chloride when moving from Type I to Type II waters, and transitional boundaries are absent between water Types II and III, and Types III and IV. Furthermore, the model lacks credibility in an area where recent geological events have radically disturbed the regional ground‐water flow regime. An alternative hydrochemical zonation model is proposed in which each water type evolves independently of adjacent water types as a function of its own unique aquifer flow history. This multistage model is supported by tritium and radiocarbon evidence and can be related to probable changes in the regional flow regime during the past 125,000 years.