Summary

This study is a review of the major and minor chemical constituents in groundwater in the Jurassic limestones of Gloucestershire, England. Particular emphasis has been placed on the identification of hydrogeochemical processes.

Groundwaters in the limestones of the Great Oolite Series and the Inferior Oolite Series, where they are unconfined, are of the Ca, HCO3 type. In areas where both limestone series are confined beneath clays there is a gradual change in the chemistry to a Na, HCO3 type. Deep within the confined limestones, particularly in the Great Oolite series, an Na, Cl component has been identified. Natural heavy metal concentrations are generally very low except where old pipework has influenced the Pb or Zn content.

Significant down-dip chemical changes in terms of redox conditions, pH, dissolved oxygen content, ion-exchange and the concentrations of minor constituents are exhibited by groundwaters in the limestones of both series. An oxidation-reduction zone has been defined in each limestone unit. In the Inferior Oolite the zone occurs some 8 km down-gradient from the onset of confined conditions. In the Great Oolite a similar zone occurs 1–2 km into the confined area. The delay in the onset of reducing conditions in the Inferior Oolite is tentatively attributed to leakage of Great Oolite water through the intervening Fullers Earth.

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