The Harrogate mineral waters exhibit a wide range of mineralization, with the most concentrated reaching half-seawater salinity and containing abundant sulphide. Comparison of new and historical chemical data shows that the major element composition of the main wells has remained essentially constant for over 200 years. The most saline wells have similar chemistry and a number of species (Na+. K+, Li+, Ca2, Mg2+, S2+) behave conservatively with Cl. The more dilute wells have the same characteristic ratios of these species to Cl and thus originate by simple dilution of the brine component. The non-conservative species (mainly HCO3, and SO2−4) are those which are affected by sulphur redox reactions. Stable isotopes show the waters to be meteoric in origin; this and the chemical characteristics of the brines (87Sr/86Sr2+,Sr2+/C1, Li+/Cl) point to a meteoric-derived formational brine as the source of the salinity with dilution by local shallow groundwater.

Sulphur isotope data show the sulphate in the dilute waters could be derived by oxidation of pyrite in Carboniferous mudstones by shallow groundwaters. Total sulphur concentrations in the saline waters are similar to the dilute waters and hence bacterial reduction of sulphate in the local shallow groundwater can account for the sulphide in the wells.

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