Groundwater chemistry and water-rock interactions at Stripa

Groundwaters from near surface to a depth of 1232 m in the Stripa granite have been sampled and analyzed for major and trace constituents. The groundwater composition consists of two general types: a typical recharge water of Ca-HCO₃ type (<300 m depth) and a deeper Na-Ca-Cl type (>700 m depth) of high pH (8–10) that reaches a maximum of 1250 mg/L in total dissolved solids (TDS). Intermediate depths show mixtures of the two types that are highly fracture-dependent rather than depth-dependent. Any borehole can vary significantly and erratically in TDS for either a horizontal or vertical direction. The general transition from Ca-HCO₃ type to Na-Ca-Cl type correlates with the depth profile for hydraulic conductivity that drops from 10⁻⁸ m/s to 10⁻¹¹ m/s or lower. Thermomechanical stress (from heater experiments) clearly shows an effect on the groundwater composition that could be caused by changing flow paths, leakage of fluid inclusions or both.

Dissolution and precipitation of calcite, fluorite and barite, aluminosilicate hydrolysis, and addition of a saline source (possibly fluid inclusion leakage) play the major roles in defining the groundwater composition. The low permeability of the Stripa granite has produced a groundwater composition that appears intermediate between the dilute, shallow groundwaters typical of recharge in a crystalline rock terrain and the saline waters and brines typical of cratonic shield areas at depth.