The petrographical and petrological characterisation of rocks, which are in permanent contact with groundwater, especially in fractured aquifers, provides important information about the input of elements into groundwater. Groundwater and meteoric water reacts with the rock until chemical equilibrium between the two systems is achieved. For example, the solution of carbonates results in the formation of karst systems over geological time periods. Also, petrogenetic minerals such as feldspar and mica are dissolved from crystalline rocks due to water-rock-interaction (Fig. 1). This causes input of different elements into groundwater.
Fig. 1: Water-rock-interaction (water on Grimsel granodiorite).
The modal mineral composition (Fig. 2), fluid inclusions (Fig. 3), and the chemistry of rocks provide information about the composition of hydrothermal palaeo waters which have circulated once in the aquifers. These information play an important role in the palaeohydrological evaluation of hydrogeological systems and in the prospection of secondary ore deposits.
Fig. 2: Thin section of a gneiss.
The analysis of isotopes of petrogenetic elements offers a valuable tool for the interpretation of water-rock-interaction. The analysis of δ13C of carbonates, the 86Sr/87Sr ratio of feldspars as well as δ34S of sulphates (e.g. gypsum) and sulphides (e.g. pyrite) provides hints concerning the genesis of these minerals. In combination with the isotopy of dissolved compontents information about possible water-rock-interaction can be derived.
Fig. 3: Fluid inclusions.
The isotope ratios of trace elements like uranium and thorium are important tools for the dating of rocks. These methods are commonly used in petrology.
The knowledge about the mineralogical and chemical composition of rocks is also important with respect to dating of old groundwaters using noble gas isotopes. Specific isotopes of the elements uranium, thorium, potassium, and lithium which are present in petrogenetic minerals in different concentrations decay to noble gases. In situ produced noble gas isotopes are released into porewater and further into groundwater. Based on the knowledge about the age of the rock, the element concentration in the rock, and the noble gas isotope concentration in the rock and in the water the age of very old waters can be estimated.