Hydrochemistry and solution rates in gypsum karst: case study from the Western Ukraine

The gypsum karst in the Western Ukraine spreads through a large territory covering more than 20,000 km(2) and is represented by a range of stages (evolutionary types), from deep-seated through subjacent to entrenched. Correspondingly, hydrogeological settings of karst development, circulation patterns and chemical characteristics of groundwaters differ substantially between the respective areas. Based on 1,800 analyses, this paper summarises hydrochemistry of the gypsum-hosting Miocene aquifer. The majority of sampling has been performed in conjunction with a study regime of gypsum solution rates by means of standard tablets. In this study, which included 53 tablet stations representing varying conditions of water-rock interaction, 644 weight-loss measurements were made over the period 1984-1992. The highest rates are characteristic of entrenched karst although active dissolution is localised along well-defined sinking streams with short underground courses, rare vertical percolation paths and the water table. Lower but still quite substantial rates are characteristic of subjacent and deep-seated (confined) karst. However, the overall dissolution removal is greater due to higher flow through the gypsum and the larger area of rock-solvent contact. The results are generalised in order to derive the approximate solution rates characterising major situations and to be suitable for modeling purposes.