Hydrogeochemical and karstological effects of the activation of water circulation within a gypsum quarry (based on the example of Criva Quarry, Moldova Republic)

Abstract

We describe the hydrogeochemical and karstological effects of the activation of water circulation within a gypsum quarry located in the Prut River valley in the northern part of the Republic of Moldova, just next to the border with Ukraine. The quarry, located near the village of Criva, has been exploiting a gypsum deposit since 1946. Due to the almost complete filling with water of the gypsum layer, which is 25 metres thick, groundwater drainage has been carried out. As a result of the deepening of the quarry to 20 m, the upper part of the aquifer has been drained. Anthropogenic activation of karst processes are visible within the rock mass and on the surface, and since 1977, these have been observed in the labyrinthine Zoluszka Cave, exposed by mining works, which is 92 km long and has a volume of 0.65 million m3. Based on the analysis of archival materials using polynomial regression analysis, the course of water drainage in the quarry was reconstructed for the years 1946–2023. In this period of 77 years, 313,003,504 m3 of water were pumped out of the quarry. In order to model the course of variation of selected characteristics over this time period, a polynomial regression method based on a third-degree equation was used. This decision was dictated by the considerable fluctuations in water drainage levels and the relatively small amount of empirical data, especially in the initial and final time periods during which the study was conducted. On the basis of water chemistry, taking into account the volume weight of gypsum, the mass of gypsum drained in the dissolved state and the volume of karst voids formed were calculated. They amount to 624,435.18 tons of dissolved rock and 328,948.12 m3 of newly formed karst voids, respectively. The average annual rate of chemical denudation under anthropogenic pressure was 4,272 m3/year and the rate of denudation under natural conditions was frequently exceeded by an estimated 50 to 4,000 times on an annual basis, depending on the age assessments of the karst in the area. Currently, karst in the aeration zone includes the labyrinthine system of karst voids of the Zoloushka cave (which are in part filled  with clayey deposits of collapsed and residual origin), and karst in the saturation zone includes the lower part of the gypsum unit with a thickness of 5 m, where new karst voids are formed. The development of karst fissures in this lower part of the gypsum, with a total volume under the cave of 165,000 m3, has caused deformation of the clay bottoms of the passages and their settlement to a depth of 2–3 m. On the surface, underground karst activation is reflected in more than 150 karst sinkholes, which began to form en masse in the quarry area after 1946. Nevertheless, the observed trend of clear degradation of the aquifer, small water inflows to the quarry  and the tendency to stabilize the aquifer level are the premises for preserving the quarry and continuing its drainage after the gypsum exploitation has ended. We recommend reclaiming the quarry and creating a recreational and tourist centre on its basis.