GEOCHEMISTRY AND FORMATION CONDITIONS OF GROUNDWATER COMPOSITION IN THE AREA OF MESOZOIC SEDIMENTARY COVER IN TATARSTAN

Mesozoic sediments cover an area of 2870 km². Mesozoic sedimentary rocks are located in the southwestern part of the Republic of Tatarstan. A thick sequence (up to 390 m) are composed of clay with thin (up to 1 m) layers of sandstone and marl. Permian polygenic sulfate-carbonate-terrigenous deposits underlie the Mesozoic massif. They are exposed in the north of the Mesozoic sediment area.The studied area (~5000 km²) is used for farming. There are no large settlements or industrial enterprises there. The paper considers the specific features of Mesozoic and Permian groundwater composition. There is a trend to improving the groundwater quality with time. The reason of this positive trend is a decreasing technogenic impact and depopulation of the territory. The lateral and vertical hydrogeochemical stratification is identified and characterized. Clay predominating in the massif appears to be the reason for the decreasing intensity of water exchange. Hydrolysis, cation exchange and leaching of carbonate and sulfate minerals add to this trend.The groundwater composition varies widely due to these processes and control specific mineralogical and geochemical features of Mesozoic massif (the saturation of organic substance, the presence of opoques, the primary marine ionic-salt compositions in local areas). Groundwater is more mineralized, with a large amount of organic substance, silicic acid, iron, boron and other micro-components in the area of Mesozoic sediment. The groundwater mineralization increases non-linearly to 6.1 g/l (mainly due to sulfates, chlorides and sodium), and the hardness increases to 50.6 mmol/l in the geological section of this area (up to a depth of occurrence 300 m) in areas with downward filtration. The water hardness decreasess to 1.5–3.9 mmol/l at depths of 100–150 m (up to 200 m) in large areas. Vertical zonality (up to a depth of 150 m) is determined mainly by the leaching processes of carbonate and sulfate rocks outside the Mesozoic development area. Mineralization increases almost according to the linear law to 3.5 g/l (mainly due to sulfates, calcium and magnesium), and hardness – up to 37.7 mmol/ l here.

The balneological properties of water are determined by the increased contents of organic substances, silicic acid, iron, hydrogen sulfide, as well as waters of the group “without specific components and properties”.