Seyfe Gölü (Kırşehir) Havzasının Hidrojeoloji İncelemesi

Abstract

Seyfe Lake is located in the south of Seyfe Closed Basin with a surface drainage area of 1481 km2; it is located within the borders of Mucur district of Kırsehir province. It is characterized by normal and strike slip faults that develop in the direction of NE-SW and NW-SE due to the fact that the basin was under the influence of a N-S compression regime. In this thesis study, it was aimed to investigate the relationship of water samples and hydrogeological units within the lake area, the reasons for the ongoing shrinkage in the lake area and to construct a representative conceptual hydrogeological model for the basin. In this regard, detailed geology, hydrology, hydrogeology, hydrogeochemistry, isotope hydrogeology and remote sensing studies were carried out on the basin scale between 2019-2021. The study area is comprised of a basement of Kırşehir Massif consisting of Paleozoic aged metamorphic units, Upper Cretaceous aged magmatic units tectonically covering the metamorphic base, Neogene aged sedimentary units that cover these units and alluvial units that cover all units with unconformity. Hydrogeological evaluations were carried out according to the permeability and conductivity properties of the lithological units that exposed the basin and scaled to 1/100.000 of the hydrogeological map of the study area. The crystallized limestone and marble levels of the Paleozoic aged Bozcaldag Formation, consisting of fragmented surfaces in the north, south and west of the basin, have gained secondary permeability with the effect of tectonism and have been identified as the main aquifer unit. Lithologic characters such as gneiss, schist, amphibole and quartzite belonging to the Paleozoic aged metamorphic series, mudstone and clayey levels of the Neogene aged Kızılırmak Formation, and the Mesozoic aged magmatic units exposed in a limited area in the northeast of the basin are hydrogeologically impermeable. Limestone, sandstone and conglomerate levels of Neogene aged Kızılırmak Formation and gravel and sandy levels with relatively high spread and thickness of alluvial unit consisting of unconsolidated clayey, gravel and sandy levels around Seyfe Lake are defined as semi-permeable units. The average amount of precipitation for the basin was calculated to be 381.45 mm/year, and the amount realized by evaporation-transpiration was calculated to be 326.32 mm/year with the Thornthwaite-Mather method. In order to interpret the hydrogeological structure in the basin, 30 springs, 26 wells and drainage channels, which act as aquifer units, were measured and sampled on-site between 2019-2021 to represent rainy and dry periods. According to the groundwater level measurements made in the wells in May 2021 and October 2021 in the basin, the depth to the groundwater varies between 2 m and 60 m. It has been determined that the general ground water flow direction in the region is towards Seyfe Lake, which is at the center of the basin. Surface and underground water represent the marble aquifers in the basin offering Ca-HCO3 water facies according to the Piper diagram. The water samples representing the drainage channel connected to the lake area offers Na-Cl facies. The water samples collected in the basin are located in the "Rock Dominance" region according to the Gibbs diagram. The main processes controlling the chemistry of the waters were determined through the dissolution of silicates by chemical decomposition of the minerals forming the rock with water due to rock-water interaction. Satellite images were used to determine the changes in the lake area due to climatic and anthropogenic effects, the amount of temperature change and overall changes in the lake surface area were determined in the historical process. Accordingly, with the MNDWI water index, which is one of the spectral water index methods, the minimum and maximum lake area between 1985-2020 was calculated to be 1.86 km2 and 66.87 km2, respectively, and it was calculated that there was a 93.78% decrease in the lake surface area. The maximum depth was calculated as 1.95 m in the north of the lake with bathymetry researches. Seasonal lake surface temperature statistics for 1990 and 2020 were created with lake surface temperature studies. Changes in lake water average temperature were calculated to be 3.5 oC from 1990 to 2020. Corine database was used to determine the effects of human-induced activities on water bodies in the basin. Consequently, the most significant changes among the 1st level land classes were observed in the water structures and wetlands classes, while it was determined that dry lands under the agricultural lands class were converted into significantly irrigated agricultural lands.

Due to the large agricultural areas throughout the basin, the climatic changes over the years and the decrease in surface waters, groundwater is used extensively by the people of the region for agricultural irrigation. The water samples were evaluated in “Wilcox” and “US Salinity” diagrams and it was determined that the majority of the water samples fell into the classes of “Excellent to - Good”, “Good to Permissible” and "C2-S1” and “C3-S1” respectively and were in the group of waters suitable for irrigation water use. According to the saturation analysis studies conducted, it was determined that the water samples in the basin were generally saturated with dolomite, aragonite and calcite minerals except for some samples, and not saturated with silvite, halide, anhydrite and gypsum minerals. This situation shows that when lithological units surfacing in the study area are taken into account, the waters come into contact with the marble units of the Paleozoic metamorphic massif during their circulation periods.

A significant part of the water samples examined according to their stable isotope contents are the waters of meteoric origin falling on the Global Meteoric Water Line (2H=8*O18+10). The water samples representing the mountainous areas in the west and southwest of the basin are located on the Local Meteoric Water Line, which is determined as 2H=8*O18+15. Southwest, southeast and northwest parts of the basin, which represents high mountain ranges, constitutes the potential feeding areas of aquifer units. The Tritium values of the sampling points in the basin vary between 0.4 and 4.52 TU. While high tritium values indicate the presence of recharge from current precipitation, low tritium contents indicate that the aquifer unit representing the water sample is fed with relatively longer groundwater source with a relatively longer residence time and a relatively longer circulation path.