Use of geochemical and hydrogeochemical data to evaluate the development of karst in the Lar Formation in the area of Gelevard Dam (south of Behshahr)

The Lar carbonate formation has a high potential for karstification, with extensive expansion in the southern parts of Behshahr. Different types of karstic phenomena such as caves and sinkholes have been identified in this formation. For more accurate study of karst in the around of Gelevard dam, 101 thin sections of rock samples related to GV1 borehole located at the left side of the dam were prepared and studied. Then, according to lithological changes and observation of karstic cavities in the cores, 20 rock samples were selected and geochemically analyzed by XRF method. Investigating the changes of trace elements, and Sr / Mn and Na / Mn ratios, showed that the penetration of surface water and the increase of meteoric diagenesis in the karstic zone increased the amount of iron and manganese (1500-90 ppm respectively) and reduced the sodium and strontium (around 390-360 ppm respectively) and their ratio to manganese. Therefore, along with karstification, the alteration of the trace elements occurs at the margin of karst, which goes back to normal by distancing from it. Hydrchemistry analysis of water samples from Gelevard reservoir and drawing of Piper and Duro diagrams shows that the water type in most springs is calcium-magnesium bicarbonate, which corresponds to the type of karst aquifers. Calculating of saturation indexes using the PHREEQC 2.6 software indicates that spring waters are often supersaturated from calcite and aragonite and also dolomite in the dry period. The study of water discharge (Q) and electrical conductivity (Ec) of some springs alongside the saturation index, shows that flow system in karst springs is often conduit- diffuse type.