The Study of Heating Mechanism and Hydrogeochemical Modeling of Semnan Thermal Springs

In this study, a temporal and spatial variation of physiochemical and isotopic characteristics of Semnan thermal springs along with five cold springs adjacent to them were investigated to identify the source and hydrogeochemical processes affecting them. Also, to investigate the probable impact of Abgarm river on the water quality of Semnan plain, the result of analysis of major ions in 35 deep wells and qanats have been used. For this purpose, common statistical methods, graph drawing, ion ratios, multivariate statistical analysis, saturation index, modeling and qualitative indices have been used. The results reveal that thermal springs, despite different temperature and salinity, have the same origin, and these differences are due to different ascending flow paths and different conductive cooling mechanism. The chemical composition of thermal waters is controlled by dolomite, halite and sulfate minerals dissolution and calcite precipitation and bacterial sulfate reduction. The concentration of major and trace elements in the thermal springs do not change in time significantly because they are derived from old groundwater with deep circulation. Seasonal change in the concentration of some trace elements is due to the seasonal variation of pH, temperature and, dilution due to the penetration of cold shallow waters with lower chloride levels in the rainy months. The temperature of the heating reservoir based on K-Mg, chalcedony, quartz and chemical equilibria approach was estimated in the range of 60 – 80 ᵒC. Hydrogeologically, a conceptual model and, spring basin were determined. The general groundwater flow direction is from the dolomite Lar Formation in Chenaran anticline toward the adjacent syncline in a confined condition and then in the result of the operation of a thrust fault, it flows to the surface. With the help of the inverse modeling, the amount of dissolution of probable minerals in this direction was estimated. Using Hierarchical cluster analysis (HCA) samples of Semnan plain water were divided into three: bicarbonate, sulfate and chloride groups. Checking the spatial distribution of the samples shows that the samples of each group fall in a specific area; indicating similar affecting processes on water quality in each group. Investigations have shown that saline
water intrusion of Abgarm river water, salts dissolution and, cation exchange are effective in increasing the salinity of groundwater in Semnan plain.

The Study of Heating Mechanism and Hydrogeochemical Modeling of Semnan Thermal Springs Keywords:

Geothermometery , Hydrogeochemistry , Inverse Modeling , Multivariate Analysis , Semnan , Thermal Spring