بررسی علل تفاوت رفتار هیدروژئولوژیکی چشمه های کارستی در مناطق خشک، مطالعه موردی: توده آهکی دوساری، جنوب شرق ایران

Investigating the causes of difference in the hydrogeological behavior of karst springs in arid regions, Case study: Dosari limestone body, SE of Iran



Introduction
In many areas in the world, karst springs constitute the main source of water supply. In recent years, the decline in groundwater storage of alluvial aquifers has led to attention to water resources of hard formations. From a hydrogeological point of view, limestone is one of the most important types of hard formations. Karstification of limestone rocks lead to the development of conduits and fractures networks, which causes the drainage to the karst springs. Karst springs are the most common way of groundwater discharge from the limestone rocks. Sometimes, karst springs with similar geological conditions shows different hydrogeological behaviors. The study of different studies of karst aquifers indicates the influence of factors such as surface geomorphology, altitude, rainfall, temperature, tectonic status, geology, etc. on the hydrodynamic characteristics of karst aquifers.
The Dosari limestone body, belonging to the Oligo-Miocene, is located in the southeast of Jiroft plain. The Dehdivan and Barkhordar springs are the most important springs in the Dosari area. Despite the same origin of the springs, there are differences in the spring discharge rates and its temporal changes. This study was conducted to investigate the hydrogeological characteristics of the Dosari limestone rocks and their springs.
Methodology
The objective of this study is to delineate the karst hydrogeology and hydrogeological behavior of karst springs of the Dosari limestone body. For this purpose, discharge data and water quality of existing springs have been used. Fractures and lineaments have also been extracted and analyzed using geological maps and satellite images. Saturation index of carbonate minerals was calculated to delineate the equilibrium status of Calcite and Aragonite minerals in the groundwater. With respect to the abovementioned data, the primary conceptual model of groundwater flow in limestone aquifer is also presented.
Discussion and Results
The dependence of Barkhordar spring discharge on rainfall is very high. The discharge of this spring reacts to rainfall very quickly. But the Dehdivan spring shows more discharge fluctuations due to rainfall events. There are relatively low temporal changes in the springs chemical parameters during the study period. The low temporal changes in the chemical characteristics of the springs indicate the similarity of the discharge regime of Dehdivan and Barkhordar springs. The discharge of Dehdivan spring has a maximum threshold, after which the fluctuations of the Barkhordar spring discharge will increase. It seems that this discharge threshold also affects the water quality of the springs. In other words, reaching the maximum discharge rate of Dehdivan spring increases the flow of water to the Barkhordar spring and changes the chemical quality of the spring water. In the case of Dehdivan spring, the reverse temporal trend of spring discharge and its electrical conductivity can be seen well. But in the case of Barkhordar spring, the trend is different. These different trends can be a reason for the delayed discharge of the Barkhordar spring. Dosari spring is recharged by rainfall, but Barkhordar spring is affected by both rainfall and discharge threshold of the Dehdivan spring.
Conclusion
According to the results, the springs recharge regime is of the diffuse type. The Dehdivan spring has less distance to the main recharge catchment of the springs and the main direction of groundwater flow in the limestone of Dosari limestone body is from the recharge area to Dehdivan spring and then the Barkhordar spring. The difference in the catchment area of the springs and the water flow paths in the aquifer have caused the difference in the discharge of the springs and its temporal changes. Increasing recharge to Dehdivan spring due to rainfall causes the flow of old stored groundwater to the Barkhordar spring. The entry of these waters into the Barkhordar spring causes abnormal changes in the spring discharge, water quality (Ions concentration and electrical conductivity) and balance status of carbonate minerals in the Barkhordar spring water. So that when the discharge rate of the Barkhordar spring increases, the amount of electrical conductivity and saturation index of carbonate minerals increases. This indicates the discharge of old water stored in the karst aquifer.
Increasing recharge to Dehdivan spring due to rainfall causes the flow of old stored groundwater to the Barkhordar spring. The entry of these waters into the Barkhordar spring causes abnormal changes in the spring discharge, water quality (Ions concentration and electrical conductivity) and balance status of carbonate minerals in the Barkhordar spring water. So that when the discharge rate of the Barkhordar spring increases, the amount of electrical conductivity and saturation index of carbonate minerals increases. This indicates the discharge of old water stored in the karst aquifer.
Key words: Groundwater, Karst spring, Discharge, Chemical parameters, Dosari limestone body.