بررسی نقش چینه‎ ‍شناسی در پتانسیل تغذیه در حوضۀ نکارود

حوضۀ نکارود با وسعت 2275 کیلومترمربع در شمال ایران قرار دارد. بخش عمدۀ آهک‌ها در حوضۀ نکارود، شامل آهک‌های ضخیم‌لایه و توده‌ای لار و آهک‌های کرتاسه است و به مقدار کمتری آهک‌های الیکا، دلیچای و آهک‌های روته نیز وجود دارد. خصوصیات مختلفی از قبیل میزان رخنمون، ضخامت و خصوصیات لیتولوژیکی سنگ‌های کارستی و ارتباط با لیتولوژی‌های دیگر در مقیاس ناحیه‌ای، کنترل‌کننده‌های زمین‌شناسی توسعۀ کارست را تشکیل می‌دهند. از میان این عوامل، ضخامت و لیتولوژی سازندهای موجود در منطقۀ مطالعه‌شده، از اهمیت بالایی برخوردار است، به‌گونه‌ای که اگر سنگ‌آهک رخنمون‌یافته ضخیم‌لایه و توده‌ای باشد، معمولاً پدیدۀ کارستی‌شدن با سرعت و شدت بیشتری انجام می‌شود. چینه‌شناسی منطقه و گسترش آهک لار در منطقه، یکی از دلایل غالب‌بودن سیستم جریان مجرایی در این حوضه است. چینه‌شناسی در توسعۀ کارست و گسترش مکانی مقدار تغذیه در سازندهای منطقۀ‌ مطالعه‌شده، نقش موثری دارد و با در نظر گرفتن تاثیر عوامل موثر بر تغذیه با استفاده از gis و سنجش از دور، وضعیت بیلان هیدروژئولوژیکی کارست در منطقه بررسی می‌شود. تعیین بیلان هیدروژئولوژیکی آبخوان‌ها، به‌ویژه در مناطق کارستی، از اهمیت بالایی برخوردار است. تخمین مقدار تغذیه نیز، یکی از مهم‌ترین پارامترهای مطالعات هیدروژئولوژیکی برای محاسبۀ بیلان است. تغذیۀ آبخوان کارستی از‌نظر کمیت و پراکندگی مکانی، به عوامل طبیعی مختلف مانند آب و هوا، توپوگرافی، پوشش گیاهی، خاک و زمین‌شناسی بستگی دارد. نقشۀ پهنه‌بندی پتانسیل تغذیه، با استفاده از چند لایۀ اطلاعاتی شامل لیتولوژی، مقدار شیب، جهت شیب، تراکم آبراهه‌ها، بارش، تراکم شکستگی‌ها و اپی کارست، پس از وزن‌دهی به روش کارشناسی و تلفیق اطلاعات در محیط سیستم اطلاعات جغرافیایی، تهیه شد. با توجه به نتایج حاصل‌شده، منطقۀ مدنظر به 5 بخش با پتانسیل تغذیۀ 12، 20، 28، 36 و 48درصد تقسیم شده است و مساحت مربوط به هر بخش به ترتیب 50، 360، 877،817، 5 کیلومترمربع است. متوسط تغذیه در کل محدوده، حدود 29% برآورد شد. بر‌اساس میزان تغذیه و متوسط بارش سالانه (430 میلی‌متر)، مقدار کل تغذیه در حوضۀ کارستی نکارود حدود 261 میلیون مترمکعب برآورد می‌شود که از این مقدار، حدود 64 میلیون مترمکعب به‌وسیلۀ چشمه و چاه‌های موجود در حوضۀ کارستی تخلیه است؛ بنابراین حجم تخلیۀ سالانه در این منطقه در مقایسه با حجم تخلیۀ سالانه، حدود 197 میلیون مترمکعب بیشتر است. با توجه به قرارگیری حوضۀ کارستی نکارود در نزدیکی دریای مازندران و وجود گسل‌های این منطقه، ‌این‌چنین اظهار می‌شود که به احتمال زیاد بخشی از آب‌های تغذیه‌شده در این حوضه، موجب تغذیۀ آبخوان مجاور‌ و بخشی نیز به دریا تخلیه می‌شود.

investigating the role of stratigraphy in recharge potential in nekarood basin

abstractthe nekarood basin is located in the north of iran and has an area of 2275 km2. in the nekarood basin, the majority of limestones include thick bedded and massive limestones of lar and cretaceous limestones, and there is also a lesser amount of elika, dalichai and ruteh limestones. various features such as the amount of outcrop, thickness and lithological characteristics of karst rocks and their relationship to other lithologies on a regional scale are the geological controls on karst development. among these factors, the thickness and lithology of the existing formations in the studied area are of great importance, in such a way that if the outcropping limestone is thick bedded and massive, the karstification process is very high. the stratigraphy of the region and the spread of the lar limestones in the region are among the reasons for the dominance of the conduit flow system in this basin. stratigraphy has an effective role in the development of karst and the spatial distribution of the amount of recharge in the formations of the studied area, and by considering the effect of factors affecting recharge using gis and remote sensing, the status of the hydrogeological budget of karst in the region is investigated. estimating the amount of recharge is also one of the most important parameters in hydrogeological studies for calculating the budget. the recharge potential map was prepared using several layers of information, including lithology, slope, aspect, density of streams, precipitation, density of fractures and epikarst, after weighting by expert method and integrating information in the geographic information system (gis) environment. according to the obtained results, the desired area is divided into five sections with 12, 20, 28, 36 and 48% recharge potential, and the corresponding area of each section is 50, 360, 877,817, and 5 km2, respectively. the average recharge in the whole range was estimated to be about 29%. based on the amount of recharge and average annual precipitation (430 mm), the total amount of recharge in the nekarood karst basin is estimated to be about 261 million cubic meters (mcm), of which about 64 mcmis discharged by springs and wells in the karst basin, so the annual discharge volume in this area is more than the annual discharge volume of about 197 mcm. considering the location of the nekarood karst basin near the caspian sea and the existence of faults in this area, it can be stated that, most likely, part of the water recharge in this basin is discharged to the sea.keywords: karst, stratigraphy, recharge potential, discharge to the sea  introductionaccording to the definition of ford and williams, karst is a basin with a specific hydrology and morphology, which was created by high dissolution of rock and high development of secondary porosity (williams ford 2007). karst groundwater is one of the most important sources of groundwater worldwide. due to the expansion and diversity of carbonate formations, more than 25% of the world’s population gets their drinking water from karst underground aquifers (williams ford 2007; liu et al. 2019; hao et al. 2021).  as a coastal karst aquifer, the nekarood karst basin can drain its water into the caspian sea through alluvium, faults and deep dissolution channels. due to the climatic conditions (high rainfall) and stratigraphy in this region, no estimation of the amount of recharge in this basin has been done so far. the main purpose of this research is to investigate the role of stratigraphy in the development of karst and the spatial expansion of the amount of recharge in the formations of the region, with considering the effect of factors affecting recharge by using gis and remote sensing, to investigate the karst hydrogeological balance in this area and the amount of recharge. materials methodsin order to investigate the role of stratigraphy on the development of karst in the nekarood basin, firstly, field studies were conducted regarding the stratigraphy and formations of the area. then hydrogeological studies were conducted in this basin. important layers of information affecting recharge, including lithology, amount of slope, aspect, density of streams, precipitation, density of fractures and epikarst have been prepared. according to the nature of each factor, their role in the amount of recharge has been determined. after weighing, these information layers were prepared using an integrated gis and map of recharge potential. then the water balance of the karst basin was estimated according to the amount of feeding, precipitation and discharge. discussion of results conclusionsin the nekarood basin, the majority of limestone includes thick bedded and massive limestones of lar and cretaceous limestones, and there are also dolomitic limestones, elika, delichai and ruteh limestones in a smaller amount. there are about 716 springs in this basin, of which four springs (sanbi, espeh o, jezi and kaocheshmeh) have a flow of more than 10 liters per second and are located in the lar formation in this basin. the lar limestone formation in this basin consists of thick bedded to massive limestones and hydrogeologically has a high potential of karstification. also, there are a lot of dissolution features as well as caves in these limestones. the existence of three important faults in this basin, namely the caspian fault, the north alborz fault and the nekarood fault, has also been one of the important factors in the development of karst in this basin. the flow system in the lar formation, which is dominated by lithology, is a conduit system type. examining the temporal changes of physical (such as discharge) and chemical (electrical conductivity) characteristics also shows that the variability of these parameters is high for the mentioned springs, which indicates the dominant conduit system in this basin and the development of the karst phenomenon in this area. to determine the recharge potential in the study area, factors affecting recharge were classified, and based on expert judgment, points from 2 to 8 were considered for them. based on this classification, wherever the factor has a score of 8, it has the greatest impact on recharge, and wherever the score is 2, it has the least impact on recharge. based on the effective factors, the average recharge percentage in this basin is estimated at 29 percent.based on the investigations conducted in the nekarood karst area, it can be seen that in some parts of this karst basin, the degree of karst development is more evident, and the dominant system is conduit flow. therefore, the main factor of karst development and the dominance of conduit flow system in parts of this basin is thick bedded and massive limestone. in order to estimate the percentage of annual recharge potential in hydrogeological balance studies, in addition to stratigraphy, the influential factors of density of faults and fractures, rainfall, amount of slope, direction of slope, karst effects, density of waterways and soil cover have also been used. using the gis, the influencing factors in the recharge potential have been evaluated and weighed, and finally, the recharge potential map has been prepared. according to the recharge potential map, the highest amount of recharge is related to the thick limestones of the lar and cretaceous layers, which include about 36 to 45% of the recharge potential. the annual recharge in this area was about 261 mcm. the total annual discharge volume of springs and wells was about 64 mcm, and the difference between recharge and discharge is about 197 mcm. considering that the groundwater tends to move towards the base level of erosion and due to the shape of the basin as well as the presence of large faults and bedrock in the region, it is likely that a part of the recharge water is discharged to the alluvium and the caspian sea. however, the estimation of the amount of recharge due to the factors affecting the recharge is associated with uncertainty, which can be reduced by using isotopic and hydrogeochemical samples in the area of water exit from the basin and achieving a favorable result.