تحليل هيدروژيومورفولوژيك آبخوان كارستي چشمه ساسان-دشت ارژن فارس

Hydrogeomorphological Analysis of Karstic Aquifer (Case study: Sasan Spring, Fars Province)

در دو دهه اخير حفاظت و مديريت صحيح منابع آب موجود در آبخوان‌هاي كارستي، به دليل اهميت ويژه آن‌ها در تامين آب شرب، مورد توجه بسيار بوده است. چشمه ساسان در جنوب غرب استان فارس، در سمت راست رودخانه شاپور و در قاعده سازند آسماري-جهرم ظاهر شده است. آب اين چشمه در شهرستان كازرون در زمينه اقتصاد كشاورزي و تامين آب منطقه بخصوص شهر بوشهر از اهميت زيادي برخوردار است. در اين تحقيق به منظور طبقه بندي آبخوان كارستي و شبيه سازي منحني فروكش آبدهي و تخمين حجم آب ذخيره شده در بخش اشباع و غير اشباع آبخوان كارستي از روش مانجين استفاده شد. نتايج نشان دهنده شيب ملايم منحني فروكش و ذخيره ديناميكي زياد كه كمتر تحت تاثير بارش قرار مي گيرد است. از نظر طبقه بندي مانجين، سيستم آبخوان كارستي منطقه نشتي، داراي عمق زياد و درز و شكاف فراوان بصورت جز يا تماما در زير رسوبات با نفوذپذيري كم محصور شده است. روش مانجين هماهنگي نسبتاً خوبي در شبيه سازي مقادير دبي هاي دوره فروكش طي سالهاي مختلف دارد و حداكثر درصد خطاي روش مانجين در برآورد حجم آب ذخيره شده در آبخوان كارستي برابر با 51/5 درصد مي باشد. بطور متوسط حدود 75 درصد از كل حجم آب ذخيره شده سالانه در آبخوان مربوط به بخش ذخيره اشباع (جريان پايه) و بقيه مربوط به غيراشباع (جريان سريع) است. در اين آبخوان كارستي، شبكه كارستي بخوبي توسعه نيافته است و سيستم از نوع افشان است. با توجه به اينكه عمدتا منطقه از آهك تشكيل شده است اشكال كارستيك در منطقه توسعه يافته اند كه نقش مهمي در جذب نزولات جوي و نفوذ آب دارند و در نهايت با توجه به قرار گيري چشمه ساسان در سازند آسماري-جهرم و حجم آب، كارست زايي در قسمت هاي پايين دست، به صورت انواع لاپيه ها نمايان مي شود.

Karstic aquifers are important sources of water supply around the world and particularly in Iran. Almost 25% of groundwater is stored in karstic formations. In Iran, karstic formations cover 11% of the land surface. More than 80% of municipal water demands in karst-dominated areas (e.g. Fars and Kermanshah provinces) are supplied from springs or wells in karstic aquifers. Karst aquifers are exceedingly heterogeneous in properties due to having double or triple porosity structures, mixed flow nature and varying conduit permeability. Hydrographs of spring flows show overall response of these aquifers to input recharge due to surface flow or precipitation. The shape of recession curve of a hydrograph for a karstic spring is influenced by the aquifer size and hydrodynamic characteristics such as infiltration rate and water flow rate passed through the aeration and saturated zones. In this study, Mangin’s analytical method was applied to classification of karstic aquifer of Sasan spring and also simulation of daily spring flow using the 11 years recorded data sets. In addition, the relation between the geomorphologic evidence and the obtained results by the analytical method was discussed. Methodology Models description The Manginʹs empirical method states that the recession curve of a spring hydrograph is composed of two parts: quick flow and delayed or base flow. This equation incorporates the Mailletʹs exponential formula to explain the base flow. The quick flow in this model estimated by an empirical function which two parameters of ? and ? play main role. The parameter of ? denotes to inverse of recession curve duration, while the parameter of ? indicate the capacity of infiltrated zone to pass the flow. Despite simulation of discharge ordinates during the recession period, the considered methods are also capable to compute the initial and total water volume stored in the karstic formations (in unsaturated and saturated zones) by integrating from discharge equation respect to time. Mangin’s classification of karst aquifers Classification of the karstic aquifer according to the Mangin’s method, He considered two indices, defining the extent of the karst phreatic zone, and characterizing the infiltration conditions. Mangin (1975) defined five fields within the space defined by and . Each field is designed according to data and and values from well known karst systems around the world. Case study Sasan karst spring located in Dashte-Arjan plain in southwest of Farst province with average height of 813 m, mean flow of 1.29 m3/s (40.7 million cubic meters per year), whereas the water type of Ca-Na-HCO3 and Ca-Mg-HCO3 are dominant. The karstic aquifer connected to this spring creats in Asmary formation. This spring is the greatest karst spring in the south of Iran which supplying the agriculture and domestic water demands of people in the region. Computing the SPI for the monthly precipitation data during 1999 to 2009 shows that the 57% of total months lay in the normal region (-1 < SPI < 1), whereas the 18% and 5% of total months are very wet (November and December) and very dry (July), respectively. The Sasan spring discharges the stored groundwater in karst formations averagely 1290 lit/s (or 40.7 million cubic meters per year). Daily precipitation and spring flow data were used for simulation of recession curve ordinates, stored water volume, and also classification of the aquifer. Maximum precipitation and spring discharge are take place in the January and March, respectively wich indicates two month lags. Results and Discussion The recession curve of the hydrographs in the first year (1999) was considered for model calibration (fitting the parameters) and other recessions for model verification. The beat fitted values of parameters including each models were obtained equal to ?= 0.1 1/day, ?=0.77 1/day. Using the fitted parameters, the discharge ordinates in recession period and also volume of storage water in karstic formations for years of 1999 to 2009 were simulated and compared with the corresponding observed values. The results of model stand for domination of fast flow through the unsaturated zone in compared to the slow flow in the saturated karst formations. The indices of k and i for the aquifer are obtained in the ranges of and which denotes to the karst system of this aquifer has a deep phreatic zone, partly or totally confined underneath impermeable sediments, and largely karstified during previous karstification phases which are named ‘‘non-functional karst systems’’. Conclusion Results of this study indicate that the Mangin’s method is capable to simulate the recession curves of the Sasan spring as well. In addition, the indices of Mangin for classification of karstic aquifer shows that the karst systems possess a complex drainage structure responsible for very long, multiannual or secular residence times. However, the paleo-conduit networks existing in their phreatic zone remain partly functional.