ارزيابي و تهيه نقشه آسيب‎پذيري آلودگي آبخوان كارستي مانشت با استفاده از مدل ريسك

Manesht Karst Aquifer Pollution Vulnerability Assessment and Mapping by Using Risk Model

ارزيابي آسيب‎پذيري و تهيه نقشه پهنه‌بندي خطر، راهكاري مهم در مديريت منابع آب كارست به‎شمار مي‎رود. با توجه به وجود ژيومورفولوژي كارست توسعه‌يافته در مناطق نيمه‎خشك غرب ايران و ديگر شرايط طبيعي منطقه، انتشار آلودگي در اين منابع كارستي سريع و گسترده است. هدف از اين پژوهش، ارزيابي آسيب‎پذيري و تهيه نقشه آسيب‎پذيري آبخوانِ تاقديسِ مانشتِ غرب ايران، در برابر انتشار آلودگي با استفاده از مدل ريسك و ارايه راهكارهاي مديريتي براي حفظ اين منابع است. اين مدل با استفاده از پنج پارامتر سنگ مادر آبخوان، ميزان نفوذ، نوع خاك، تحول كارست و اپي‌كارست، به ارزيابي آسيب‎پذيري منابع آب كارست در برابر آلودگي مي پردازد. نتايج نشان مي‌دهد بخش‎هايي كه آسيب‎پذيري متوسط، زياد و خيلي زياد دارند، 73/96 درصد مساحت منطقه را در بر گرفته‎اند كه از آسيبپ‎پذيري بالاي اين آبخوان در برابر آلودگي محيطي حكايت دارد. مناطق با آسيب‎پذيري بالا، در راس تاقديس و در محدوده سينكول‌ها و چاله‎هاي بسته قرار دارند و اين امر مويد نقش ژيومورفولوژي كارست و توسعه كارست در ميزان آسيب‎پذيري بالاي آبخوان است. اين مدل از كارايي مطلوبي براي انجام مطالعات آسيب‎پذيري منابع آب كارست برخوردار است. در اين راستا عوامل مديريتي، همچون تعيين حريم سينكول‌ها و چاله‌هاي بسته و جلوگيري از ورود عوامل آلاينده به اين مناطق، جلوگيري از تخريب جنگل‎ها، انجام محاسبات هيدرولوژيكي و تعيين حريم آبخوان براي اقدامات مديريتي پيشنهاد مي‎شود.

Introduction Protection of Karst water resources is one of the most important steps in water resources management (Afrasiabian, 2007, 673). Karst aquifers are highly susceptible to pollution since the rapid turnover time and storage capacity of the system is low, then the interference of natural processes such as absorption, degradation and filtrations has less effect (Mudarra and Andreo, 2011, 263). One of the most important steps in Karst water resources management is preparing aquifers vulnerability map. Over the past decades, Zagros karst water resources have been tolerated pressures due to different factors such as increased operations, human pollution and climate change. These factors are in terms of quantity and quality. Manesht anticline contains precious karst water resources which supply Eyvan and Karzan springs and Kangir River which is the main water source of drinking water and cultural water for Eivan Gharb city and villages around. Thus, preparing the vulnerability map of the aquifer seems essential. The purpose of the article is preparing vulnerability map of Manesht aqifer and performance assessment of RISKE model in order to prepare the karst aquifer vulnerability map. RISKE model evaluates the karst aquifers vulnerability by using five important and environmental factors which are litho logy, infiltration rate, soil, karst and epikarst development. Methodology In first step, the case study area has been determined, in order to prepare the karst and epikarst evolution maps, site visit has been done. After the visit, Karst evolution map has been drawn in order to rate the E and K factors. RISKE model is calculated by combining five thematic layers which are litho logy, infiltration rare, soil, Karst and Epikarst developments and then by overlaying the information layers. In the mentioned overlay, vulnerability index has been considered as a linear combination of the five factors. In the model, a point between 1 to 4 has been assigned to each factor based on the environmental characters of the region. Final vulnerability has been calculated by using lg index which is the total weight of five overlaid factors. Ig=?Ri+BIj+?Sk+?Kl+?Em Inthe formula (Ig) comprehensive index of vulnerability is multiplied to ?, B, ?, ? and ?, then each coefficient weight is obtained based on dyadic model 1. Comprehensive index of vulnerability is obtained by overlaying information layers and by using formula (Ig) in which sensitivity analysis has been performed by using the relative importance of the criteria I > E=K > R=S and finally protection index of F has been prepared for vulnerability calculation and final vulnerability map. Results and Discussion For evaluating Aquifer rock factor, the most score has been allocated to Asmari formation (carbonate) (table 1). In evaluating infiltrations condition factor, topographic slope and Karst development has been chosen as the effective factors. The most score has been allocated to pave grounds in top of anticline due to less steep and more developed karst. (Table 2). In soil factor evaluation, the most score has been allocated to rocky soils with coarse texture and low thickness (Table 3). In factor evaluation of “Degree of Karstification”, when the karst is more developed, the pollution penetration is more; the most score has been allocated to top areas of anticline which has more developed karsts (table 4). Scoring Epikarst criteria has been done based on karst development of the region and field visits and the most score has been allocated to top of anticline which has more developed karst (Table 5). Protection amount index (F) has been calculated as follows: F = 0.09R 0.5I 0.08S 0.16K 0.18E Then final vulnerability map of the region has been calculated by F index and overlaying the layers. Conclusion The final vulnerability map has four classes: Low, medium, high and very high vulnerability. Regions with medium, high and very high vulnerability include 96.73 % of the area which shows the vulnerability of the aquifer to environmental pollution. Regions with high vulnerability are in top of the anticline and in sinkhole. This matter confirms the role of Karst geomorphology and karst development in high vulnerability of the aquifer. This model is suitable for vulnerability studies of the karst water resources.