بررسي آسيب‌پذيري آبخوان دشت نيشابور با استفاده از روش دراستيك در محيط GIS

Evaluation of the vulnerability of Neishabour aquifer by DRASTIC method using GIS

شناسايي و تهيّه نقشه پهنه‌بندي مناطق آسيب‌پذير آبخوان، يعني مناطقي كه امكان نفوذ و پخش آلاينده‌ها از سطح زمين به سيستم آب زيرزميني وجود دارد، يك اَبزار مديريتي مناسب براي جلوگيري از آلودگي منابع آب زيرزميني مي‌باشد. با توجّه به اهميت دشت نيشابور در تامين آب كشاورزي، شرب و صنعت منطقه، از مدل دراستيك جهت اَرزيابي آسيب‌پذيري آبخوان استفاده شده است. در مدل دراستيك از مشخصه‌هاي موثر در اَرزيابي آسيب‌پذيري آبخوان آب زيرزميني شامل عمق سطح ايستابي، شبكه تغذيه، جنس آبخوان، نوع خاك، شيب توپوگرافي، مواد تشكيل دهنده منطقه غير اشباع و هدايت هيدروليكي استفاده مي‌شود كه به صورت هفت لايه در نرم افزار ArcGis تهيّه شدند و با وزن‌دهي و رتبه‌بندي و تلفيق هفت لايه فوق، نقشه نهايي آسيب‌پذيري آبخوان نسبت به آلودگي تهيّه گرديد. با انطباق يون نيترات بر روي نقشه نهايي دراستيك، مشخص شد كليه نقاطي كه داراي نيترات بالا هستند، در محدوده آلودگي زياد قرار گرفته‌اند كه دقّت و صحّت مدل را مورد تاييد قرار مي‌دهد. با توجّه به نقشه پهنه‌بندي به‌دست آمده حدود 55/23 درصد از منطقه مورد مطالعه در محدوده آسيب‌پذيري خيلي كم، 16/17 درصد آسيب‌پذيري كم، 21/19 درصد آسيب‌پذيري متوسط، 72/28 درصد آسيب‌پذيري زياد و 36/11 درصد در محدوده آسيب‌پذيري خيلي زياد قرار مي‌گيرد. نتايج اين تحقيق نشان مي‌دهد بيشترين درصد پتانسيل آسيب‌پذيري مربوط به كلاس زياد است و مناطق جنوبي و غربي دشت از پتانسيل آسيب‌پذيري زيادي برخوردار بوده و نواحي شمالي كمترين پتانسيل را دارند

1. Introduction Identifying and preparing the aquifer zoning map of vulnerable areas, where the pollutants from surface water can infiltrate and spread into groundwater system, is an important management tool for preventing groundwater resource contamination. One of the most important vulnerable aquatic resources against the contamination is the aquifer. The purpose of this research is making the vulnerability map of the region and also detecting the regions with the highest amount of ground water contamination characters in the study area. Regarding to the importance of groundwater resources and its uses for different purposes such as drinking, agriculture and industry, we do the research and also study on the aquifer vulnerability. Also, protecting the region is necessary in developing optimal management of ground water resources. 2. Study Area Neishaboor plain, with an area of 3351/38 square kilometers, is one part of the aquatic basin of the central desert. The plain is located in the southern mountains of Binalood situated in northeastern part of central desert and its climate is mid-dry. The plain is located in the longitude of to eastern hemisphere and the latitude of to northern hemisphere. The highest height of Neishaboor Plain is 3200 m above the sea level over Binalood Mountains and the lowest point is 1050 m above the sea level in the exterior part of the plain. The rainfall is reported to be 292 mm as an average for the plain area. 3. Material and Methods In this research, we have used different information including topographical maps or numerical topographical maps with 1:25000 scale, geometrical maps with 1:100000 scale, soil map, meteorological statistics, hydrology groundwater level, the result of pumping test, digging logarithm and observational well. We also utilized the wells and their data. The statistics of geographical studies maps and water consumption are all included in the research based on khorasan Razavi water stock company. Seven factors or characters that are measurable by the hydrological system are determined in this method. These factors included groundwater depth, net recharge, aquifer media, soil type, topography, impact of vadose zone and the hydraulic conductivity. These characters have numerically been estimated, so a special rate from 1 to 10 is specified to each factor relating the potential contaminants. 4. Results and Discussion Regarding to the layers in DRASTIC model and their special weight, the weight overlapping function was used by the Raster calculator menu in ArcGIS software to combine and compare the layers and integrate the data. The final map of the aquifer in comparison to the contamination was categorized into 5 boundaries or ranges, including extremely low vulnerability, low vulnerability, medium vulnerability, high vulnerability and extremely high vulnerability. One of the flaws noticed in DRASTIC model is that the categorizing and parameter evaluation is fairly expertized and the model should be calibrated by the distribution concentration of natural contaminants such as Nitrate, which mainly leads to special vulnerability of the aquifer. In this research, we have used the data related to Nitrate concentration driven from 57 samples of ground water wells to estimate the model again. 5. Conclusion Regarding to the zoning map, we found that nearly 23.55 percent of the studied Region is located in the lowest vulnerability range, 17.16 percent is located in the low vulnerability range, 19.21 percent is located in the average vulnerability range, 28.72 percent is located in high vulnerability range and finally 11.36 percent is located in extremely high vulnerability range. The results show that the highest vulnerability potential range is high in the southern and western parts of the plain while the northern areas demonstrate the lowest potential. Comparing and contrasting the nitrate ion on the final map of DRASTIC model, it was concluded that areas with the highest nitrate ion concentrations are located in extremely high contaminate force range, emphasizing the precision and accuracy of the model.