Identify the vulnerability potential of groundwater quality of the Birjand Plain using DRASTIC Model and its calibration using AHP method

Water is the most important parameter in the development of human societies. One of the pillars in the development of water resources is the investigation of environmental conditions and environmental compatibility of the project.. Identify areas vulnerable to pollution can play a major role in development strategies. In most area of Iran، which is located in the arid and semi-arid region، groundwater is the most important water supply for agricultural، domestic and industrial uses. In most areas، the risk of groundwater contamination is considered to be a serious restriction for this source. Therefore avoid groundwater contamination in groundwater resources management is essential. One of the methods to identify vulnerable areas is the use of qualitative indicators. Among the qualitative indicators، the DRASTIC index of vulnerability to groundwater pollution has many applications. This index is obtained by combining seven different parameters. Each of the parameters of the model، investigate the potential and the possibility of accepting the contamination of the aquifer and each parameter has a unique weight. So far، most studies which have been done with this indicator have led to aquifer vulnerability mapping and calibration and the model calibration and optimization of input coefficients is less studied. This study was conducted to investigate the vulnerability of Birjand aquifer and increase the accuracy of the model. In this study for estimate the vulnerability to contamination، different hydro-geological data were used. These data are including depth to water table، net recharge، aquifer environment، soil environment، topography، environment unsaturated and hydraulic conductivity. Then، plain vulnerability maps were computed using DRASTIC index with combination of these data. One of the most important and influential parameter in environmental contaminations is nitrate. In this project، in order to evaluate the vulnerability of the Birjand aquifer obtained by DRASTIC model، observed data of nitrate in Birjand aquifer which was tasted in 1390 were used. According to the concentration of the Nitrate in the observation wells; model was calibrated using Analytical Hierarchy Process (AHP). For this purpose، parameters of the DRASTIC model، with respect to the inconsistency rate as stated، was modeled by AHP method using of software provided by the Export choice modeling، and calibration weight and then analysis these weights was done using the incompatibility coefficient. DRASTIC layers was obtained Using interpolation and classification tools in the Arc Gis 10. 2 software. According to the preliminary results، depth to the water table and slope parameters has the highest weight in aquifer data. Also، recharge rate of the aquifer in two parts of urban areas، due to the recycled water، has higher weight than in other parts of the aquifer. Also، according to drilling logs، influence testing، pedological testing and the aquifer environment classified in two classes، the soil environment in four classes and unsaturated zone in tree classes. Considering to the high levels of recharge due to return flow were divided in urban areas and agricultural land and according to the test results of pedological and influence، hydraulic conductivity parameters in three classes. After obtaining the required parameters for vulnerability assessment، vulnerability map of the Birjand. Aquifer was obtained using DRASTIC model. Incompatibility factor was chosen as one of the major constraints to optimize the coefficients and weights DRASTIC model. Based on the obtained results and the value of using AHP method incompatibility factor of less than 0. 08 is selected as the best option for analysis. Figure classification of Birjand aquifer vulnerability was presented based on DRASTIC with weight of AHP modelare including four categories very low، low، moderate and moderate to high. The results show the sensitivity of the aquifer in the outlets due to the high water table. The results provided by the DRASTIC model showed that the model was not accurate enough to identify vulnerable areas and it is need to calibrate the weights of models. Therefore، in this study using Analytical Hierarchy Process (AHP) and observational data of nitrate، model was calibrated in the Birjand aquifer. The results showed that the modified DRASTIC model has the higher accuracy in compare with common DRASTIC model، and there is a good correlation between improved weights using AHP method and the observed nitrate concentration in observation wells.