GIS-based DRASTIC and modified DRASTIC techniques for assessing groundwater vulnerability to pollution in Torghabeh-Shandiz of Khorasan County, Iran

The condition of groundwater pollution in Torghabeh-Shandiz of Khorasan County has become a major problem, especially in regard to the expanding population and human activities. The aim of this study of assessing the groundwater vulnerability of this region using a GIS, DRASTIC, and modified DRASTIC techniques. The layer of seven parameters (depth to the water table, net recharge, aquifer media, soil media, topography, the impact of vadose zone, and hydraulic conductivity) was converted to thematic maps with GIS. The DRASTIC map was combined with the land use map for producing the modified DRASTIC map to assess the effect of land use activities on the groundwater vulnerability. The result of the difference area% between DRASTIC and modified DRASTIC methods shows that the percentage of low vulnerability areas have decreased by 27.4%, by applying this modified method. In contrast, the areas of moderate to very high vulnerability increased by 19.8% and 5.4%, respectively. In DRASTIC and modified DRASTIC methods, the very high vulnerability zone is present towards the northeast region. The river flows from the northeast region of the watershed allows more recharge of water, which may drain the fertilizers from the surrounding agricultural lands along with it to the groundwater and hence leads to groundwater vulnerability of this region. The very low vulnerability and low vulnerability zones are present in the western and central portions of the watershed. To check the reliability of the modified DRASTICindex map in the field condition, groundwater samples were collected for the analysis of nitrate, which is found as one of the pollutants in groundwater resulting due to use of fertilizers during agriculture. The calibration results suggested that the modified DRASTICindex significantly affects the study area. Nitrate is important to obtain better results in the vulnerability map, considering that most of the lands in the study area are agricultural.