A GIS, DRASTIC techniques for assessing groundwater vulnerability in Torghabeh –Shandiz watershed of Khorasan county

The state of groundwater pollution in Torghabeh-Shandiz watershed of Khorasan County has become a major problem, particularly in respect of the increasing population, and the agricultural, industrial and commercial development. The present work attempts to interpret the groundwater vulnerability of this watershed using the DRASTIC model together with the geographic information system (GIS). The data which correspond to the seven parameters (depth to water level, net recharge, aquifer media, soil media, topography, the impact of vadose zone and hydraulic conductivity) of the DRASTIC model were collected and converted into thematic maps with GIS. The study showed that the DRASTIC index shows lies in the range of 68 to 225 which were classified into five zones, very low, low, moderate, high, and very high-risk zone of groundwater contamination. The DRASTIC map illustrates that an area of 15.9% is a very low vulnerability, 20.29% low vulnerability, 26.33% moderate vulnerability, 24.34% high vulnerability and 13.14% very high vulnerability in the study area. 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. To check the reliability of the DRASTIC index map in the field condition, groundwater samples were collected for the analysis of nitrate (NO3) which is found as one of the pollutants in groundwater resulting due to use of fertilizers during agriculture. The presence of high nitrate concentration over high to moderate risk pollution zone in the study area validated satisfactorily.