Mapping ShahrekordAquifer Vulnerability Using DRASTIC Model and its Verification with Seasonal Variationsof Nitrate and Phosphate concentrations

Vulnerability assessment is applied to identify susceptible areas of aquifers to contamination. Drastics model was designed based on seven components: Depth to water table (D)، Feeding pure (R)، Aquifer media (A)، Soil environment (S)، Topography (T)، Unsaturated environment (I)، and Hydraulic conductivity (C). The aims of this study are: evaluating the Shahrekord aquifer vulnerability using DRASTIC model in GIS environment، verification of DRASTIC model and investigating the effect of nitrate and phosphate concentration seasonal variations on the accuracy of prediction by Drastic model. In order to determine the influence layers، a score was given to each of parameters and the final maps were added together. The final vulnerability map show that 11/5%، 79/6%، and 8/9% of the study area have very low، low and moderately pollution potential (vulnerability)، respectively. Comparison of vulnerability map with maps of seasonal variations of nitrate and phosphate concentrations show that the results of DRASTIC model somewhat depends on sampling time. Therefore، correlation of nitrate or phosphate with DRASTIC map shows significant differences during the years. The greatest and lowest correlation occursin spring and at the end of summer، respectively. Thus، the sampling time must be considered in Drastic model.