Mapping the risk of groundwater pollution using GIS (Case study: Sarab plain)

The protection of the quality of groundwater is very important considering its vast usages in agriculture, industry and as drinking water. Removing groundwater pollution is an expensive process and needs a long time. Therefore, it is necessary to prevent the pollution of groundwater resources using proper methods. One of the usual methods is to investigate areas with high potentials to get polluted. The aim of this study choose a suitable method to identify areas sensitive to pollution in Sarab plain.
Two models DRASTIC and SINTACS were used in “Sarab” plain to evaluate the pollution of groundwater resources. First, the vulnerability maps of the aquifer were provided for each model. To do this, the information layers of “depth of groundwater table”, “net recharge”, “aquifer media”, “soil media”, “topography”, “the effect of vadose zone” and “hydraulic conductivity of the aquifer” were prepared in GIS. As the next step, the land use parameter was combined with the vulnerability maps, to prepare risk assessment maps. Also, to consider hydrogeological condition of the studied area, AHP method was used to correctly evaluate the weights and the ranks of the effective parameters and sub-parameters in DRASTICLU and SINTACSLU modified models. The correlation coefficient between the concentration of the Nitrate in groundwater and the calculated indexes for the studied area was calculated using linear regression.
Based on DRASTIC map, 82.62% of the studied area has medium vulnerability potential. However, 14.43% and 12.94% of the area have low to medium and medium to high vulnerability potential respectively. On the other hand, based on SINTACS maps, the vulnerability potential in 76.48% and 23.52% of the studied area is medium to high and low respectively. Also, according to the results of DRASTICLU index, 24.10%, 48.97% and 24.86% of the studied area are evaluated with low, medium and high risk respectively. However, based on SINTACSLU map, 55.35%, 40.82% of Sarab plain are located in areas with medium and high risk respectively. Besides, according to AHP-DRASTICLU index 7.81%, 50.86%, 37.52% and 3.74% of the studied plain are evaluated as areas with very low, low, medium and high risk respectively. AHP-SINTACSLU index shows similar results to AHP-DRASTICLU index.
To be able to choose the best index to evaluate the potential of vulnerability and risk for Sarab plain, the correlations between Nitrate concentration in groundwater on one hand and the amount of each of the used indexes were investigated. According to the results, DRASTIC index with R2=0.40 has the highest correlation among the investigated indexes. However, regarding risk evaluation, AHP-DRASTICLU index has the highest correlation, with R2=0.56. Therefore, to evaluate pollution vulnerability and risk in Sarab plain, these two indexes are the most accurate and trustable.