Several hydrogeologists were asked to provide ratings and weights to each DRASTIC parameter considering the off-diagonal relationship in one half of each matrix. The rating and weight of each parameter were appointed by ranking their significance and suitability. A pairwise-comparison matrix within each level was constructed to empower the decision maker to estimate the contributionof eachfactor independently. Thereafter,the expert opinions were applied into a square matrix to derive the optimized ratings and weights of DRASTIC. As a general rule, CR ≤ 0.1 should be kept for the matrix to be consistent. Table 4 shows the consistent ratios of the parameters, which are all less than 1.