Comparison of comprehensive index method and fuzzy comprehensive method in the evaluation of groundwater quality: A case study in Zunyi City

The comprehensive index method and the fuzzy comprehensive method are widely used in groundwater quality evaluation. The groundwater environment in karst area is vulnerable and the potential pollution sources are complex. In order to better understand the effect of comprehensive index method and fuzzy comprehensive method in assessment of karst groundwater in Zunyi City, Guizhou Province, a typical karst area was selected for study. Applying comprehensive index method and fuzzy comprehensive method to evaluate and contrast analysis groundwater quality of nine representative water points in Zunyi, the results showed: The shallow groundwater quality in Zunyi is generally good, 33 % of the water was grade III or higher; but the individual regional groundwater quality is poor, mainly the NO2-, NH4+, Mn, Na+, Cl-, SO42-, total dissolved solids, total hardness (CaCO3) and Se exceeded the standard. There were six water points whose evaluation results were the same and all were grade II. The remaining three water points were different; in the comprehensive index method all were grade IV, while in the fuzzy comprehensive 2 water points were grade V and one water point was grade III. The main reasons for these differences are the maximum value of the single index are emphasized when calculate the comprehensive value F and the weight of evaluation indexes are not taken into account in the comprehensive index method; however, these disadvantages of the comprehensive index method are resolved in the fuzzy comprehensive method, the proximity of indexes to water quality classification limitation is carefully depicted, and the impact weight of all indexes to groundwater quality are quantified visible, so in the groundwater quality evaluation, the fuzzy comprehensive method is superior to the comprehensive index method.