Fuzzy Synthetic Evaluation Of Groundwater Quality Based On GIS In Ordos Cretaceous Artisan Basin

Groundwater quality evaluation is one of the most important parts of groundwater resources evaluation in Ordos Cretaceous Artisan Basin. According to the results of groundwater quality evaluation, we can clearly know about the groundwater quality and its distribution in the study area. The evaluation results could be a proper guidance for sustainable exploitation and scientific management of groundwater resources in the study area. And it is also of great importance to prevent the groundwater quality deterioration and establish the scheme of groundwater resources protection. At the same time, a scientific and reasonable result can also guarantee the safety of domestic and productive water consumption.In this research, groundwater quality in study area was evaluated based on the Groundwater Quality Standard (GB/T 14848-1993). First, pH, hardness, TDS, sulfate, chloride, total iron, nitrate, nitrite, ammonia, fluorid, copper, manganese, zinc, arsenic, hexavalent chrome, cadmium and lead were chosen as the evaluation indexes based on groundwater resources present situation as well as actual demands of water consumption in study area. And then Nemerow composite index method was used to evaluate the groundwater quality. And in view of the disadvantages of theGroundwater Quality Standard and Nemerow composite index method in practical application, unreasonable threshold values of some indexes in Groundwater Quality Stardard are modified. At the meanwhile fuzzy synthetic evaluation method was used to evaluate the groundwater quality.Groundwater quality evaluation results were expressed on the platform of ArcGIS. And taking geological,hydrogeological conditions in study area into account, spatial distribution regularities of groundwater quality were analyzed. The evaluation results by these two methods mentioned above show that groundwater quality is quite well in most areas of northern cretaceous basin and vary slightly at different depths except that in the west and north boundaries of northern Cretaceous Basin where the groundwater quality is mainly the IV or V grade because the overlying Tertiary formation has a relatively high salt content. For example, in Wulanmulunt River--Wuding River groundwater subsystem, shallow groundwater, middle groundwater and deep groundwater quality are mostly II grade, so groundwater in this subsystem can be used for not only domestic drinking but also industrial and agricultural production. Groundwater quality in the southern Cretaceous Basin is worse than that in the northern part. For example, in Malian River—Jinghe River groundwater subsystem, water quality of middle and deep groundwater is mainly VI or V grade. And almost all of organic pollutants in groundwater do not exceed the limitation of the Standard by analyzing the water samples collected from the places where organic pollutions would most likely take place (e.g. the residential area, irrigated area, some chemical plants).Finally, the evaluation results of two methods mentioned above were compared. The comparing results indicated that the distributions of II,III,V grade groundwater in the study area by those two methods are almost thesame while the distributions of I and IV grade groundwater are different. And the main reasons leading to difference are as follows:(1)The result of Nemerow composite index method is mainly influenced by the index ( Fm ax) that contributes most to groundwater pollution. And influences of other evaluation indexes are not considered. So once the concentration of one index among the 17 indexes mentioned above exceeds the limitation of the Standard, the groundwater quality evaluation result would most likely to be IV or V grade. However, the fuzzy synthetic evaluation results are more objective because weights of different indexes are well considered.(2)Unreasonable thresholds of some indexes in the Groundwater Quality Standard are modified when using fuzzy synthetic evaluation method.(3)By building membership functions, fuzzy synthetic evaluation method well depicts the fuzzy boundary when classify the quality grade of indexes. While Nemerow composite index method classifies the quality grade of index only by comparing the measured concentration with threshold values specified in Groundwater Quality Standard. So the fuzzy synthetic evaluation method is more objective.(4)In fuzzy synthetic evaluation, the groundwater quality is determined by a matrix operation of the weighted vector with the fuzzy evaluation matrix. While the result of Nemerow composite index method is determined by simply comparing the comprehensive evaluation score (F) with five groundwater quality categories thredholds specified in Groundwater Quality Standard. This also shows that fuzzy synthetic evaluation method is more objective than Nemerow composite index method.The comparison results show that fuzzy synthetic evaluation method is more objective in study area. As long as membership function is built properly, the weights of evaluation indexes are reasonable and the fuzzy operator is well chosen, the groundwater quality evaluation result by fuzzy synthetic method can accurately reflect the actual situations of groundwater quality.