Application of single factor method and fuzzy variable method in water quality assessment
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Abstract:
The rapid social-economic development has caused serious water environmental pollution,and water quality assessment is the basis of environmental water management.However,different assessment methods may yield different water quality grades results.The downstream reaches of the Liao River,Daliao River,and the estuary were focused on Water quality assessment was conducted based on several pollutants,and the applicability of the different methods was discussed.This study may provide scientific references for water quality assessment and management for the terrestrial-marine system. In September 2019 and June 2020,30 sites in the downstream reaches of the Liao River,Daliao River,and the estuary were sampled.Dissolved oxygen (DO),total phosphorus, ammonia nitrogen,and permanganate index were selected as the evaluation indexes for rivers,and dissolved oxygen (DO),active phosphate,and inorganic nitrogen were selected as the evaluation indexes of estuarine waters.The single factor method and fuzzy variable method were selected for water quality assessment.The single factor method defines the grade of the worst single index as the comprehensive water quality grade.The fuzzy variable method uses the concept of opposite fuzzy sets to construct the relative subordinate degree matrix and to determine the weight of each index.Finally, the comprehensive relative subordinate degree was used to evaluate the water quality grade.Two methods were applied to determine the weights of indexes,i.e.,the ordered binary comparison method and the entropy weight method. Results of single factor method showed that in the rivers,DO and permanganate indexes were the major pollutants in 2019,and the water quality varied from Grades Ⅱ to Ⅳ for all sampling sites.In 2020,the total phosphorus was the major pollutant,and 17 sampling sites have the water quality inferior to Grade V.For the estuary,water quality for all sampling sites was inferior to Glade Ⅳ due to the high mass concentrations of inorganic nitrogen.As for the fuzzy variable method,the water quality of the river water varied from Grade Ⅱ to Ⅳ,the estuary water were Grade Ⅱ and Ⅲ based on the ordered binary comparison method.Likewise,the water quality grades based on the entropy weight method were better than the ordered binary comparison method.In comparison,the water quality grades obtained by the fuzzy variable method were generally superior to single factor method both for the rivers and estuary water samples.The results proved that the fuzzy variable method can comprehensively consider the impact of each index on water quality grade by the index weight.The single factor evaluation method often produces "over-protection" results,but it can better reflect the strict requirements of water environment management and is more simple and feasible to implement. Overall,the results indicated that the water quality grades obtained by the fuzzy variable method were generally superior to single factor method.The major pollutants of river water were DO,permanganate index and total phosphorous,and the water quality grades were Ⅳ and inferior to Ⅴ based on the single factor method, while Ⅰ to Ⅲ based on fuzzy variable method.The major pollutants in the estuary were inorganic nitrogen.The single factor method is assessed based on the grade of the worst index,while the fuzzy variable method weighs the impacts of all indexes.Water quality grades using different assessment methods should be comprehensively considered in water environmental management.
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