Comparison of two methods in groundwater quality assessment in typical karst areas: taking Disu underground river as an example

The appropriate evaluation method of groundwater is the key to ensure the scientific and reasonable evaluation results. Taking Disu underground river system basin area in typical southwest karst area as an example, the fuzzy comprehensive index method and the water quality index method were applied to evaluate the groundwater quality of the representative sampling points of the underground river system. The results showed that the groundwater quality in the whole basin was relatively good and the percent of water points whose quality was worse than Class Ⅲ of Standard for Groundwater Quality (GB/T 14848-2017) was 21.43% and 32.14%, respectively, according to the two assessment methods. The main indicator exceeding the standard was ${\mathrm{NO}}_3^{-}$ , whose maximum detection concentration was 2.3 times of the Class Ⅲ water quality limit, and the over-standard points mainly concentrated in Disu village where the activities of human beings were intense. Among the 28 points, 15 points got the same evaluation results by the two methods and the other points were only one water quality grade different. The water quality index method could meet the requirements of water quality classification and quantitative evaluation, but for the water samples with different over-standard indexes, the comparability was poor. The fuzzy comprehensive index method could precisely reflect the approaching degree of the actual concentration and water quality classification limit of the indexes, quantify the impact weight of all groundwater quality evaluation indexes and make the result more accurate. On the other hand, the calculation for this method was complex, with bad maneuverability, unable to identify the main over-standard indexes, and the impact of the indexes that posed a greater threat to human health and ecological environment may be covered up when quantifying all the evaluation indexes. Therefore, in practical applications, the appropriate evaluation method should be selected according to the monitoring data and evaluation purpose, so that the evaluation results could not only reflect the actual situation of water body, but also meet the needs of management.