Abstract: In this study, a groundwater quality evaluation model was established to assess the groundwater quality reasonably and accurately in the Zhangye Basin by using random forest model (RF). Based on the pH, Cl^-, SO₄^2-, NO₃^-, Na⁺, NH₄⁺, and total hardness observation values of 81 groundwater sampling points in the basin, a comprehensive evaluation of groundwater quality for the whole study area was made. Results indicated that the water quality in the study area can be mainly classified into class Ⅱ, Ⅲ, and Ⅳ. Specifically, water quality in most of the local Ganzhou District was class Ⅱ because groundwater was difficult to be contaminated by surface for the deep water table. However, with shallow water level and poor water quality, most areas in Linze county and Gaotai county had class Ⅲ water quality, especially some areas in Gaotai county even reached class Ⅳ for the highest water level and being located in downstream of the river. Moreover, according to the index of importance, the main factor affecting the groundwater quality in the study area was found to be NO₃^- and the order of the other ions was NH₄⁺, SO₄^2-, Na⁺, Cl^-, total hardness and pH successively. In order to test the validity of the developed model, comparisons were made to the support vector machine (SVM) model and the artificial neural network (ANN) model. Results showed that performances obtained by the three aforementioned models were satisfactory and RF model performed much better than the SVM and ANN models.

Key words: water quality evaluation, groundwater, random forest, Zhangye Basin