A good understanding of the spatial and temporal variations of the hydrochemical characteristics of sandstone aquifers is essential for quality assessments of the groundwater. This paper investigates the spatial-temporal changes in the characteristics of water from sandstone aquifers in an underground mine, including its hydrochemistry and quality. Based on multi-criteria decision-making and a geographic information system, a water quality index of the water from sandstone aquifers is established by combining ordered weighted averaging and maximizing the deviation in a GIS environment. It is argued that mining activities affect the hydrochemical characteristics and water quality. The method is validated through a case study of the Chensilou coal mine in Henan Province, China, in which the spatial and temporal changes of the water quality in the sandstone aquifers from 2001 to 2016 are analyzed, and the factors that affect the water quality are elaborated. It is found that from 2001 to 2016, which is known as the “golden decade” of coal production, the water quality was better in 2001, which is basically in agreement with the distribution of the elements found in the natural hydrochemistry of the groundwater. In 2006, the chemical composition of the groundwater water changed due to a large amount of human-induced activities, so that the hydrochemistry of the phreatic water was more complex than that of the confined water. The groundwater quality in the studied area gradually improved in 2016 due to investments that restored the environmental balance, and the water quality in the sandstone aquifers was improved as opposed to the situation in 2006.