Hydrogeochemical processes of groundwater system and its mining-motivated effect in mining areas of Western China

Abstract

The exploitation of coal resources will destroy the aquifer structure, disturb the groundwater system and produce a new water cycle model. As the focus of coal supply and production, the western mining area is short of water resources and fragile in ecology. High-intensity and large-scale mining aggravates this disturbance and makes the hydrochemical process such as water-rock interaction more intense. The coal mining-motivated effect of groundwater system hydrochemistry behind it is a key scientific problem related to the prediction accuracy of coal mine safety mining and the protection of groundwater environment in green mining. In view of this, taking the Caojiatan coal mine in western Yushen mining area as an example, using the principles and methods of hydrogeochemistry, from the perspective of ‘what it is’, ‘why it is’ and ‘how it changes’, the results of mining-motivated effect, the process of mining-motivated effect, and the evolution trend of hydrochemistry are systematically studied. The results show that the groundwater in the study area can be divided into five clusters. Cluster Ⅰ represents the shallow groundwater dominated by Quaternary and weathered bedrock aquifers in the west wing of the mine field after coal mining. Cluster Ⅱ represents the mixing of groundwater before and after mining. Cluster Ⅲ represents the groundwater before mining. Cluster Ⅳ and Cluster Ⅴ mainly represent the groundwater of Yan’an Formation after mining. After coal mining, the proportion of HCO3–Ca and HCO3–Mg in the groundwater samples of the fourth and fifth sections of Zhiluo Formation and Yan’an Formation increased, the shallow groundwater quality after mining in the west wing of the mine field was the best as a whole, and the water quality of each aquifer has a tendency to evolve well and is not sensitive to the response of coal mining. The groundwater in the study area is controlled by the ion exchange, and the groundwater in the fourth and above aquifers of Yan’an Formation before coal mining is controlled by the dissolution of carbonate and silicate rocks. The shallow groundwater in the west wing of the mine field after coal mining is controlled by the dissolution of carbonate rocks. The groundwater in the aquifers of the Zhiluo Formation and the fourth and fifth sections of the Yan’an Formation is mainly controlled by the dissolution of silicate rocks and the oxidation of FeS2. The groundwater in the aquifers of the third and lower sections of the Yan’an Formation is mainly controlled by the dissolution of evaporated salt. Coal mining accelerates the circulation speed of groundwater and the hydraulic connection between aquifers. The resulting dilution effect and the discharge measures after mine water treatment are the reasons for the hydrochemical characteristics and water quality evolution of the aquifers in the fourth section of Yan'an Formation and above. In the future, we should continue to do a good job in the discharge of mine water after treatment, and pay attention to the trend of groundwater characteristics in the fifth section of Zhiluo Formation and Yan’an Formation evolving to shallow groundwater, so as to avoid misjudgment of the results of water inrush sources.

Keywords

• groundwater hydrochemistry
• self-organizing maps (som)
• entropy-weighted water quality index (ewqi)
• complex groundwater system in mines
• yushen mining area
• jurassic coalfield