Evolution mechanism of a groundwater system in the opencast coalmine area in the typical prairie

The Yimin coal mine is located in the Hulunbuir Prairie in northeastern China. The development of the mineral exploration of many years has caused tremendous influence on the regional groundwater environment and also threatened the vulnerable prairie ecological system. In this paper, the groundwater system in the Yimin coal mine area is taken as the research object. Characteristics of the groundwater system before and after the mining are compared in order to reveal the interaction mechanism of groundwater and environment under the intensive manual intervention in a prairie mining area. A comprehensive study of the temporal and spatial evolution of groundwater flow system and geo-chemical characteristics in the coal mining area is carried out by means of groundwater flow field comparison, mathematical statistics, comparison of contour map and Piper diagrams. The results show that the open pit coal mining increases access of O 2 to the subsurface and changes the original reduction environment into an oxidation environment, which facilitates sustained pyrite oxidation, acid production, and subsequent reactions. As a result, the pH value decreases, total hardness (TH) and concentrations of SO 2- 4, Fe 2+, Fe 3+, Ca 2+ and Mg 2+ increase significantly. In addition, the hydrochemical type of groundwater changes from HCO 3—Na ·Ca ·Mg type to HCO 3 ·SO 4—Na ·Ca ·Mg type. Under the influence of groundwater drainage of the opencast coal mining, the groundwater circulation is accelerated, which leads to an evolution of desalination of groundwater, and the chemical indicators such as TDS, COD and Na ++K + show a decreasing trend in the coal mine area.