# Groundwater Mixing Process Identification in Deep Mines Based on Hydrogeochemical Property Analysis
## 1 Introduction  
With shallow coal resources having been exhausted, deep mining has become an unavoidable trend [1]. As mining activities are conducted deeper and deeper, the hydrogeological conditions of the mine area become increasingly complex, which eventually leads to hazardous events (such as water inrush). Water inrush results in a large amount of groundwater bursting into a work-plane, destroying the mining area and killing miners [2; 3; 4; 5]. Karst collapse columns are typical geological structures in coal fields in Northern China, which can induce water hydraulic connection between adjacent aquifers and threaten to mining activities. Usually, karst develops on the top of Ordovician limestone and underlies the coal bearing strata, and karst will further form into huge karst caves. When the overlying strata lose support, they will collapse and form karst collapse columns under the pressure of gravity [6; 7].  
Seismic exploration is a traditional method for evaluating the risk of karst collapse columns. Moreover, karst collapse columns can be identified by means of hydrogeological modeling. In [6], a three-dimensional (3-D) groundwater flow system was established to reveal different vertical hydraulic connection paths or channels. Meanwhile, the omens of water inrush caused by karstcollapse columns arouse researchers' attention [8]. In [9], the discontinuous deformation analysis (DDA) method was proposed, based on the 3-D seismic technique, to calculate the deformation field and analyze the subsidence mechanism of karst collapse columns. A damage-based hydro-mechanical model was established in [10] to reveal the seepage property and water inrush mechanism of karst collapse columns. This model was then evolved into a fluid-solid coupling model solved by COMSOL Multiphysics (Version 3.5a, COMSOL AB, Stockholm, Sweden, 2008) [7]. However, there is yet to be an effective and accurate method to locate karst collapse columns and determine water conductivity.  
Hydrogeochemical methodologies have been widely used to solve hydrogeological problems in mining [11; 12; 13; 14], such as assessing the environmental effects of acid mine drainage (AMD) [2; 15; 16; 17; 18; 19; 20], tracing the pollution sources in mining areas [21; 22; 23], and studying the leachate of mine tailings [24; 25; 26; 27]. In order to investigate the evolution of the compositions in mining groundwater and the groundwater quality under the influence of ADM, the PHREEQC hydrogeochemical model and the expanded Durov diagram were performed in [28]. In [29], water-rock interaction and hydrogeochemical evolution within a carbonate aquifer system was determined through data evaluation methods.  
However, although existing techniques and models have been proposed to identify karst collapse columns and determine their water conductivity, the accuracy is still an issue. To this end, a new hydrogeochemical method is proposed in this work by properly integrating the piper diagram and PHREEQC model. For the first time, this new hydrogeochemical method is applied to the Liuqiao Second Mine of the Hengyuan Colliery in Huaibei, North China to identify the groundwater mixing process through karst collapse columns, to determine the specific location of water conducted karst collapse columns, and to attempt to recognize specific karst collapse columns at risk of water inrush.