Predictive Study Of Water Discharge In A Phosphate Mining Pit By Visual MODFLOW

Phosphorus contains compound fertilizer which is used in highly amounts by agriculture. The demand for phosphorus ore and the exploitation degree of phosphorus mine are also greater and deeper. Along with the mining process of phosphorus ore, groundwater gushes constantly from aquifer, which is very critical to predict the changes of groundwater flow field and groundwater discharge from mining.There were many methods used in prediction of groundwater discharge. Such as hydrogeological comparison method, analytical method of steady well flow, analytical method of unsteady flow, method of groundwater discharge curve equation, water balance method, and analytical method. However, those methods were endowed with limitation in the prediction of groundwater discharge from the fractured aquifer in the mining area. As the wide application of computer, groundwater numerical simulation software had been extensive used both domestically and internationally. In this study. Visual MODFLOW, a software characterized by visualization, applicability, high simulation, convenient and flexible, was used to represent the conditions of recharge, runoff, drainage and hydrogeological environment, simulated the dynamic influence of phosphate rock mining on the groundwater environment, and forecasted the distribution of groundwater flow field and change of groundwater discharge in fractured aquifer in mine.The phosphorus mine in this study is located in Leibo County, Phosphate bed submitted stratiform and medium thin ore body. It deposited in Lower Cambrian Maidiping group. The study area experienced a sub-tropical monsoon climate with obvious rainy season and dry season. Mountain ranges and rivers incised deeply in the area. Near the mining area, LiutongRiver and AyigeRiver flowed though the north side and west side of this mine, respectively, then both of them flow into Jinshanjiang River. In this area, folds and faults developed very well, main faults (F1 and F2) had a direct impact on the deposit filling water. Atmospheric precipitation was a main recharge of groundwater.The domain of the model was determined as 9.50 km×10.27 km based on the mining area, sequence, mining elevation, serving period, hydrogeological conditions and the reason of deposit filling water. Rivers and natural watersheds were determined as the boundaries of this model. The model was divided into 6 layers, reflecting Quaternary pore permeable layer, karst fissure medium aquifer in Longwangmiao Group, Canglangpu Group Upper, Qiongzhusi Group Upper, karst fissure strong aquifer in Maidiping Group and the Sinian system Upper of the Dengying Group, respectively. In the calibration of steady state flow model, sensitivity analysis was carried out and parameters and, boundary conditions were adjusted to make sure the modelled groundwater level reached the measured values in observation wells. From the sensitivity analysis of the steady state flow model, we knew that river boundary infiltration parameter, rainfall infiltration recharge and conductivity were significant effected groundwater field distribution in the model while initial water level and water storage coefficient did not. In order to obtained the dynamic change of groundwater discharge, a transient flow model was carried out based on the steady state flow model calibration. In the transient condition, hydrogeological parameters were further optimized according to the dynamic measured data of observation wells and sensitivity analysis. The sensitivity’ analysis of transient flow model showed that rainfall infiltration, boundary conditions, water storage coefficient and initial water level affected the dynamic groundwater level fluctuations significantly. The calibrated transient flow model was then used to forecast the distribution groundwater flow field and change of groundwater discharge.In this study, the groundwater discharge rate was predicted with the equivalent pumping wells. These equivalent pumping wells allowed the groundwater drop down to the mining elevation namely the pumping rate in the equivalent pumping wells is the groundwater discharge rate. Through equivalent pumping wells continuous adjusted the groundwater discharge rate, the water level of model was reduced to quasi mining elevation, and the water discharge of the mining height was determined. Model prediction results showed that:when mining to 1650 m and 1540 m elevation, both groundwater flow of simulation as the same as the current conditions basically,and high lying area was mainly flowed to a low elevation river area of groundwater flow, but part region bias pumping area. When mining to the elevation of 1540 m and 1650 m, the maximum groundwater discharges were 800 m3/d and 1920 m3/d, respectively. Model further predicted dynamic changes of groundwater discharge while the water line dropped to the elevation of 1650 m, the total of groundwater discharge were 37310 m3/d,9340 m3/d and 3030 m3/d in wet, normal and drought seasons, respectively. In this study, Visual MODFLOW was used to conduct quantitative analysis on fractured aquifer flow field changes and groundwater discharge under the different mining scenarios. It can provided theoretical basis for phosphate water gushing and mine safety.