Beijing Xishan Ordovician Karst Water Simulation And Analysis Of Groundwater Exploitation Environmental Effects

Beijing Xishan Ordovician karst water is one of the most importment back-up water sources in Beijing. The karst water distributes in Beijing Xishan Mountain and piedmont area, and be-longs to Dashi River basin of Daqing River system and Yongdinghe River system. The water quality is excellent and the mining potential is high. To exploit the water source scientigically and legitimately and ease the geological environment problems caused by the groundwater over-exploitation, some meteorological and hydrological data, basic geological data, hydro-geological data and other related data are collected and managed by the GIS software. Based on the Hydrogeology, groundwater dynamics theory, combined with the analysis of groundwater level data, hydrogeological drilling and profilem, pumping test results, water chemistry and iso-topic results, the system features of the Karst water’s supply, run-off and excretion are re-searched and makes the hydrogeological condition more distinct. On the basis, the conceptual model and numerical model of Ordovician karst water as main are builted with RS, GIS and nu-merical simulation. With the model, the resources of the Ordovician karst water is evaluated, and the best exploitation plan is determined. The results can prove scientific basis for grounderwater exploitation. In the process of building model, some conclusions are acquired as follows:1.The runoff pattern analysis of Ordovician karst water affected by the geological structureNumerous folds and faults distribute in the study area and control the boundary of Ordovi-cian karst water. By analysing the related data, with the groundwater level curve, groundwater chemistry methods and numerical simulation, the affection of the Xiangyu syncline and the Yongding River fracture to the run-off patterns of Ordovician karst water is paid close atten-tion.The results show that: the karst water can not go through the syncline and the Yongding River fracture is an important water channel. About75%of the water will go along the fault to Babaoshan fracture, and the other25%of the water will run along the north owing of the syn-cline to Wenquan area.2. Resources assessment of Ordovician karst waterThe resources evaluation results of Ordovician karst water using the numerical model show that:from2000to2012, the average annual recharge is about11777×104m3/a, which Yongding riverleakage recharge accounts for about76%, the average annual of precipitation supply is about2472×104m3/a; the annual recoverable resources of about8422×104m3/a, the most recov-erable resources is about11699x104m3/a and the least recoverable resources is only about36572×104m3/a. Recently, the karst water has been exploited redundantly and the average annual water balance is about-2868×104m3/a. 3. The research of optimum exploitationAccording to the different status of the target,3programs are designed including status quo exploitation, ideal exploitation (according to the average annual amount of recoverable resource and reducing exploitation yearly. All the programs are simulated using the numerical models.The results show that:from2013to2030, in program1, about34962×l04m3water will be exploited every year, but the water level drops quickly, the water balance of Ordovician karst water is about-4465×104m3/a and that of Quaternary groundwater is about-442×104m3/a; the ground-water level in program2and program3is rising and both water balance is positive,which indi-cates that program2and program3will not cause a large drawdown. Consider the quantity of exploitation, the gorounderwater level and the balance of the groundwater sysytem, the amount of water supplied in program3is the most (28521×104m3/a), and program2is about27988×104m3/a, so program3can be used as the best mining plan.4. Analysis of Environmental impaction caused by groundwater exploitationThe geological environment problems in study area include land subsidence in Shahe, Or-dovician Karst Water pollution in Yuquan Mountain and karst collapse risk in Wali. The differ-ent simulation results show that:As status puo mining plan,the confined water level of Quaternary is falling in Shahe area and the annual rate is about1.7m/a, exceeds the critical rate of decline(lm/a) and will induce land subsidence disaster; Ordovician karst water and Quaternary water level decline in Yuquan Mountain area, the annual rate is about1.2m/a, the Ordovician karst water and Quaternary groundwater can recharge each other, which will lead to the pollution of Ordovician karst wa-ter.The recharge water is about144x104m3/a from Ordovician karst water to Quaternary groundwater, and about64×10m/a from Quaternary groundwater to Ordovician karst water; The groundwater level drops fastest (0.87m/a), the groundwater level below the Wali conglom-erate gravel rock roof elevation, karst collapse risk will be high.According to program2, the confined water level decline has been effectively curbed in Shahe area, and the water level rise slowly; Under the program, Ordovician karst water and Quaternary groundwater level in Yuquan Mountain is recovering, the recovery rate was0.08m/a and0.02m/a respectively. There is no water from Quaternary groundwater to Ordovician karst water, and about925×10m3/a from the Ordovician karst water to Quaternary groundwater;The final level of the groundwater in Wali area is high up to10.6m, higher than Wali conglomerate roof elevation, which means the karst collapse risk is low.Under program3,the confined water level rises rapidly, reaching0.3m/a, which indicates that the risk of the occurrence of ground subsidence is smaller;the Ordovician karst water level and the Quaternary groundwater level continue to recover in YuQuan mountain, the water from Ordovician karst groundwater to Quaternary water reached1450×104m3/a, Ordovician Karst Water pollution risk will reduce; the final water level is about9.7m in Wali area, and the karst collapse risk is also very small.