| Groundwater flow system in Yangzhuang basin is relatively enclosed with one narrow outlet like springs,featuring well-functioned passage and storage.Currently,the exploitation of the karst groundwater resources in Yangzhuang basin have already reached a relatively high level,with large and medium groundwater source fields built,such as Houshiwan,Xishilou,Weizhuang and Yangzhuang,and so on.Numerous studies demonstrated that the human activities(like groundwater over-exploitation,pollution)bring serious negative effects on the karst ecosystems,such as the groundwater level decrease,and no flow of karst springs.Furthermore,in recent years,the quality of karst groundwater in Yangzhuang basin was found to become worse and worse.Especially,the nitrate pollution excess the standard value covers more than 1/3of the basin,and the concentrations is 2.5 to 3 times greater than the standard value in some areas,posing great threat to urban water supply safety.Up to date,the problems caused by the human activities have caused great attention of the governments and scientists,and many investigations,observations,and mapping works have been conducted.However,most of previous studies investigate the processes in the surface water and groundwater separately.For example,the surface water body is simply taken as the source and sink of the groundwater flow system,ignoring the fluidity of surface water,which may lead to some deviation in the evolution rules and prediction of groundwater flow and solute transport in the aquifer of Yangzhuang basin.Therefore,the surface water and groundwater are integrated into a whole body to simulate and predict the water and chemicals transport from the following three aspects:water resources management(water quantity),contaminant transport(water quality)and aquifer remediation(treatment),improving water supply safety of the groundwater flow system in Yangzhuang basin.From the perspective of water resources management(water quantity),the full-coupled model of SWAT and MODFLOW is applied to simulate groundwater flow,where the SWAT model is used to describe the flow process of the surface water and MODFLOW is used to describe that of the groundwater.After the calibration of the SWAT-MODFLOW model,it will be combined with the management model in Yangzhuang basin to evaluate the water resources management.Under the optimization target of maximum exploitation in 10 ground water resources in the basin and the constraint condition that the groundwater level was not lower than the controlled groundwater level,evaluations are conducted on exploitable resources under different conditions to come up with responding optimized exploitation proposals and ultimately,to provide scientific support for high efficient and sustainable utility of groundwater resources in Yangzhuang basin and to provide water resource guarantees for regional ecological safety and economic development.From the perspective of contaminant transport(water quality),the reactive transport model was established based on groundwater flow system in the coupled model,and was used to predict the migration mechanism of typical pollutants(nitrate)in Yangzhuang basin.The influence factors were analyzed using this model.As for the aquifer remediation(treatment),the hydraulic interception technology and bio-remediation technology were used to investigate the remediation efficiency for the nitrate pollution.The main conclusions of this study are as follows:?A coupled simulation model of groundwater and surface water in Yangzhuang basin was established(groundwater quantity)A coupled model of SWAT-MODFLOW was established to describe the interactions between the groundwater and the surface water in Yangzhuang basin.The parameter identification and verification of the model were conducted based on monthly measured flow of Guanzhuang hydrologic station in the outlet of basin together with dynamic data of 25 observation wells of the basin.The results showed the calibration is reliable,which demonstrated that this model could be used to represent the conceptual model of groundwater-surface water interactions.?Optimal exploitation volume of groundwater resources in Yangzhuang basin(groundwater quantity)The coupled model of SWAT-MODFLOW is double-coupled with the optimal management model of GWM in this study to solve the problems related to groundwater-surface water management.The groundwater resources in Yangzhuang basin is evaluated systematically from two aspects:the maximum exploitation volume of groundwater resources in entire basin and maximum exploitation volume of groundwater source fields.The result shows that the exploitable volume in Yangzhuang basin is 5 745×10~4m~3/a(15.74×10~4m~3/d);the optimal volumes in the groundwater source fields including Songzhuang,Houshiwan,Nanzhuang,Weizhuang,Xishilou,Xupo,Longshantou,Yangzhuang,Dongyu,Duanzhuang and Wanghang are 0.18,1.4,2.6,1.86,3.8,1.7,0.4,0.4,0.5,1.4 and 1.5×10~4 m~3/d,respectively.Under the condition when finishing the reservoir building in Yangzhuang,its exploitable volume could reach7285×10~4 m~3/a(19.96×10~4m~3/d);the optimal volumes in the groundwater source fields including Songzhuang,Houshiwan,Nanzhuang,Weizhuang,Xishilou,Xupo,Longshantou,Yangzhuang,Dongyu,Duanzhuang and Wanghang are 0.18,2.06,3.40,2.24,4.15,1.86,0.72,1.31,0.63,1.75 and 1.66×10~4m~3/d,respectively.?Prediction of nitrate pollution in groundwater of the Yangzhuang Basin(groundwater quality)Based on the water flow system by the coupled model,the solute transport model of the basin was further established.The numerical simulation of nitrate transport in the groundwater of Yangzhuang basin was carried out,and the main influencing factors were systematically analyzed.According to the prediction results of nitrate concentration,if no actions were taken to control the contaminant transport,the nitrate concentration in the groundwater of the Yangzhuang fault block will increase,which is mainly caused by the excessive use of nitrogen fertilizer by local farmers and the random accumulation of farm manure in the basin.At the same time,according to the prediction results,the concentration of nitrate in the groundwater in the Lulujing area is high.This is mainly because many large-scale poultry farms have been built in the past,even though these poultry farms that pollute the groundwater have been dismantled around 2010.Meanwhile,the necessary site soil remediation was not carried out at that time,and it will take a long time to completely be remediated in the polluted groundwater.Therefore,we suggested that relevant departments should take necessary actions to control the surface pollution sources of agricultural fertilization,and remedy saturated soils and groundwater contaminated by previous farms to reduce the possible impact of residual pollutants on groundwater.?Optimal hydraulic capture volume of groundwater pollutants based on stochastic modeling(pollution remediation)As the spatial scale of Yangzhuang basin is large,Equivalent Porous Media(EPM)is adopted in the coupled SWAT-MODFLOW model to simulate the migration of groundwater and pollutants in karst aquifer in Yangzhuang groundwater flow system.Such a method has a high accuracy in regional scale but it could not precisely indicate the heterogeneity in the aquifer partially,which may lead to an unreliable result in smaller scales.Therefore,based on the defined macro parameters for the regional groundwater model,stochastic simulation was conducted to represent the permeability of reality to study the effect of the heterogeneity in on-site remediation test field for nitrates pollutants in Yangzhuang basin on the hydraulic capture system of groundwater pollutants.It comes out that optimal hydraulic capture system estimated by the traditional deterministic method could not capture pollutants thoroughly,with a stable average risk rate as high as 23%.According to the study of nitrates remediation in Yangzhuang basin,different hydraulic capture volumes show a logarithmic relationship with stable average risk rates.The larger the volume is,the lower risk is caused by the spatial variability of permeability coefficient.The corresponding stable average risk rates of different volumes are considered as relative risk values of determined pumpage of the capture system.In the reality,the capture value should be defined from the curve according to the economical reasonability and acceptable risk in order to optimize the capture volume from the perspective of risks and lost.?Elimination of nitrates in the Karst groundwater through bio-remediation technology(pollution remediation)Different types of carbon source and bioreactors are applied in the experiment to conduct on-site remediation of nitrate pollution.Effects of nitrogen removal are compared and selected.The result showed that the reaction effect of liquid carbon could reached as high as 34.5%;that of solid carbon was 46.5%.Remediation experiment with liquid carbon was vulnerable to the environment due to the absence of reaction carrier,which showed a huge fluctuation when the rainfall happened or the wells were exploited nearby.When the supply of the liquid carbon stopped,the content of nitrates in the observation well would return to natural value in 24 hours.Remediation with solid carbon showed a slower reaction rates than that with liquid carbon.But with a better carrier,the reaction was relatively stable,showing a better effect and lasting a longer time.After removing the reactor with solid carbon,the content of nitrates in the observation well would return to natural value in 48 hours.In conclusion,the conclusions of this study provided the technical support for the aquifer remediation of nitrates pollution in the middle and south of Shandong province,for the improvement of groundwater quality and for the reasonable utility of groundwater resources to ensure water safety for urban and rural citizens. |
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