Numerical Simulation Of Groundwater Vulnerability In Tanghe Reservoir And Its Surrounding Areas

With the continuous development of C hina's economy,the problem of groundwater pollution is becoming more and more serious.Groundwater vulnerability reflects the sensitivity of groundwater system affected by external conditions,and groundwater vulnerability assessment is one of the preconditions to protect groundwater.Tanghe sewage reservoir is located in the plain area of Baoding C ity,Hebei Province.It has been nearly 40 years since it was put into use.Because there are no anti-seepage measures at the bottom of the reservoir,industrial sewage is discharged into the reservoir through pipes and canals,which has seriously polluted the groundwater in the surrounding areas and caused serious harm to the ecological environment.Given the serious pollution risk to groundwater resources in Tanghe wastewater reservoir and its surrounding areas,this paper conducts a special vulnerability evaluation of Tanghe wastewater reservoir and its surrounding areas by adopting a vulnerability evaluation tool based on numerical simulation.In addition,results of water quality assessment and health risk assessment are used to verify the reasonableness of the special vulnerability evaluation.The main findings are as follows.(1)This paper analyzes the characteristics of groundwater flow field in Tanghe sewage reservoir and its surrounding areas,and the relationship between surface water and groundwater recharge,runoff and discharge.According to the physical and geographical conditions of the study area,the water level is measured on site.According to the collected hydrogeological data,the groundwater flow model of Tanghe sewage reservoir and its surrounding areas is constructed by using the MODFLOW module of GMS software,and the identification and certification of the model are carried out.The simulation results show that the water equilibrium is negative,indicating that the groundwater recharge in the study area is less than the discharge,and the water level is declining.(2)A vulnerability assessing system is thereby established.According to the actual situation of Tanghe wastewater reservoir and the surrounding areas,this paper analyzes the formation mechanism of groundwater vulnerability in the study area.Based on the traditional DRASTIC model,five essentia l indexes are selected to assess the vulnerability of groundwater.The indexes are depth of burial,integrated lithology of the envelope,net recharge,hydraulic conductivity,and aquifer thickness.At the same time,taking the influence of human activities into account,the evaluation indicator for land use is added.According to the distribution of each evaluation index in the study area,each index of different nature and unit is quantified by using the score value.The paper also sets up the corresponding scoring system,according to the distribution of each assessing index in the study area.(3)The weight values of the six assessing factors are calculated by using the Analytic Hierarchy Process(AHP).Including groundwater depth,comprehensive lithology of vadose zone,net recharge,hydraulic conductivity,aquifer thickness and land use type.The results of numerical groundwater simulation are used to obtain the partition results of three assessing indicators: groundwater depth,hydraulic conductivity,and net recharge.Combining with the spatial data processing and editing and visualization functions of Arc GIS,this paper vectorizes the factors in the groundwater vulnerability assessing index system and extracts attribute data.The vulnerability partition map of each assessing factor is obtained by applying the spatial superposition analysis technique.The attribute data of each factor is multiplied by their weights and then is superimposed in order to obtain the evaluation attribute data,and the groundwater vulnerability partition map of Tanghe wastewater reservoir and its surrounding areas is generated.The results show that the groundwater vulnerability of Tanghe wastewater reservoir and its surrounding area are most influenced by the depth of groundwater,followed by the type of land use.In addition,the western part of the study area has lower groundwater vulnerability compared to the eastern part.(4)The water quality assessment results are compared with the groundwater vulnerability assessment results to verify the rationality of the vulnerability assessment results.The single factor evaluation method,Nemerow index method and fuzzy comprehensive evaluation method are used to evaluate and analyze the water quality of the water quality monitoring wells in the study area,and the three evaluation results are compared and analyzed;the special vulnerability evaluation results in the study area are compared with the water quality evaluation results to verify the rationality of the vulnerability evaluation results.The evaluation results show that: the water quality evaluation results obtained by Nemerow index method and fuzzy comprehensive evaluation method are almost the same,and the results of fuzzy comprehensive evaluation method are more reasonable;the fuzzy comprehensive evaluation results show that among 87 water quality monitoring wells,there are 19 class I water,2 class II water,7 class III water,4 class IV water and 55 class V water,accounting for 21.8%,2.3%,8% and 32.2% of the total number of monitoring wells respectively And 63.2%,and the proportion of type V water is relatively large;type V water is generally distributed in medium vulnerable area,high vulnerable area and high vulnerable area.(5)The USEPA health risk evaluation model is used to evaluate the risks to the health of adults and children in the study area.The results show that if people drink or contact groundwater,the health risk of non-carcinogenic heavy metals in groundwater in the study area does not exceed 1,indicating an acceptable level.