基于饱和-非饱和流溶质运移数值模拟的地下水水源地硝酸盐污染风险评价研究

Process simulation method is effective in assessing the groundwater pollution risk of nitrate in the water source field. And groundwater flow and solute transport numerical simulation is key for process simulation method. There is few research results on combined model of unsaturated zone and aquifer until now because the migration process of pollutants in the groundwater system have not been fully considered and the characterization of relative parameters is difficult. Taking the groundwater source field of eighth water plant in Beijing city as an example and nitrate as the typical pollutant, this project will focus on demonstrating the spatial distribution of nitrate in the pathway of unsaturated zone and aquifer and determining the main factors of nitrate fate and transport process by integrating the indoor experiments, field experiments and groundwater chemical analysis. Then, it will build the saturated-unsaturated flow and solute migration simulation model by TOUGHREACT/EOS9 software respectively by integrating the unsaturated zone and the aquifer. Besides, the quantitative parameters of solute fate and transport in the unsaturated zone and phreatic aquifer will be obtained for the model by static adsorption experiments, nitrification and denitrification rate tests and field in-situ pumping injection test. Lastly, this project will assess and predict the groundwater pollution risk as well as verifying the accuracy of assessment results by statistic analysis. The results obtained from this project can facilitate to the quantitative assessment of groundwater pollution risk in the water source field and have obviously practical significance of implementing the groundwater pollution risk and quality control.

过程模拟法是地下水水源地污染风险评价的有效方法。建立地下水流和溶质运移数值模型是过程模拟法的关键。目前将包气带和饱和带进行联合模拟的研究较少，原因在于没有充分考虑污染物在地下水系统中的迁移转化过程，且表征转化过程的参数获取难度较大。本项目以北京市水源八厂为研究对象，选取硝酸盐为特征污染物，将包气带和饱和带有机结合，通过野外原位监测、水文地球化学分析等方法，分析硝酸盐在包气带→饱和带路径上的时空分布特征，揭示硝酸盐迁移转化过程；构建饱和-非饱和流溶质迁移数值模型，采用静态吸附实验、反硝化速率测定实验和野外原位抽注试验确定模型所需的硝酸盐转化参数，运用TOUGHREACT/EOS9软件模拟计算表征地下水水源地污染风险的指标；定量评价和预测地下水污染风险，并验证评价结果准确性。该研究为定量评价地下水水源地污染风险提供新的研究方法，对实现地下水水源地风险防控和环境质量控制具有重要现实意义。

地下水水源地污染风险评估是保障水源地供水安全的重要工具。采用过程模拟法评估可提高风险评价结果精度。由于水文地质条件和污染物迁移转化过程的复杂性，目前较少考虑污染物在地表污染源到抽水井之间整个地下水系统的对流、弥散和各种迁移转化过程，也缺乏联和饱和带和非饱和带数值模型的综合应用。本研究选取北京水源八厂作为研究对象，识别了硝酸盐在地下水系统中迁移转化过程及反硝化过程的影响因素，重点测定了不同深度不同介质不同生物地球化学环境的包气带反硝化强度以及含水层反硝化速率等关键参数，构建了基于饱和-非饱和流溶质联合数值模型的地下水水源地污染风险定量评价方法，评估了评价区地下水硝酸盐污染风险，预测了南水北调来水和地下水调整开采布局情景下的地下水水源地污染风险变化。结果显示，（1）研究区不同深度和介质的包气带反硝化过程包括NO3-N浓度上升、下降、上升3个主要阶段。10m内包气带垂向空间的反硝化强度整体水平较低（0.0017-0.0233 mg•（kg•d）-1），在垂向空间上呈“S”型分布。包气带反硝化强度垂向分布影响因素包括正相关的黏粒、硝酸盐、亚硝酸盐、微生物多样性与反硝化功能基因数量等。（2）研究区27.5m深浅层含水层反硝化速率（以N计）为 348.81μg kg-1 day-1-564.07μg kg-1 day-1之间。（3）评价区地下水污染风险以较低和中等为主。潜水含水层风险高和较高区主要分布在地下污染源强高的风险源周边以及离风险源较近的抽水井附近。第二承压含水层各污染风险分区分布情况与潜水含水层近似,总体低于潜水。（4）预测情景下不会因现有地表污染源产生地下水水源地污染风险。本研究通过扭住“双源”（重点污染源和水源地），兼顾 “地表-地下，包气带-含水层的污染协同防控”，解决了地下水水源地污染风险评估结果精准性和动态性，为水源地尺度地下水污染风险精准分级提供了新思路。研究成果可为水源地风险源排查与风险防控、地下水水源地污染风险管控等地下水污染防控工作提供技术支撑。