地下含水层中过硫酸盐运移数值模拟及其影响半径综合模型研究

Numerical Simulations of Persulfates Transport in Aquifers and Developments of Prediction Models of Persulfate Influence Radius

过硫酸盐被广泛应用于有机污染土壤和地下水的原位化学氧化(ISCO)修复.过硫酸盐在地下含水层中发生复杂的物理和化学过程,其运移受地下含水层介质和氧化药剂输送技术参数的影响,过硫酸盐影响半径(即地下水中1 g/L过硫酸盐的边界处)预测与评价的技术难度大.本研究围绕ISCO过硫酸盐单孔连续注射情景,采用FEFLOW软件,开展了过硫酸盐在砂质、粉质含水层等代表性场地含水层中运移数值模拟研究.考虑了11个涉及地下含水层介质、氧化剂消耗和输送技术参数的影响,筛选了相关的参数及其组合项,分别表征地下水弥散、过硫酸盐衰减及二者复合作用对含水层中过硫酸盐影响半径的影响.基于模拟结果数据,构建了地下含水层中过硫酸盐影响半径时空演变的综合模型,涵盖了氧化剂注射作用、地下水弥散、过硫酸盐衰减及二者复合作用的影响,其拟合优度R2为0.990(n=2 083),对流-弥散方程的模型外部验证结果 R2为0.992(n=11).参数敏感性分析显示,地下含水层介质的有效孔隙度、过硫酸盐注入流量及注入时间、含水层厚度等参数对扩散阶段过硫酸盐影响半径的影响较大,它们的影响随扩散时间呈现不同的变化趋势.单次连续注射研究显示,代表性砂质含水层中过硫酸盐第30天的纵向影响半径为5.2~7.7 m,而代表性粉质含水层中过硫酸盐的纵向影响半径为0.7~3.8 m.该综合模型可应用于不同地下含水层中过硫酸盐输送技术及其水动力辅助输送技术参数的匹配优化,提高过硫酸盐原位化学氧化修复效果.

In situ chemical oxidation(ISCO) using persulfates is widely used for remediation of soil and groundwater contaminated by organic pollutants. During ISCO, injected persulfates transport in aquifers and undergo complex physical and chemical processes,depending on the aquifer characteristics and injection systems. How the transport and distribution of persulfates in aquifers vary with aquifers and injection systems remains poorly understood. As a result, it is difficult to predict the radius of influence(ROI) of persulfates in aquifers, denoted as the border at which 1 g/L persulfates occur and is the lower limit to ensure the performance of persulfates. In this study, we developed continuous single-well injection system scenarios in distinctly different aquifers with different injection parameters,and carried out numerical simulations of persulfate transport and distributions in aquifers using the numerical model FEFLOW. A total of11 parameters were considered, involving aquifer characteristics, persulfate consumption and/or injection systems. Different combination equations of individual and/or interaction items of the 11 parameters were proposed to represent the impacts of hydrodynamic dispersion,persulfates consumption and their interactions on persulfate ROI in aquifers. By using the combination equation and mathematical simulating result data, we developed a comprehensive model of the temporal and spatial distribution of persulfate ROI in aquifers, with the determining degree(R~2) of 0.990(n=2083). The external verification of transport of persulfates in aquifers had a R~2 value of 0.992(n=11),based on the Advection-Dispersion Equation of persulfates in aquifers used by the FEFLOW model. Sensitivity analyses indicated that effect porosity and thickness of the aquifers and the persulfate injection flow rate and injection time had a large positive or negative impacts on persulfate ROI. Their impacts decreased or increased in magnitude over time, and the decreasing or increasing rates drastically varied with aquifers. The effects of injection and hydrodynamic parameters on persulfate ROI in typical aquifers were discussed to determine the best design of alternative injection approaches. Simulations of continuous single-well injection system scenarios showed that persulfate ROI values were 5.2-7.7 m in sandy aquifer on the 30thday, as compared with 0.7-3.8 m in silty aquifer. The finding may have practical implications for the evaluation and optimization of persulfate based ISCO, based on the site-specific conditions including aquifer permeability and heterogeneity as well as natural oxidant demand.