Sensitivity analysis of upscaling prediction of the mass flux at DNAPL contaminated sites
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摘要:
重非水相液体(DNAPL)污染问题日益严重。为评估DNAPL污染场地的环境风险, 常采用升尺度模型推估DNAPL污染源区溶解相的质量通量(溶解通量)。由于升尺度模型中的参数较多, 调查成本较高, 因此需筛选模型中的关键参数, 指导实际污染场地设计合理的观测数据采集方案。首先对升尺度模型中6个参数(地下水平均流速
q 、标准化浓度C 0/C eq、离散状DNAPL质量比例GF 0、初始时刻离散状DNAPL贡献的通量比例f g、拟合参数β 1及β 2)开展全局敏感性分析, 识别其中关键参数, 进而采用局部敏感性分析定量化关键参数的变化对通量预测的影响。研究结果表明, 参数q、C 0/C eq、GF 0和f g对通量预测有较大影响。q 和C 0/C eq在整个衰减过程中敏感性均相对较高,GF 0和f g随着衰减过程的进行, 敏感性不断增高, 分别在衰减中后期和后期达到峰值; 对于不同结构的污染源区,q 或C 0/C eq增大时, 通量的增幅基本不变。随着污染源区中离散状DNAPL和池状DNAPL间的质量比例(GTP )增大,GF 0或f g增大时, 其对通量预测的影响不断增大或减小。因此在预测溶解通量时需将调查成本重点应用于q 和C 0/C eq; 在合理设计污染源区修复方案时, 应重点调查GF 0; 在预测污染源区寿命时,f g为重要调查对象; 对于所有结构的污染源区,q 和C 0/C eq均为重要调查对象, 对于GTP 较大的污染源区, 应将调查成本重点应用于GF 0, 对于GTP 较小的污染源区, 应重点调查f g。Abstract:Dense nonaqueous phase liquid (DNAPL) contamination is a growing problem. To assess the environmental risk of DNAPL-contaminated sites, the mass flux of the dissolved phase (mass flux) in the source zone of DNAPL contamination is often extrapolated using upscaling models. Due to the large number of parameters in the upscaling model and the high cost of investigation, the key parameters in the model need to be screened to guide the design of a reasonable observation data collection scheme for actual contaminated sites. In this paper, a global sensitivity analysis was first conducted on six parameters (mean groundwater velocity
q , standardized concentrationC 0/C eq, the mass ratio of gangliaGF 0, the fraction of the mass flux attributable to the ganglia dissolutionf g, and fitting parametersβ 1 andβ 2) in the upscaling model to identify the key parameters, and then a local sensitivity analysis was used to quantify the impact of changes in the key parameters on mass flux prediction. The results showed that the parametersq ,C 0/C eq,GF 0 andf g had a large impact on the mass flux prediction. The sensitivities ofq andC 0/C eq were relatively high throughout the depletion process, while those ofGF 0 andf g increased continuously with the depletion process, reaching peaks in the middle and late stages of depletion, respectively. For source zones with different structures, the increase in mass flux was essentially constant whenq orC 0/C eq increased. As the ganglia-to-pool (GTP ) mass ratio increased in source zones, its effect on the mass flux prediction continued to increase or decrease whenGF 0 orf g increased. Therefore, the investigation needs to focus onq andC 0/C eq when predicting the mass flux, onGF 0 when reasonably designing the remediation plan of the source zone, and onf g when predicting the lifetime of the source zone. For all structural source zones,q andC 0/C eq are the most important to investigate, and the investigation cost should be focused onGF 0 for source zones with largeGTP andf g for source zones with smallGTP .-
Key words:
- DNAPL /
- mass flux /
- upscaled model /
- global sensitivity analysis /
- local sensitivity analysis
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图 1 污染通量的升尺度预测[12]
Figure 1. Upscaling prediction of the mass flux
表 1 模型参数设置
Table 1. Setting of the model input parameters
参数 取值 研究区范围/m 50×25×15 网格尺寸/m 1×1×1 观测断面位置/m x=50 生成参考场的地质参数 lnKi的协方差函数 q(x, x′)=σKi2exp(-|x-x′|/I2) lnKi的相关长度/m Ix, Iy, Iz=18, 6.25, 1 lnKi的方差 σlnKi2=2.0 lnKi的均值/ln(m·s-1) μlnKi=-8.0 平均粒径/μm 295 流体性质 水 DNAPL 密度/(kg·m-3) 1 000 1 496 黏滞性/(Pa·s) 0.001 0.000 89 DNAPL污染源区 泄露位置/m (25, 12.5, 0.5) DNAPL类型 三氯乙烯 DNAPL的残余饱和度 0.20 Brooks-Corey模型参数λ 2 DNAPL的总质量/kg 2 000 泄露速率/(m3·s-1) 5×10-20~5×10-4 GTP 1.1~36.5 运移参数 水力梯度 0.01 孔隙度/% 30 纵向弥散度/m 0.5 横向弥散度/m 0.05 分子扩散系数/(cm2·s-1) 1.6×10-5 表 2 模型参数范围设置
Table 2. Setting of the range of model parameters
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