Numerical analysis for estimating residual DNAPL by single-well “push-pull” partitioning tracer tests
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摘要:
目前,刻画场地重非水相液体(dense non-aqueous phase liquid, DNAPL)污染常用的钻孔取样和井间分溶示踪试验方法成本高昂。相比而言,单井注抽试验节省经费,且对污染源区的扰动少,但该试验方法推估DNAPL残留量的准确性尚未得到定量验证。针对该问题,基于数值方法分析了示踪剂类型、注抽速率、污染源区结构等因素对单井注抽试验推估DNAPL残留量精度的影响。结果表明:(1)选用分溶系数比2,2-二甲基-3-戊醇(2,2-dimethyl-3-pentanol, DMP)低的己醇进行示踪,示踪剂回收更加充分,推估污染物残留量的平均精度增幅可达35.11%;(2)当注入速率从100 m3/d提高至130 m³/d、抽出速率从120 m3/d提高至150 m3/d,示踪剂接触的污染源区面积更大,均质源区对应的污染物残留量平均精度从42.45%提高到60.26%,非均质源区对应的平均精度从27.69%提高至48.72%;(3)污染源区结构复杂程度的增加会阻碍示踪剂的运移,非均质源区对应的平均精度比均质源区降低了13.15%;(4)单井注抽示踪试验更适用于离散状为主的污染源区,其平均精度比池状为主的源区增加了15.74%。单井注抽试验结合数值分析可有效推估残留非水相液体的分布,建议在DNAPL污染场地精细调查中使用,可为场地风险评估和修复方案的制定提供参考。
Abstract:The drilling methods and the partitioning inter-well tracer tests which are often used to characterize dense non-aqueous phase liquids (DNAPL) source zones cost a lot. Compared with common methods, the single- well “push-pull” partitioning tracer tests not only save money, but also reduce the disturbance to the contaminated site, which can be used to estimate the residual DNAPL. However, the accuracy of this method in estimating residue DNAPL has not been quantitatively verified. To solve the problem, this paper analyses the influence of several factors on estimating the accuracy of residual DNAPL based on the numerical method, including tracer type, the injection and pumping rate and contaminant source zone architectures. The results display that Hexanol with a smaller partition coefficient than DMP can be selected for tests, as the tracer are recovered more fully, which can result in the average accuracy of residue DNAPL estimated to increase by 35.11%. When the injection rate of the tests increases from 100 m³/d to 130 m³/d and the pumping rate increases from 120 m3/d to 150 m3/d, the tracer can expose to a larger contaminant source area so that the average accuracy of the homogeneous source zones increases from 42.45% to 60.26% while the average accuracy of the heterogeneous source zones increases from 27.69% to 48.72%. The increase of the complexity of the source zone architectures will hinder the tracer migration, and the average accuracy of the heterogeneous source zones is 13.15% lower than that of the homogeneous source zones. The single well “push-pull” partitioning tracer test is more suitable for the ganglia-dominated contaminant source zones, and the average accuracy of the ganglia-dominated contaminant source zones is 15.74% higher than that of the pool-dominated source zones. The single-well “push-pull” partitioning tracer tests combined with numerical analysis can effectively estimate the distribution of residual NAPL, and are recommended for use in the fine investigation of DNAPL contaminated sites, so as to provide a reference for the risk assessment and the site remediation.
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表 1 不同泄漏速率下生成的污染源区设置
Table 1. Cases of contaminant source zones with different leakage rates
算例设置 污染源区结构 泄漏速率/(m3·d−1) 实际残留量/m3 GTP 算例1 均质 0.3 1.97 1.96 算例2 非均质 0.3 2.57 2.35 算例3 均质 0.9 3.02 0.90 算例4 非均质 0.9 3.49 0.71 表 2 模型参数
Table 2. Model parameters
研究区参数 取值 PCE的密度/(kg·m−3) 1.62 PCE的黏滞性/(mPa·S) 0.89 水力梯度 0.002 孔隙度 0.34 纵向弥散度/m 0.03 横向弥散度/m 0.009 DMP对PCE的分溶系数 27.5 己醇对PCE的分溶系数 8.5 表 3 毛管压力及相对渗透率参数
Table 3. Capillary pressure and relative permeability parameters
毛管压力模型
(Brooks-Corey)毛管压力
端点值C1 9.6 C2 19.9 毛管压力
指数λ1 −0.52 λ2 2 相对渗透率
模型
(Corey-Type)残余饱和度 Sw 0.24 So 0.2 端点相对
渗透率Pw 0.486 Po 0.65 相对渗透率指数 nw 2.85 no 2.7 注:下标1和2分别表示源区生成阶段和单井注抽阶段的参数;下标w和o分别代表水相和油相。 表 4 单井注抽示踪模拟设置
Table 4. The setting of single-well “push-pull” tracer simulation
模拟条件 注入阶段 抽取阶段 注入速率
/(m3·d−1)注入
天数/d抽取速率
/(m3·d−1)抽取
天数/d算例1、3 情景1 100 30 120 30 情景2 130 30 150 30 算例2、4 情景3 100 30 120 30 情景4 130 30 150 30 -
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