Numerical analysis for estimating residual DNAPL by single-well “push-pull” partitioning tracer tests

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.