Sensitivity analysis of upscaling prediction of the mass flux at DNAPL contaminated sites

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 concentration C0/Ceq, the mass ratio of ganglia GF0, the fraction of the mass flux attributable to the ganglia dissolution fg, and fitting parameters β1 and β2) 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 parameters q, C0/Ceq, GF0 and fg had a large impact on the mass flux prediction. The sensitivities of q and C0/Ceq were relatively high throughout the depletion process, while those of GF0 and fg 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 when q or C0/Ceq 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 when GF0 or fg increased. Therefore, the investigation needs to focus on q and C0/Ceq when predicting the mass flux, on GF0 when reasonably designing the remediation plan of the source zone, and on fg when predicting the lifetime of the source zone. For all structural source zones, q and C0/Ceq are the most important to investigate, and the investigation cost should be focused on GF0 for source zones with large GTP and fg for source zones with small GTP.

Keywords

• dnapl
• mass flux
• upscaled model
• global sensitivity analysis
• local sensitivity analysis