Assessing the joint impact of DNAPL source-zone behavior and degradation products on the probabilistic characterization of human health risk

The release of industrial contaminants into the subsurface has led to a rapid degradation of groundwater resources. Contamination caused by Dense Non-Aqueous Phase Liquids (DNAPLs) is particularly severe owing to their limited solubility, slow dissolution and in many cases high toxicity. A greater insight into how the DNAPL source zone behavior and the contaminant release towards the aquifer impact human health risk is crucial for an appropriate risk management. Risk analysis is further complicated by the uncertainty in aquifer properties and contaminant conditions. This study focuses on the impact of the DNAPL release mode on the human health risk propagation along the aquifer under uncertain conditions. Contaminant concentrations released from the source zone are described using a screening approach with a set of parameters representing several scenarios of DNAPL architecture. The uncertainty in the hydraulic properties is systematically accounted for by high-resolution Monte Carlo simulations. We simulate the release and the transport of the chlorinated solvent perchloroethylene and its carcinogenic degradation products in randomly heterogeneous porous media. The human health risk posed by the chemical mixture of these contaminants is characterized by the low-order statistics and the probability density function of common risk metrics. We show that the zone of high risk (hot spot) is independent of the DNAPL mass release mode, and that the risk amplitude is mostly controlled by heterogeneities and by the source zone architecture. The risk is lower and less uncertain when the source zone is formed mostly by ganglia than by pools. We also illustrate how the source zone efficiency (intensity of the water flux crossing the source zone) affects the risk posed by an exposure to the chemical mixture. Results display that high source zone efficiencies are counter-intuitively beneficial, decreasing the risk because of a reduction in the time available for the production of the highly toxic subspecies.Peer ReviewedPostprint (author's final draft

Mixing in Multidimensional Porous Media: A Numerical Study of the Effects of Source Configuration and Heterogeneity

We investigate transport of an inert solute in multidimensional porous media characterized by spatially variable hydraulic conductivity. Through the use of a GPU-accelerated solute transport simulator based on the Random Walk Particle Tracking technique, we show how different factors such as the degree of heterogeneity, flow dimensionality and source zone configurations impact mixing. Solute mixing is quantified in terms of the temporal evolution of the plume’s statistics (mean, variance and probability density function) and the dilution index. Our analysis show that mixing is strongly affected by the above mentioned factors. We also compare the probability distributions obtained from the numerical simulations with the beta distribution. Despite the discrepancies at very low concentrations, our results show that the fitting with the beta distribution is improved for transport in three-dimensional settings originating from a vertical planar source. To improve the fit at low concentrations, we utilize two one-to-one variable transformation, namely the logarithm and power law transformations. Results demonstrate that the Pareto-type IV and the uniform distributions are capable to capture the lower tail of the cumulative distribution function. Numerical results are validated against existing analytical solution for both homogeneous and heterogeneous media.FPJdB acknowledges the financial support provided by the NSF grant number 1654009.Peer reviewe

Assessing the joint impact of DNAPL source-zone behavior and degradation products on the probabilistic characterization of human health risk

The release of industrial contaminants into the subsurface has led to a rapid degradation of groundwater resources. Contamination caused by Dense Non-Aqueous Phase Liquids (DNAPLs) is particularly severe owing to their limited solubility, slow dissolution and in many cases high toxicity. A greater insight into how the DNAPL source zone behavior and the contaminant release towards the aquifer impact human health risk is crucial for an appropriate risk management. Risk analysis is further complicated by the uncertainty in aquifer properties and contaminant conditions. This study focuses on the impact of the DNAPL release mode on the human health risk propagation along the aquifer under uncertain conditions. Contaminant concentrations released from the source zone are described using a screening approach with a set of parameters representing several scenarios of DNAPL architecture. The uncertainty in the hydraulic properties is systematically accounted for by high-resolution Monte Carlo simulations. We simulate the release and the transport of the chlorinated solvent perchloroethylene and its carcinogenic degradation products in randomly heterogeneous porous media. The human health risk posed by the chemical mixture of these contaminants is characterized by the low-order statistics and the probability density function of common risk metrics. We show that the zone of high risk (hot spot) is independent of the DNAPL mass release mode, and that the risk amplitude is mostly controlled by heterogeneities and by the source zone architecture. The risk is lower and less uncertain when the source zone is formed mostly by ganglia than by pools. We also illustrate how the source zone efficiency (intensity of the water flux crossing the source zone) affects the risk posed by an exposure to the chemical mixture. Results display that high source zone efficiencies are counter-intuitively beneficial, decreasing the risk because of a reduction in the time available for the production of the highly toxic subspecies.Peer Reviewe