A three-dimensional numerical model for linking community-wide vapour risks

• New numerical model to simulate vapour intrusion coupled to groundwater transport.
• Allows vapour intrusion from spatially and temporally variable groundwater plumes.
• Many simulations conducted in a 1000 m × 500 m × 14 m heterogeneous domain.
• The exceedance of standards depends strongly on distance downgradient and cross-gradient.
• Substantial source reduction often required, but less so for community receptors.

A three-dimensional (3D) numerical model that couples contaminant transport in the saturated zone to vapour transport in the vadose zone and vapour intrusion into buildings was developed. Coupling these processes allows the simulation of vapour intrusion, arising from volatilization at the water table, associated with temporally and spatially variable groundwater plumes. In particular, the model was designed to permit, for the first time, 3D simulations of risk at receptors located in the wider community (i.e., kilometre scale) surrounding a contaminated site. The model can account for heterogeneous distributions of permeability, fraction organic carbon, sorption and biodegradation in the vadose and saturated zones. The model formulation, based upon integration of a number of widely accepted models, is presented along with verification and benchmarking tests. In addition, a number of exploratory simulations of benzene and naphthalene transport in a 1000 m long domain (aquifer cross-section: 500 m × 14 m) are presented, which employed conservative assumptions consistent with the development of regulatory guidance. Under these conservative conditions, these simulations demonstrated, for example, that whether houses in the community were predicted to be impacted by groundwater and indoor air concentrations exceeding regulatory standards strongly depended on their distance downgradient from the source and lateral distance from the plume centreline. In addition, this study reveals that the degree of reduction in source concentration (i.e., remediation) required to achieve compliance with standards is less if the risk receptor is in the wider community than at the site boundary. However, these example scenarios suggest that, even considering community receptors, sources with initially high concentrations still required substantial remediation (i.e., > 99% reductions in source concentration). Overall, this work provides insights and a new tool for considering the relationships between contaminated site source zones and community-wide risk assessment that allows for development of policies and technical approaches for contaminated site management. It is anticipated that this coupled model not only will allow significant convenience, for example in running suites of Monte Carlo simulations for complex scenarios, but will also allow the investigation of vapour intrusion under conditions where soil gas concentrations may change over the same timescale as an evolving plume.