An integrated transient Tier-3 human health risk assessment framework based on multispecies reactive transport modelling

Tier-3 human health risk assessments for groundwater contamination typically rely on numerical models with time-invariant parameters. However, when physical and biochemical properties evolve in time, parameters must be dynamically updated, but no integrated tool with this capability has been presented so far. To fill this gap, a tool called TRANSRISKRT3D was developed to incorporate time-varying first-order reaction rates into the multispecies reactive transport model RT3D for transient Tier-3 analysis. We applied it to an alluvial aquifer contaminated by chlorinated solvents. Initially managed with pump-and-treat, the site later transitioned to sequential in-situ bioremediation, significantly enhancing contaminant removal in certain areas. Considering volatile organic compounds inhalation risk over a defined exposure period, TRANSRISKRT3D captured how evolving remediation strategies influence spatio-temporal risk dynamics. Scenario analyses showed that early implementation of in-situ bioremediation would have halved the area with "unacceptable" risk levels (R > 10⁻⁴), underlining the importance of timing in remediation planning.