On the intrusion and recovery of ocean-sourced 3H and 14C in coastal aquifers considering ocean-surge inundation

Abstract Human activities, such as discharging nuclear wastewater into the ocean, can put coasts in the risk of being exposed to radioactive contaminations. The radionuclides may potentially intrude into coastal aquifers from the ocean and endanger the groundwater resources, either laterally through subsurface or vertically through land surface during ocean surge induced inundation. This study attempted to investigate the temporal and spatial scales of these intrusion processes using an experimental site near Bremerhaven, northern Germany. A fully coupled surface-subsurface flow and transport model was employed to simulate the groundwater flow and the transport of TDS (total dissolved salt), 3H and 14C, under conditions with or without an ocean surge event. Scenarios of different exposure schemes were designed by varying the exposure duration and radionuclide concentrations. The intrusion and post-exposure recovery were assessed via contaminated volume and area, intrusion length, and recovery time. Results show that, depending on exposure duration, the intrusion of 3H and 14C can cause serious contamination in the coastal aquifer, with up to 18.9 km2 land area being contaminated and reaching up to 3.3 km landward from the shore. 3H shows a ‘harder intrusion, easier recovery’ pattern due to its higher decay rate. The full recovery may take up to 70 years and 217 years (considering 30-years-exposure) for 3H and 14C, respectively. An ocean surge event is more likely to exacerbate the intrusion, but it does not significantly change the full recovery time. Shorter exposure duration should be favored to minimize the influences on the coastal aquifer when the total exposure quantity is fixed. This study highlights the severity of radioactive contamination in coastal aquifers and the difficulty of recovery. Thus, cautious decision should be made concerning the possible impacts of releasing radioactive wastewater on coastal life and human beings.