MODELING TRITIUM TRANSPORT, DEPOSITION AND RE-EMISSION

The atmospheric release of tritium oxide (HTO) potentially impacts human health, typically through inhalation or absorption. Due to HTO's similarity to water, vegetation will absorb HTO by solution in the leaf water and then re-emit it, creating a number of secondary sources of HTO. Currently, models used for emergency response at Savannah River Site incorporate the transport and deposition of HTO but do not provide estimates for its potential re-emission from vegetation or soil surface though re-emission could result in prolonged exposure and greater than predicted dose for an individual downwind. A simple model of HTO transport, deposition and re-emission has been developed to examine the potential increase in exposure and dose. The model simulates an initial release of HTO that moves with a mean wind and expands through diffusion as a Gaussian puff. Deposition is modeled using previous estimates of deposition velocity for HTO and re-emission is modeled using a time constant that describes how quickly HTO is transferred between the surface and atmosphere. Additional puffs are created to simulate re-emission of HTO as well as horizontal diffusion across model grid cells. An evaluation of field data indicates that the use of a re-emission module tends to improve model predictions through improved prediction of peak concentration magnitude and location. When considering dose, nearly all of the released material is included in the dose calculation when re-emission is included. Although exposure to HTO through re-emission occurs over a few hours, the incremental increase in dose is relatively small because the atmospheric concentration of re-emitted HTO is much lower than the initial release