The use of dynamic modeling in assessing tritium phytoremediation

To minimize movement of tritium into surface waters at the Mixed Waste Management Facility at the Savannah River Site, tritium-contaminated groundwater released to the surface along seeps in the hillside is being retained in a constructed pond and used to irrigate forest acreage that lies over the contaminated groundwater. Management of the application of tritium-contaminated irrigation water needs to be evaluated in the context of the large amount of rainfall relative to evapotranspiration, the strong seasonality in evapotranspiration, and intra-annual and interannual variability in precipitation in this region. A dynamic simulation model of water and tritium fluxes in the soil-plant-atmosphere continuum was developed to assess the efficiency (tritium transpired/tritium applied) of several irrigation management strategies. The model was parameterized using soil-water content data measured at 18 sites for the first year of the project and evaluated using tritium activity measurements made at the same 18 sites over 2.5 yr. The model was then used to evaluate several irrigation strategies. The 25-yr efficiencies (tritium transpired/tritium applied) of the irrigation strategies were related to the quantity of irrigation water applied. There was a strong (r² = 0.99) negative linear relationship between irrigation water applied and efficiency. When a quasi-steady state has been reached in the system, the annual efficiencies of all the irrigation strategies were negatively correlated with annual rainfall. Quantification of these relationships allows irrigation managers to choose irrigation strategies based on desired long-term system efficiency, which differ with climate and irrigation strategy.