Application of TREECS Modeling System to Strontium-90 for Borschi Watershed near Chernobyl, Ukraine

Abstract The Training Range Environmental Evaluation and Characterization System (TREECS™) ( http://el.erdc.usace.army.mil/treecs/ ) is being developed by the U.S. Army Engineer Research and Development Center (ERDC) for the U.S. Army to forecast the fate of munitions constituents (MC) (such as high explosives (HE) and metals) found on firing/training ranges, as well as those subsequently transported to surface water and groundwater. The overall purpose of TREECS™ is to provide environmental specialists with tools to assess the potential for MC migration into surface water and groundwater systems and to assess range management strategies to ensure protection of human health and the environment. The multimedia fate/transport models within TREECS™ are mathematical models of reduced form (e.g., reduced dimensionality) that allow rapid application with less input data requirements compared with more complicated models. Although TREECS™ was developed for the fate of MC from military ranges, it has general applicability to many other situations requiring prediction of contaminant (including radionuclide) fate in multi-media environmental systems. TREECS™ was applied to the Borschi watershed near the Chernobyl Nuclear Power Plant, Ukraine. At this site, TREECS™ demonstrated its use as a modeling tool to predict the fate of strontium 90 ( 90 Sr). The most sensitive and uncertain input for this application was the soil-water partitioning distribution coefficient ( K d ) for 90 Sr. The TREECS™ soil model provided reasonable estimates of the surface water export flux of 90 Sr from the Borschi watershed when using a K d for 90 Sr of 200 L/kg. The computed export for the year 2000 was 0.18% of the watershed inventory of 90 Sr compared to the estimated export flux of 0.14% based on field data collected during 1999–2001. The model indicated that assumptions regarding the form of the inventory, whether dissolved or in solid phase form, did not appreciably affect export rates. Also, the percentage of non-exchangeable adsorbed 90 Sr, which is uncertain and affects the amount of 90 Sr available for export, was fixed at 20% based on field data measurements. A Monte Carlo uncertainty analysis was conducted treating K d as an uncertain input variable with a range of 100–300 L/kg. This analysis resulted in a range of 0.13–0.27% of inventory exported to surface water compared to 0.14% based on measured field data. Based on this model application, it was concluded that the export of 90 Sr from the Borschi watershed to surface water is predominantly a result of soil pore water containing dissolved 90 Sr being diverted to surface waters that eventually flow out of the watershed. The percentage of non-exchangeable adsorbed 90 Sr and the soil-water K d are the two most sensitive and uncertain factors affecting the amount of export. The 200-year projections of the model showed an exponential decline in 90 Sr export fluxes from the watershed that should drop by a factor of 10 by the year 2100. This presentation will focus on TREECS capabilities and the case study done for the Borschi Watershed.