Radionuclide Coefficients for the BNL Site: For the Saturated and Unsaturated Zones

The partition coefficient (K{sub d}) is defined as the concentration of a species of interest on the solid phase divided by the concentration of that species in the liquid phase, at steady-state. Therefore the lower the K{sub d}, the less sorption takes place. It is a parameter that is readily incorporated in chemical transport models. The K{sub d} approach to the partitioning of contaminants between soil and water in groundwater flow systems, is based on the assumption that sorption reactions are in equilibrium and are reversible. As a contaminant plume advances along flow paths, the movement of the front is retarded (relative to the movement of the groundwater) as a result of transfer of part of the contamination mass to the solid phase. If the input of contaminant mass to the system is discontinued, contaminants will be transferred back to the liquid phase as lower-concentration water flushes through the previously contaminated zone. Several sets of soil/water partition coefficients (K{sub d}) have been measured using BNL site specific soils and groundwater (with tracers added). They were all reported earlier through internal memos or reports. The K{sub d}s obtained for various radionuclides are determined experimentally with soils taken from different locations and depths around site. In each experiment the site specific soil and water taken from that same geoprobe boring were used. Generally tracer radionuclides were used. However, for samples from the BGRR area, water taken from the sump was used for Cs-137 and Sr-90 Kd experiments. Values of K{sub d} in the saturated zone are summarized in Table 1, as ranges and an average. Soil sample identifications, locations, and descriptions are provided in Table 2 and Figure 1. Other experiments, specific to the unsaturated zone at the BGRR, are discussed below. In addition, each data source, including methods, is reproduced in Appendix A, including figures documenting isotherm experiments for the various soil samples used. The values of n above are the number of different locations or depths from which soil was sampled for K{sub d} evaluations. Each K{sub d} value (and n) may be the result of three to five individual experiments which are used to construct an isotherm. Average values reported above include skewed to high values. This is the result of preferential uptake of some elements on the fine-grained detritus, weathering products and Fe oxides found at the surface. The one set of measurements of K{sub d} of surface soil was made with soil from the 650 sump outfall. Best estimates for samples at the surface and at depth (in the saturated zone of the subsurface) are also given above. Please bear in mind that these values represent an average of samples from selected locations. Overall, for samples at depth, I believe they approach reasonable sitewide averages. However, the surface soil values should only be applied to the 650 sump outfall. Locally, values may vary depending on mineralogy and grain size of the soil, as well as composition of the water flowing through it