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<br />Model Foundations <br /> <br />. Behavior of RFETS Detention Ponds as SedimenUltion Basins <br /> <br />Stormwater runoff from the Industrial Areais captured by the system of detention ponds located in the Site <br />buffer zone. Each pond. acting as an individual sedimentation basin. causes particulate matter suspended in <br />the stormwater to settle out of the water column. Water-quality data for the ponds compared to data for <br />stormwater inflows supports this conclusion by clearly indicating a significant improvement in quality after <br />the stormwater passes through the ponds (RMRS. 1995). <br /> <br />Association of RadiDnuclides with Susl1ended Solids <br /> <br />Data from stormwater collected at RFETS froIi11991 through 1995 indicate radionuclides associate with <br />solids suspended in the stormwater (measured as total suspended solids; TSS) (RMRS. 1995). Based on <br />these studies and for the purposes of this model. the following characteristics regarding Pu in soils are <br />postulated or assumed: <br /> <br />1. Pu forms a strong association with soils. <br />2. Pu transport is generally slow and aided by the presence of pedogenic factors which increase <br />perviousness of the soil. <br />3. Environmental deposits of Pu at the Site occur and decrease in activity quickly within a fOOl of <br />the soil surface. <br />4. Pu occurs disproportionately attached to smaller particles. perhaps dependent, in part, on the <br />greater availability of adsorption sites per unit weight for smaller versus larger particles. <br />5. Surficially localized Pu is potentially available for transport by wind and water erosional <br />forces. <br /> <br />Radionuclide Removal Model <br /> <br />Re/ationshil1 Between Stonnwater Flows and Radionuclide Activities <br /> <br />Increasing intensity of precipitation events generate higher stormwater flows that result in increased TSS <br />transport caused by ditch and wetland scouring. sheet flow on bare soils. and raindrop impact. <br />Consequently. since radionuclides are documented to associate with suspended solids. relationships <br />between stonnwater Pu activity and flow rates should exist. These relationships can be used to estimate '.he <br />Pu loading into the ponds for any given inflow rate. <br /> <br />When examining the relationship between flow and Pu activity using all data collected from 1991 to 1995. <br />no good statistical correlations could be determined. However. by separating and excluding certain data <br />points that appear to be from different populations. the fit in Figure 3 was generated. As the figure <br />indicates. at flows less than or equal to 0.23 cfs, the Pu activity is expected to be below 0.05 pCifL. <br /> <br />Particle Settlin~ and Associated Actinide Removal <br /> <br />In order to model the capacity of the detention ponds to remove radionuclides by settling. the differe"t sizes <br />of suspended particles. and their respective settling velocities. must be known for a given volume of <br />stormwater. The distribution of suspended sediment particle sizes in stormwater in the A- and B-Series <br />drainages was the subject of a study conducted in 1992 (AMAX, 1992). Suspended sediments were sieved <br />and filtered to determine the distribution of particle si~es ranging in diameter from 1 micron ( Ix 10-9 ;Il) to <br />more than 19,000 microns (19 em). Table 1 shows the distribution of particle sizes for samples conec~ec. at <br />site SW023 (upstream from the B-Series ponds). These data were plotted and an equation was develc?"c <br />for a unique particle size distribution curve. <br />