HEC-6 was modified for research purposes at the Oak Ridge National Laboratory's Environmental Sciences Division in order to track particle-reactive contaminant fate by introducing contaminant influx, sorption/desorption, radioactive decay and bed-sediment layering.
The HEC-6-R model was applied to the Clinch River/Watts Bar Reservoir (CR/WBR) system in East Tennessee, which received Cesium-137 releases from the Oak Ridge National Laboratory due to manufacturing and research activities between the mid-1940s and recent times (Olsen, et.al.).
The HEC-6-R model simulations were carefully calibrated against long-term sediment accumulation measurements reported by the Tennessee Valley Authority for the 1946-1951, the 1951-1956, the 1956-1961 and the 1961-1991 periods (TVA Report).
The HEC-6-R model simulations were validated against Cesium-137 concentration measurements in the Clinch and the Lower Watts Bar section of the CR/WBR system recorded in 1962, 1977 and 1987 (Oakes, et.al., 1982; Oakes, et.al., 1987; Olsen et.al.).
In the following animations, predicted Cesium-137 bed-sediment contamination is shown from 1944 to the year 2021. Historically-measured daily water discharges from 1944 to 1991 are used in the simulation to move the contaminated sediments from their source in White Oak Creek Dam through the Clinch into Lower Watts Bar. In addition to erosion and deposition, sorption and desorption of the contaminated sediments can take place. From 1991 to the year 2021, the 1981-1991 discharges were repeated three times for the necessary future predictions.
In the animations, the Clinch River is shown in the top frame while Lower Watts Bar is shown in the bottom frame at a somewhat smaller scale. A number of overlapping model reaches are visible in both the top and bottom frames, making the area of connection clear. The split frame was necessary to make color changes visible in the Clinch.
The first simulation shows HEC-6-R predictions of total Cesium-137 present in each model reach over time on a linear color scale. After initial contamination of the bed sediments in the late 1940s and early 1950s, a large release of Cesium-137 moves downstream in the mid-1950s and appears in particular reaches that are predominantly deposition areas. During the early 1960s, erosion of contaminated sediments and radioactive decay are particularly noticeable in many reaches as Cesium-137 activity decreases. The simulation then settles into a long period where the Cesium-137 undergoes primarily radioactive decay.
In the second simulation, Cesium-137 accumulation for each reach area was divided by the area to give activity in pCi/cm2. In order to make contaminant levels visible in the animation, the Cesium-137 inventories were log transformed.