Sediment Movement and Deposition using Cesium-137 Tracer

ABSTRACT RADIOACTIVE isotopes were released into the atmosphere with nuclear weapons testing during the decade 1955-1965. One isotope, 137Cs, is adsorbed by the finer soil particles and further movement by chemical processes is limited. Therefore, the measurement of residual 137Cs levels provided a method for estimating soil movement, by physical processes, along a sloping cornfield. Recent post-test average 137Cs concentrations on a 0.6 ha (1.5 a) hillside transect declined to 70 nCi/m2 or 45% of the initial 155 nCi/m2 of 137Cs fallout on the field surface. Thus, 55% of the 137Cs and a corresponding amount of cultivated soil have been removed. The 137Cs-computed average annual soil loss for this transect was 60 Mg/ha (27 t/a). The portion of eroded soil removed from the field was 60% of the average annual soil loss, based on the measured sediment yield of 32 Mg/ha/y (14 t/a/y) plus deposition of 4 Mg/ha/y (2 t/a/y) in the waterway upstream of the gaging site. This compared favorably with the sediment delivery of 58% determined photogrammetrically, using the measured sediment yield for the 1965-1981 period. Soil loss determinations using 137Cs and photogrammetric methods were comparable and provided information for a first attempt sediment accounting. Hillside net soil displacement rates were estimated to be about 25 Mg/ha/y (11 t/a/y). Long-term USLE computations (gross erosion) indicated hillside soil displacement twice this rate for our study watersheds of 30.2 ha (74.5a) and 33.5 ha (82.8a).