Using Radioactive Fallout Cesium (137Cs) to Distinguish Sediment Sources in an Agricultural Watershed.

Radioactive fallout Cesium (¹³⁷Cs) has been used for quantifying sources of accumulating sediment in water bodies and to determine the rates and pattern of soil erosion. The objectives of this research are to use ¹³⁷Cs as a tracer to determine patterns of soil erosion and deposition of eroding soils and to identify sediment sources and relative contributions at the watershed outlet. The study area is WE-38, a 7.2-km² subwatershed of east Mahantango creek, a tributary of the Susquehanna River about 40 km north of Harrisburg, PA. Soil samples were collected across transects in different land cover/use patterns across the watershed. Within each transect samples were collected at six locations (two at the top of the hill, two at the toe slope and two in the middle) to a depth of 50 cm at five contiguous depth increments of 10 cm. Additional samples were also collected from the watershed outlet, stream banks, land slides and gullies and reference areas. The samples were measured for ¹³⁷Cs concentrations using Gamma-ray analyses Canberra-2000 Genie-2000 Spectroscopy System. Results on quantifying the erosion rates within each transect, land cover class/use, and over the entire watershed will be discussed in detail.