Skip to main content
ARS Home » Research » Publications at this Location » Publication #313554
Title: Dating sediment in a fast sedimentation reservoir using 137Cs and 210Pb

Author
item Zhang, Xunchang
item ZHANG, GUANGHUI - Beijing Normal University
item Garbrecht, Jurgen
item Steiner, Jean

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2015
Publication Date: 5/29/2015
Citation: Zhang, X.J., Zhang, G., Garbrecht, J.D., Steiner, J.L. 2015. Dating sediment in a fast sedimentation reservoir using 137Cs and 210Pb. Soil Science Society of America Journal. 79:948-956.

Interpretive Summary: Over 10,000 reservoirs have been constructed in agricultural watersheds in the United States since the 1940s to control flooding and sediments. Reservoir sedimentation records provide a unique opportunity to retrospectively study the effects of land use changes and climate variations on sediment production, if sediment chronology could be properly determined. The objectives of this study are to (i) evaluate the applicability of four 210Pbex models for dating deposited sediments in fast-sedimentation environments, (ii) estimate the sedimentation rates, and (iii) relate the sedimentation rates to changes in soil conservation measures, land uses, and climate. Three sediment profiles were sampled at 5-cm intervals in a flood control reservoir in the Fort Cobb Reservoir Experimental Watershed in southwestern Oklahoma. Radioactivity was measured with a gamma spectrometer for radioisotopes 137Cs and 210Pb. Four 210Pbex dating models were tested. Results showed that three numerical models [constant flux-constant sedimentation (CFCS), constant initial concentration (CIC), constant rate of supply (CRS)], which are widely used in the lacustrine and marine environments under slow-sedimentation conditions, were not applicable to fast-sedimentation flood control reservoirs because of considerable 210Pbex input with sediment which is assumed negligible in the models. The modified CRS model, which uses 137Cs age markers as references, predicted sediment chronology reasonably well. The average sedimentation rates were 3-5 times higher during the 1957-1963 time period than during the 1964-2011 period. The significant sediment reductions coincided with the implementations of numerous conservation measures in the watershed, including check dams, terraces, changing cropping patterns, and progressive adoption of no-till and conservation tillage systems. The findings will be useful to erosion scientists and soil conservationists who are interested in estimating sedimentation rates in flood control reservoirs.

Technical Abstract: Over 10,000 reservoirs have been constructed in agricultural watersheds in the United States since the 1940s to control floods and sediments. Reservoir sedimentation records provide a unique opportunity to retrospectively study the effects of land use changes and climate variations on sediment production, if sediment chronology could be properly determined. The objectives of this study are to (i) evaluate the applicability of four 210Pbex models for dating deposited sediments in fast-sedimentation environments, (ii) estimate the sedimentation rates, and (iii) relate the sedimentation rates to changes in soil conservation measures, land uses, and climate. Three sediment profiles were sampled at 5-cm intervals in a flood control reservoir in the Fort Cobb Reservoir Experimental Watershed in southwestern Oklahoma. Radioactivity was measured with a gamma spectrometer for radioisotopes 137Cs and 210Pb. Four 210Pbex dating models were tested. Three models [constant flux-constant sedimentation (CFCS), constant initial concentration (CIC), constant rate of supply (CRS)], which are widely used in the lacustrine and marine environments under slow-sedimentation conditions, were not applicable to fast-sedimentation conditions because of considerable 210Pbex input with sediment which is assumed negligible in the models. The modified CRS model, which uses 137Cs age markers as references, predicted sediment chronology reasonably well. The average sedimentation rates were 3-5 times higher during the 1957-1963 time period than during the 1964-2011 period. The significant sediment reductions coincided with the implementations of numerous conservation measures in the watershed, including check dams, terraces, changing cropping patterns, and progressive adoption of no-till and conservation tillage systems.