Determination of lake sediment accumulation rates in an agricultural watershed using lead-210 and cesium-137

Quantifying the effectiveness of erosion control practices in watersheds remains a difficult problem. Determination of recent sediment accumulation rates for lake sediments in agricultural watersheds using radioisotopes, such as lead-210 (Pb-210) and cesium-137 (Cs-137), is potentially a valuable means of assessing the effectiveness of soil conservation practices at the watershed scale. The predominance of sediment arriving in runoff from the watershed, variable sedimentation rates, and mechanical mixing of soil in nearby fields all present challenges in the conversion of radioisotope data to sedimentation rates. Four sediment cores from Beasley Lake, Mississippi, were used to demonstrate the application of the Constant Initial Concentration (CIC) model for calculating sediment age from the distribution of Pb-210 bottom sediments. The activity of Cs-137 was used to supplement the Pb-210 data by providing a benchmark date within the core to calibrate the CIC model. Three of the four cores showed reductions in sediment accumulation rate within the 30 years prior to core collection in 2008 and 2011 by at least 50% relative to rates from before the adoption of soil conservation measures in the watershed. The most recently resolved rates were approximately 0.5 cm y(-1) (0.2 in yr(-1)). The study demonstrates the application of the CIC model for developing sediment accumulation chronologies in agricultural catchments.