Historical sediment record of 137Cs, δ-HCH, and δ 13C reflects the impact of land use on soil erosion

Abstract

This paper reports the concentrations of Cs-137, hexachlorocyclohexane (HCH), dichlorodiphenyltrichloroethanes (DDT) and its main degradation products, delta C-3, and organic carbon in pond sediments (0-210 cm, sectioned by 2-20 cm interval) and surface soils (the 0-3 cm horizon) collected in 2010 from Chenjia catchment, which is located in Yanting county in the hilly central Sichuan of China. alpha-, beta-, and gamma-HCH, DDT, and DDD were not detected throughout the sediment profile. Trace concentrations of delta-HCH (0.89-29.31 ng g(-1)) and p,p'-DDE (1.85-6.02 ng g(-1)) were detected only in top 40 cm sediment. The Cs-137 fallout peak in 1963 (corresponding to the 55-60 cm depth), the sedimentary signature left by the last year of HCH use in 1989 (an additional indicator at 20-25 cm), and the obvious original channel bed prior to the construction of the pond in 1956 were used as temporal markers to estimate changes in average sedimentation rate between different periods due to changes in land use. Continuous, marked decrease in average sedimentation rate (i.e., 3.79, 1.35 and 1.07 cm year(-1) in 1956-1963, 1963-1989, and 1989-2010, respectively) over time was observed, probably due to the reforestation, abandoning of steep sloping farmland for afforestation and natural re-vegetation (implementation of the Grain for Green Program), and the conversion of part of gently sloping farmland terraces to orchard land since the 1980s, especially since the 1990s. This was corroborated by the observed decrease (more negative) in delta C-13 of sediment towards the surface, which indicates increased relative contribution of eroded soil particles coming from slopes with increased tree cover in sediment source area. Combined use of Cs-137, delta-HCH, and delta C-13 record in sediments has been demonstrated to be a powerful approach to reconstruction of response in sedimentation rate to historical land use changes.