The enrichment of¹³⁷Cs in the soil loss from small agricultural watersheds

Enrichment ratios (ER) are widely used to predict loss of sorbed nutrients or pesticides with runoff sediment, while ER is frequently neglected in studies which quantify past erosion from global fallout ¹³⁷Cs losses. The ER of ¹³⁷Cs (ER-¹³⁷Cs) in the soil loss and the subsequent depletion of ¹³⁷Cs at the soil surface were determined for eight small watersheds (1.6-16.8 ha) with different soils and land use. Due to preferential loss of the clay fraction, the upper 5 mm of the soil surface was significantly depleted of ¹³⁷Cs after a heavy storm. A total of 31 watershed-events were investigated with soil losses ranging between 1.2 and 480 kg-ha^-1 and sediment concentrations between 1.98 and 54.1 g-L^-1. The correspondent ER-¹³⁷Cs (mean: 1.72, range: 0.40-4.95) was positively correlated to the ER of clay, organic carbon, total nitrogen and calcium-acetate-lactate-extractable phosphorus (P_CAL). A close correlation between ER-¹³⁷Cs and ER-P_CAL was also found for sediment samples of detention ponds, where most of the ER values were less than 1.0 due to depletion. Therefore, ER-PCAL seems to be a suitable estimate of ER-¹³⁷Cs for both, erosion and deposition processes. Our findings strongly support the need for considering ER-¹³⁷Cs, when ¹³⁷Cs data are used to assess rates and pattern of soil redistribution. Otherwise, soil loss will be overestimated in a range of about factor 2 in many cases.