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Sampling site distribution map. Two hundred forty-five sampling sites were set, as the sand and gravel in the river bottom zone, and 147 sediment samples were collected in the Songhua River catchment. Sampling was conducted from June to August in 2010. R1, the mainstreams of Songhua River; R2, the Nen River; R3, the second Songhua River; R4, the Mudan River.
