Quantification of natural backgrounds and anthropogenic contaminants in a pristine Arctic environment : The Anadyr River basin, Chukotka Peninsula, Russia

The Anadyr River, located in northeastern Siberia, is the second largest river emptying into the Bering Sea. The river is a true Arctic river, straddling the Arctic Circle along much of its eastward course to the sea. The Anadyr basin is a relatively pristine watershed, lacking major industrial or agricultural activities, although mining of the extensive mineral resources could potentially be a source of contamination. This study examines the first samples from the Anadyr drainage basin to quantify background and anthropogenic contaminant levels of metals and radionuclides. Contamination by anthropogenic radionuclides or trace metals is not evident in the Anadyr River basin, nor in the tributari- es to the Anadyr. The natural background from detrital material (i.e., 40K, 210Pb, 232Th, 226Ra ; >400 Bq/kg) is at least two orders of magnitude higher than the signal attributable to anthropogenic (fallout) radionuclide- s (e.g., 137Cs; [sim ]3 Bq/kg). Low specific activities (average 1.6 Bq/kg) and inventory (44.2 mBq/cm2) of fallout 137Cs are attributable to the dominance of coarse-grained sediments within the drainage basin, and the annual transport of clay-sized particles (and adsorbed activity) to the estuary. Textural analyses document that sediments in the basin and estuary are dominated by sand-sized material [larger than 4 (63 µm)], which typically comprises >90% of the sediment by weight. Cores collected in shallow portions of Onemen Bay (<2.5 m deep) are characterized by a thin (<1 cm thick), high-porosity layer of fine-grained sediment overlying hard-packed sand. Metals show enrichment over crustal levels in As and Sb (by 2-6×). The ratio of As/Sb and reported mineralization within the drainage basin indicate that these are naturally occurring concentrations. The Anadyr River basin is a relatively pristine Arctic environment against which to compare potentially contaminated Arctic sites.