Remediation of streams impacted by non-point source contaminants requires an understanding of both the areas within a watershed that are contributing contamination to streams and the pathways of contaminant migration to streams. From 1998 to 2002, we studied the migration of 9 0 Sr in the Borschi watershed, a small (8.5 km 2 ) catchment three km south of the Chernobyl Nuclear Power Plant, Ukraine. Fuel particles, distributed in a heterogeneous pattern across the watershed, are weathering and releasing 9 0 Sr from the fuel matrix. Depletion of 9 0 Sr, evaluated in comparison to the immobile fission product europium-154, is occurring in the channel and wetland sediment. Channel sediments are uniformly depleted in 9 0 Sr with depth. In wetland sediments, there is a zone of depletion in the top10 cm and a zone of accumulation at depths from 10 to 25 cm. Estimates of 9 0 Sr depletion are used to map the effective source area that has contributed 9 0 Sr loading to the main channel. The effective source area includes channel bottom sediments, a wetland in the central region of the watershed, and periodically flooded soils surrounding the wetland. The total depletion from the effective source area is estimated to be 36+/-7x10 1 0 Bq. Based on observations of stream flow rate and water quality in 1999-2001, the annual 9 0 Sr removal rate from the watershed is estimated to be 1.4+/-0.2x10 1 0 or 1.5% of the inventory per year. When extrapolated over a 15-year period following the Chernobyl accident, the last value is in reasonable agreement with the estimated depletion of the source area based on 9 0 Sr/ 1 5 4 Eu ratios. The 9 0 Sr yearly leaching rate considering the whole watershed is 0.2% while the 9 0 Sr leaching rate considering the effective source area is an order of magnitude higher. Most of the 9 0 Sr release in the watershed has originated from an effective source area of 0.62 km 2 , or 7% of the watershed area.