The effective source area of 90Sr for a stream near Chernobyl, Ukraine

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 90Sr in the Borschi watershed, a small (8.5 km2) 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 90Sr from the fuel matrix. Depletion of (90)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 90Sr with depth. In wetland sediments, there is a zone of depletion in the top 10 cm and a zone of accumulation at depths from 10 to 25 cm. Estimates of 90Sr depletion are used to map the effective source area that has contributed (90)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 +/- 7 x 10(10) Bq. Based on observations of stream flow rate and water quality in 1999-2001, the annual 90Sr removal rate from the watershed is estimated to be 1.4 +/-0.2 x 10(10) 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 90Sr/154Eu ratios. The 90Sr yearly leaching rate considering the whole watershed is 0.2% while the 90Sr leaching rate considering the effective source area is an order of magnitude higher. Most of the 90Sr release in the watershed has originated from an effective source area of 0.62 km2, or 7% of the watershed area.