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Migration of Strontium-90 in surface water, groundwater and sediments of the Borschi Watershed, Chernobyl Freed, Rina

Abstract

Effective stream remediation of non-point source contaminants, such as Chernobyl fallout, requires an understanding of the areas within watersheds that are contributing contamination to streams, the pathways of contaminant migration to streams, and the mechanisms controlling concentration changes in streams. From 1998-2002, the migration of ⁹⁰Sr was studied in the Borschi watershed, a small (8.5 km2) catchment, three km south of the Chernobyl Nuclear Power Plant. Estimates of ⁹⁰Sr depletion from soil cores (based on the ratio of ⁹⁰Sr to the relatively immobil ¹⁵⁴Eu) were used to map the effective source area that has contributed ⁹⁰Sr loading into the main channel. The effective source areas include the channel bottom sediments, a wetland in the central region of the watershed, and periodically flooded soils surrounding the wetland. The estimated ⁹⁰Sr leaching rate considering the effective source areas agrees with the estimate based on monitoring observations of stream water quality and flow rate in 1999- 2001, 2.0% per year. In approximately 44 years, 90% of the remaining ⁹⁰Sr could be removed from the effective source areas. We hypothesize that during discharge periods, the pore waters in the wetland represent the ⁹⁰Sr concentration of advecting groundwater while during stagnant periods, the pore waters represent the concentration of ⁹⁰Sr in equilibrium with the sediment. This proposed explanation is supported using PHREEQC in a dual porosity mode. Using independent estimates of the model parameters, the pore water concentration profdes could be successfully matched with the assumption of advective transport during the discharge period and diffusive transport of ⁹⁰Sr during near-stagnant conditions. Changes in the ⁹⁰Sr concentration of the Borschi stream are correlated with the elevation of the water table in the vicinity of the wetlands. The elevation of the water table is a surrogate variable for the area of submerged soil. As the area of submerged soil increases, more of the contaminant in the upper soil horizon is saturated and more ⁹⁰Sr is released into the stream. In contrast to the prevailing assumption that the mechanism of ⁹⁰Sr migration to streams is overland flow during storm events, over 70% of the annual flux occurs during baseflow conditions.

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