GROUNDWATER-BORNE 90SR IN BONE OF TREE SWALLOW NESTLINGS

Many studies have shown that the mineral content of surface waters and soils can depend on the transport of solutes by groundwater. Only a few studies have traced groundwater contaminants to the terrestrial animals of a receiving ecosystem. It is well known that ⁹⁰Sr is the most mobile of the fission products in soils and groundwater and that it bio-concentrates in bone. Better understanding of the ecological behaviour of ⁹⁰Sr in field sites can reduce uncertainties in determining potential effects of ⁹⁰Sr and can influence decisions about the management of contaminated sites. The environmental behaviour of ⁹⁰Sr also relates to the geochemistry and ecological retention of environmental calcium, especially in Ca-deficient ecosystems.

Tree swallow (Tachycenita bicolor) nestlings have long been used to determine uptake of contaminants from surface-water sediments from which their food (aquatic insects) is derived. We placed swallow nesting boxes at the Chalk River Laboratories as close as possible to the discharge areas of groundwater containing ⁹⁰Sr and where surface-water sediments had ⁹⁰Sr concentrations as high as 26 Bq/g. Nesting boxes were also placed in areas having intermediate to background (less than 1Bq/g) concentrations. We analyzed the bone of 12-day old nestlings for gross beta radioactivity, which at high concentrations can be attributed to ⁹⁰Sr. Bone from nestlings closest to ⁹⁰Sr-plume discharges had the highest concentrations of ⁹⁰Sr in sediment and in the bone of tree swallow nestlings. The highest measured beta activity of tree swallow bone was 29 Bq/g. Beta activity of 29 Bq/g produces a calculated skeletal dose rate of 9 µGy/hr. This is 23% of the most conservative radio-ecological benchmark value of 40 µGy/hr. These results indicate that ⁹⁰Sr groundwater plumes at Chalk River Laboratories can be traced to birds that feed on emergent aquatic insects but the levels are low enough that past and current ⁹⁰Sr plume discharges do not appear to be detrimental.