SREL Reprint #3715

Empirical evidence that mean soil contaminant concentration is not a conservative indicator of external exposure to wildlife

Edda Bæk1, Thomas G. Hinton2, Lavrans Skuterud3, James C. Beasley4, Brit Salbu2,
Vikas C. Baranwal5, and Ole Christian Lind2

1Norwegian Radiation and Nuclear Safety Authority (DSA), P.O Box 329 Skøyen, NO-0213 Oslo, Norway
2Norwegian University of Life Sciences, Centre for Environmental Radioactivity (CERAD), Faculty of
Environmental Sciences and Natural Resource Management, P.O. Box 5003, 1433 Ås, Norway
3Orkland kommune, P.O. Box 83, NO-7301 Orkanger, Norway
4University of Georgia, Savannah River Ecology Laboratory and Warnell School of Forestry and
Natural Resources, P.O. Drawer E, Aiken, SC, USA
5Geological Survey of Norway (NGU), P.O. Box 6315 Torggarden, NO-7491 Trondheim, Norway

Abstract: Adequate information on external exposure is often a limiting factor in ecological risk assessments because of a lack of exposure data for free-ranging animals as they interact with a heterogeneous distribution of contaminants. Instead, most wildlife exposure data are estimated from model simulations. However, empirical exposure data are rarely available to verify model results, or to test fundamental assumptions inherent in exposure assessments. We used novel, GPS-coupled radiation dosimeters on reindeer (Rangifer tarandus) to quantify their external exposure for five months in a Chernobyl fallout affected mountainous area of Norway. The exposure data were then used to test a fundamental assumption in screening-level risk assessments: Mean soil contaminant concentrations can be used to conservatively estimate chronic external exposures to representative animals. External exposure and GPS coordinates were obtained hourly and compared to model simulations of exposure based on airborne surveys of 137Cs soil contamination. Our objectives were to determine 1) if modelled dose estimates based on mean 137Cs soil concentrations were conservative, and 2) if modelled dose estimates could be improved by adjusting for spatial–temporal heterogeneity of contaminants using 137Cs concentrations within reindeers’ GPS-defined home ranges and core areas. We found that external exposure modelled from mean soil contaminant concentrations underestimated exposure by 70% compared to empirical measurements from GPS-dosimeter collars worn on individual animals. Using area-weighted mean contaminant levels within the animals’ home range improved the model estimates, but still underestimated the field dosimeter data by 53%. The best estimate of external exposure was obtained when high-use areas within their home range, along with spatial heterogeneity of the contaminant, was accounted for by using area-weighted mean soil contaminant levels within GPS-derived core areas. Accounting for temporal use of core areas within animals’ home range provided a 24% improvement. A comparison of our analyses to similar work performed on Chernobyl wolves found the two studies concur that risk assessors should be cautious in relying on the paradigm of using averaged soil contaminant levels to provide conservative estimates of external exposure to representative individuals. Results of modelled external exposure based on mean soil contaminant concentrations could misguide assessors and cause inappropriate management decisions.

Keywords: Screening-level risk assessment, Model validation, External exposure, Free-ranging reindeer, GPS-coupled dosimeter, Cs-137 Chernobyl fallout

SREL Reprint #3715

Baek, E., T. G. Hinton, L. Skuterud, J. C. Beasley, B. Salbu, V. C. Baranwal, and O. C. Lind. 2023. Empirical evidence that mean soil contaminant concentration is not a conservative indicator of external exposure to wildlife. Ecological Indicators 152.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).