Do site-specific radiocarbon measurements reflect localized distributions of 14C in biota inhabiting a wetland with point contamination sources?
Introduction
Exposure of non-human biota to radionuclides or other contaminants depends upon a number of factors, including the distribution of a given contaminant in the environment relative to the utilization of the environment by resident biota (Sample et al., 1997a,b). Potential effects to biota following contaminant exposure must then be assessed based on the development of defensible linkages between the probability of exposure and the probability of corresponding effects (e.g., Garnier-Laplace et al., 2004, 2006, 2008). In many cases, contaminants, such as radionuclides, can show highly localized distributions in natural systems, particularly in areas that are influenced by point sources or by processes that result in accumulation of contaminants (e.g., sediment focusing, Rowan et al., 1995; Stephenson et al., 1995). Therefore, a key question becomes how do localized distributions of contaminants in the environment lead to organism exposure, and ultimately, to potential effects to receptor biota? To address this question, targeted sampling can be carried out in areas that show varying contaminant concentrations that span the range between background and elevated levels, so that concentrations of contaminants in environmental media can be compared to those measured in resident receptor biota (Environment Canada, 1997).
Duke Swamp is a wetland environment located on the property of Atomic Energy of Canada Limited (AECL)’s Chalk River Laboratories (CRL) site (Fig. 1). The swamp receives radionuclides, such as 14C and tritium, from Waste Management Area (WMA) “C”, and 90Sr from other WMAs primarily through groundwater influx (Killey et al., 1998; Zach et al., 1998). In areas where groundwater originating from the WMA reaches the surface, localized areas show relatively elevated concentrations of these radionuclides (King et al., 2003; King-Sharp et al., 2005). As a result, groundwater flow patterns may influence exposure to and assimilation of radionuclides into tissues of resident flora and fauna species.
Previous experimental studies of 14C uptake by vegetation in Duke Swamp (Milton et al., 1998a; King-Sharp et al., 2005) demonstrated that most of the 14C in the vegetation was taken up from atmospheric 14CO2 that had been released by degassing of the groundwater as it discharged to the surface; root uptake accounted for less than 1% of the 14C inventory in the plants. An exponential decrease in 14C specific activities (the 14C activity concentration relative to the total C concentration, e.g. Bq 14C kg−1 per kg C kg−1) in the vegetation with height above ground surface was found, probably as 14CO2 and 14CH4 disperse from the source (ground surface) and are diluted with atmospheric C (Fig. 2; Sheppard et al., 1991; Milton et al., 1998a; King et al., 2003; King-Sharp et al., 2005).
Although some work has been conducted previously to quantify 14C levels in environmental media (including air, groundwater, surface water and plants) in Duke Swamp (e.g., Sheppard et al., 1994; Killey et al., 1995; Milton et al., 1995, 1996, 1998a; Evenden et al., 1998; Killey et al., 1998; King et al., 1998), past focus has been limited to the area where the highest radionuclide concentrations were expected to occur in the Swamp. Therefore, in 2001, a detailed sampling survey was carried out in Duke Swamp, to quantify 14C levels in surface vegetation and to a lesser extent, in soils. A subset of the sites sampled was then selected for more detailed study to assess whether 14C levels measured in sedentary species (e.g., plants and fungi) reflect those measured in mobile organisms (e.g., animals). This paper analyzes the results of a series of studies, which were previously only available in reports that are not readily available (King-Sharp et al., 2005; Yankovich et al., 2005a, 2008a, 2005b, 2005c, 2008b). The objectives of the present paper were:
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To quantify 14C concentrations in flora and fauna inhabiting a wetland ecosystem (Duke Swamp) receiving inputs from a localized groundwater 14C source;
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To determine whether 14C specific activities in receptor animals reflect the localized nature of 14C in surface vegetation and environmental media in Duke Swamp;
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To compare 14C specific activities in receptor animal tissues to those measured in animal stomach contents; and
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To compare 14C specific activities in predators versus prey species.
Section snippets
Description of study area
Duke Swamp is a circa 0.1 km2 area of wetland consisting of marsh, fen and swamp habitats (CH2M Gore and Storrie Limited, 1998).
The marsh community can be described as narrow-leaved, emergent marsh habitat containing Canada bluejoint grass (Calamagrostis canadensis), common cattails (Typha latifolia), marsh fern (Thelypteris palustris), and marsh St. John's wort (Triadenum fraseri). The shrub layer consists largely of speckled alder (Alnus incana ssp. rugosa) and leatherleaf (Chamaedaphne
Sample collection and processing
Sampling was carried out during 2001 at Site 35 and 2002 at all six sampling sites to collect key environmental media samples, and representative receptor biota. Additional biota samples were collected in 2003 to fill in data gaps that remained following the 2001 and 2002 sampling campaigns. With the exception of small mammals, all sampling was carried out within a five-meter radius. To obtain a sufficient sample of small mammals trapping was conducted over a 5–10 m radius.
In addition to the
Results and discussion
Measured 14C specific activities for each sample type are provided in Table 1, Table 2, Table 3, Table 4, with a statistical summary in Table 5. Predictive relationships between animals and mosses, and between animals and air can be found in Table 6.
Conclusions and recommendations
Duke Swamp is a wetland ecosystem that receives 14C via groundwater originating from upgradient WMA “C” on AECL's CRL site. This groundwater reaches the surface of the swamp, resulting in the occurrence of maximum 14C levels over a relatively small estimated area, covering approximately 146 m2 at Site 35 (Yankovich et al., 2005a).
In general, the highest 14C specific activities in all sample types occurred at Site 35, followed by Site 27, the lowest levels were found at Site 9 in the swamp, and
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