Comparative evaluation of several small mammal species as monitors of heavy metals, radionuclides, and selected organic compounds in the environment

The merit of using small mammals as monitors of environmental contaminants was assessed using data from the literature and results of a monitoring study at selected sites on the Oak Ridge National Laboratory (ORNL) research reservation operated for the U.S. Department of Energy in Oak Ridge, Tennessee. Heavy metals, radionuclides, and organic chemicals were included in the evaluations. In the literature review, exposure to contaminants was determined from tissue residue concentrations, biochemical assays, and cytogenetic assays. In general, there was a relationship between concentrations of contaminants in the soil and concentrations in target tissues of several species. This relationship was most obvious for the nonessential heavy metals, cadmium and lead.

Each species' suitability as a monitor for a specific contaminant or type of contaminant was evaluated and subsequently ranked. A relationship between contaminant uptake and trophic level emerged. Carnivores had the highest concentrations of contaminants, followed by omnivores and herbivores which had the lowest concentrations. Target tissues for specific contaminants were also ranked.

In the field study, ten species of small mammals were trapped at selected field and reference sites to monitor for exposure to mercury, strontium-90, and benzo[a]pyrene. Residue analyses and a hemoglobin-adduct assay were performed on several species including the shorttail shrew (Blarina brevicauda), the white-footed mouse (Peromyscus leucopus), and the cotton rat (Sigmodon hispidus).

Accumulation of mercury in kidney tissue and strontium-90 in bone was related to the degree of contamination of the environment as well as trophic level of the species. Both shorttail shrews and white-footed mice trapped at the mercury-contaminated site had significantly higher concentrations of mercury in kidney tissue than those trapped at the control sites. However, the mean concentration in kidney of the insectivorous shrew was nearly 33 times that of the omnivorous mouse.

Strontium-90 was present in the bone of all species trapped at the radionuclide-contaminated sites, but was highest in the herbivore, Reithrodontomys humulis, which inhabited the grassy site. For the white-footed mouse, there was a gradient effect of strontium-90 accuMulation among the highly contaminated, intermediate, and uncontaminated sites.

The hemoglobin-adduct assay was evaluated as an indicator of subchronic exposure to benzo[a]pyrene in the laboratory and chronic exposure in the field. Concentrations of benzo[a]pyrene-hemoglobin adducts in C3H mice fed benzo[a]pyrene in the laboratory for seven weeks were initially high, but declined to low, barely detectable, levels with time. Concentrations of benzo[a]pyrene-hemoglobin adducts in small mammals exposed to low concentrations of benzo[a]pyrene in the field were low and variable among individuals and between species.