Development of Ecologically Protective Soil Concentrations Using Food-Chain Modeling in Conjunction With Site-Specific Soil and Tissue Concentrations. Anita Balaraman, Rachel Bonnefil, and Doug Lipton, LFR Levine-Fricke, 1900 Powell Street, Emeryville, CA 94608
This study used site-specific soil and tissue concentrations to validate a conventional ecological risk assessment that used literature-derived exposure values to estimate risk to ecological receptors at an upland site adjacent to the San Francisco Bay. The purpose of the study was to establish target soil concentrations for the chemicals of concern (COCs) that would be protective of ecological receptors at the site; the primary COCs were cadmium, copper, lead, nickel, selenium, and zinc. The study was designed to reflect likely exposure routes for certain representative receptor species: American kestrel, house mouse, and red-tailed hawk. The exposure of house mice to COCs would be through contaminated plant material, invertebrates and ingestion of minor amounts of soil while feeding. The house mice represented populations of omnivorous small mammals. Kestrels have been known to feed primarily on large insects, lizards, and small mammals; red-tailed hawks feed primarily on small mammals. Kestrels represent raptors that forage on site; a higher trophic level carnivore. Red-tailed hawks were considered because of their high trophic position and their use of rodents as a food item. Site-specific tissue concentrations for plants- seeds/ stems & leaves, crawling and flying invertebrates, reptiles and house mice were measured and co-located to the extent possible with soil collected from 12 sampling stations throughout the 135 acre site. Biotransfer factors (BTFs) were calculated for each COC from these site-specific soil and tissue samples and then input into a food chain model to estimate dosage to the receptors. The dosages were compared to two Toxicity Reference Values (TRVs) for each COC, a more environmentally conservative "low" TRV based on no-effects and a less conservative "high" TRV based on a low-effects. Hazard quotients (HQs) were then calculated to compare modelled dosages relative to the low and high TRVs. The dose model for each receptor was used in a back-calculation to derive ecologically protective target soil concentrations that produce upper HQ (dose/low TRV) and lower HQ (dose/high TRV) less than 1. Emphasis was placed on the bioavailability of the inorganic chemicals at site, and the relevance of TRVs (which are often derived from more soluble and available forms of the COC) to site-specific conditions. Based on extensive literature review, bioavailable dosages were calculated for some inorganic COPCs (lead and nickel) which were more representative of site-specific conditions. Using this approach, ecologically protective target soil concentrations for cadmium, copper, lead, nickel, selenium, and zinc were derived for the site. These proposed protective soil concentrations represent the highest concentrations at which no adverse ecological effects are expected to occur as a result of exposure to the respective COCs under the site’s existing conditions and habitat.
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