Evaluation of Exposure Metrics for Effect Assessment of Soil Invertebrates

Abstract

Risk and hazard assessments for the soil environment are performed on the basis of the total content of a contaminant in the dry bulk soil. Presently, scientific evidence is emerging and indicating that pore water may be a more relevant exposure medium for uptake of chemicals by biota and plants in soil. To deduce the degree to which pore water concentrations are indeed a better metrics for quantifying uptake of organic chemicals by terrestrial biota (mostly invertebrates), a literature search was performed and the available evidence in favor of any metrics was gathered in the context of a mandate of the European Food Safety Authority. It is concluded that knowledge on uptake routes of contaminants by soil invertebrates is far from complete. Overall it is clear that uptake of organic contaminants depends on species, soil type, and the chemical properties. The mode of exposure of soil invertebrates is determined by the way animals are in contact with their local environment. Morphology, physiology, and behavior are important factors in this respect, as is the mode of uptake of food, water, and oxygen. The contribution of oral uptake may vary within a specific taxon but for soil organisms in close contact with the soil solution, pore water–mediated uptake is in general the dominant pathway and it is commonly modified by soil specific ageing and speciation, and by specific factors of the organisms, such as nutrition status. Residual uptake appears to be the most important uptake route following pore water–mediated uptake. It is likely that in this case, too, pore water is involved as carrier in or at the surface of the soil in which the chemicals are dissolved. Intraspecies (especially between different life stages) and interspecies variances (e.g., size and ecological preferences) will most likely modify the actual contribution of potential exposure pathways, and a distinction must be made between hard-bodied and soft-bodied organisms. Hard-bodied organisms rely for uptake of oxygen and water on specialized organs, whereas water (pore water) and oxygen are mainly taken up via the skin in soft-bodied organisms. Hard-bodied animals are nevertheless in contact with pore water, as shown for spiders, woodlice, and collembolans. Uptake of nutrients and chemicals is possible for all invertebrates via their food, and this may be an important route in case of food sources in which high concentrations of chemicals are present. The assimilation efficiency will however depend on species-specific properties of the digestive tract and no general conclusions are to be generated in this respect.

KEY WORDS:

• effect assessment
• exposure
• exposure pathways
• hazard assessment
• invertebrates
• organics
• pesticides
• plants
• risk assessment
• soil
• uptake routes

ACKNOWLEDGMENTS

Peter Melis (RIVM) is kindly acknowledged for performing the literature search. Part of the work reported here is included in a scientific opinion of the Panel on Plant Protection Products and their Residues (PPR Panel) of the European Food Safety Authority (2009), which is freely available on http://www.efsa.europa.eu/.