The power of size. 2. Rate constants and equilibrium ratios for accumulation of inorganic substances related to species weight

Abstract

Most of the thousands of substances and species that risk assessment has to deal with are not investigated empirically because of financial, practical, and ethical constraints. To facilitate extrapolation, we have developed a model for concentration kinetics of inorganic substances as a function of the exposure concentration of the chemical and the weight and trophic level of the species. The ecological parameters and the resistances that substances encounter during diffusion in water layers were obtained from previous reviews. The other chemical parameters (the resistances for permeation of lipid layers) were calibrated in the present study on 1,062 rate constants for absorption from water, for assimilation from food, and for elimination. Data on all elements and species were collected, but most applied to aquatic species, in particular mollusks and fish, and to transition metals, in particular group IIB (Zn, Cd, Hg). Their ratio was validated on 92 regressions and nine geometric averages, representing thousands of (near-)equilibrium accumulation ratios from laboratory and field studies. Rate constants for absorption and elimination decreased with species weight at an exponent of about -0.25, known from ecological allometry. On average, uptake-rate constants decreased with about the reciprocal square root of the exposure concentration. About 71 and 30% of the variation in absorption and elimination was explained by the model, respectively. The efficiency for assimilation of elements from food appeared to be determined mainly by the food digestibility and the distribution over egested and digested fractions. (Near-)equilibrium accumulation and magnification ratios also decreased with the reciprocal square root of the exposure concentration. The level of the organism-solids concentrations ratios roughly varied between one and two orders of magnitude, depending on the number of elements and species groups investigated. Metal concentrations did not increase at higher trophic levels, with the exception of (methyl-)mercury. Organism-solids concentration ratios for terrestrial species tended to be somewhat lower than those for their aquatic equivalents. Food web accumulation, expressed as organism-organic solids and organism-food concentrations ratios, can therefore be only partly explained by ecological variables. The model is believed to facilitate various types of scientific interpretation as well as environmental risk assessment.

Contaminant Levels in Rainbow Trout, Oncorhynchus mykiss, and their Diets from Missouri Coldwater Hatcheries

“…The relatively low accumulation from diet to fillet tissue was expected given that gastrointestinal absorption in animals is typically less than 15% for lead and cadmium and excretion can be fairly rapid (Goyer 1996). Similar low assimilation efficiencies for these metals in aquatic organisms were observed by Hendriks and Heikens (2001).…”

Comparing priority setting in integrated hazardous substance assessment and in life cycle impact assessment

“…PCBs, DDT/DDE, and cesium-137 were not identified as they have no current emissions and the observed concentrations are caused by chemical residence in sediments. Lead, mercury, and cadmium are bioaccumulative heavy metals (Hendriks and Heikens 2001). The current LCIA models do not include ecotoxicity from secondary poisoning through food web bioaccumulation, which results in the underestimation of the toxic potential of food web accumulative substances.…”

Organotin accumulation in an estuarine food chain: Comparing field measurements with model estimations

“…It has been successfully applied to estimate accumulation of priority substances, i.e., organic chemicals (Veltman et al, 2005; and metals (Hendriks and Heikens, 2001), in aquatic and terrestrial communities.…”

“…This complexation may result in lower elimination rates compared to neutral organic substances. Moreover, elimination rates of organotins may be more comparable to rate constants of metals than of organic substances, as metals are known to bind to proteins as well (Hendriks and Heikens, 2001).…”