Bioaccumulation of organic chemicals in contaminated soils: evaluation of bioassays with earthworms.

Earthworms live in close contact with the soil and can thus be considered representative for the bioavailability of chemicals at contaminated sites. Bioavailability can either be assessed by analyzing earthworms from contaminated locations or by exposing laboratory-reared specimens to soil samples from the field (bioassays). In this study, we investigate the relevance of bioassays by using an extended experimental design (to identify signs of depletion of the bioavailable phase by the earthworms) and by using two species of earthworm (the standard test species Eisenia andrei and the field-relevant Aporrectodea caliginosa). Furthermore, bioassay results are compared to body residues of worms collected from the field site: a heavily polluted polder, amended with dredge spoil. We focused on telodrin, dieldrin, hexachlorobenzene, and eight PCBs. With our bioassay design, it was shown that depletion was unlikely, although more subtle effects could have occurred (e.g., changes in sorption during the experiments). E. andrei is a good choice for bioassays because its body residues correlate well to those in A. caliginosa, as well as to those in the field-collected worms. Nevertheless, E. andrei accumulated slightly more than the other species and appeared to be more sensitive to the conditions in soil from one of our sites.

Solid-phase microextraction to predict bioavailability and accumulation of organic micropollutants in terrestrial organisms after exposure to a field-contaminated soil.

The risk posed by soil contaminants strongly depends on their bioavailability. In this study, a partition-based sampling method was applied as a tool to estimate bioavailability in soil. The accumulation of organic micropollutants was measured in two earthworm species (Eisenia andrei and Aporrectodea caliginosa) and in 30-microm poly(dimethylsiloxane) (PDMS)-coated solid-phase micro extraction (SPME) fibers after exposure to two field-contaminated soils. Within 10 days, steady state in earthworms was reached, and within 20 days in the SPME fibers. Steady-state concentrations in both earthworm species were linearly related to concentrations in fibers over a 10,000-fold range of concentrations. Measured concentrations in earthworms were compared to levels calculated via equilibrium partitioning theory and total concentrations of contaminants in soil. In addition, freely dissolved concentrations of contaminants in pore water, derived from SPME measurements, were used to calculate concentrations in earthworms. Measured concentrations in earthworms were close to estimated concentrations from the SPME fiber measurements. Freely dissolved concentrations of contaminants in pore water, derived from SPME measurements, were used to calculate bioconcentration factors (BCF) in earthworms. A plot of log BCFs against the octanol-water partition coefficient (log Kow) was linear up to a log Kow of 8. These results show that measuring concentrations of hydrophobic chemicals in a PDMS-coated fiber represents a simple tool to estimate internal concentrations of chemicals in biota exposed to soil.

Elucidating the routes of exposure for organic chemicals in the earthworm, Eisenia andrei (Oligochaeta).

Earthworms take up organic compounds through their skin as well as from their food, but the quantitative contribution of each route is unclear. In this contribution, we experimentally validate an accumulation model containing a separate compartment for the gut. Uptake from the gut is modeled as passive diffusion from the dissolved phase in the gut contents. For the experiments, we exposed Eisenia andrei in artificial soil spiked with tetrachlorobenzene, hexachlorobenzene, and PCB 153. Apart from the standard accumulation and elimination experiments, we ligatured the worm (using tissue adhesive) to prevent feeding. Model fits were good, thus supporting the validity of the model. The contribution of the gut route increased with increasing hydrophobicity of the chemical, and for PCB 153 the gut route clearly dominated. Despite the importance of the gut route, the final steady-state body residues did not exceed equilibrium partitioning predictions by more than 25%. Rate constants for exchange across the skin and the gut wall could be separately identified. The rate constant across the skin decreases with K(ow) but was generally higher than data derived from water-only exposure. The relationship with hydrophobicity was less clear for the rate constant across the gut wall.