Environ. Sci. Technol. 2000, 34, 4335-4340
Availability of Polycyclic Aromatic Hydrocarbons (PAHs) and Polychlorinated Biphenyls (PCBs) to Earthworms in Urban Soils MARTIN KRAUSS,* WOLFGANG WILCKE, AND WOLFGANG ZECH Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany
We studied the uptake of 20 PAHs and 12 PCBs from 25 field-contaminated soils by Lumbricus terrestris L. and tested the suitability of the equilibrium partitioning theory (EPT) to describe the uptake. The bioavailability of PAHs and PCBs was assessed by exhaustive (hexane-acetone) and nonexhaustive extraction methods (methanol-water 1:1 and 0.5 M NaOH). The latter extracted on average 6-68% and 2-42% of the total PAH concentrations and 7-81% and 9-22% of the total PCB concentrations, respectively. The extractability of an individual compound decreased with increasing octanol-water partitioning coefficient (KOW) and was higher for the PCBs than for the PAHs. The EPT described the uptake of PAHs and PCBs well. After 15 d of exposure, the concentrations in earthworms were near steady-state. We found close correlations between concentrations in all extracts and concentrations in earthworms after 15 d of exposure (total: r ) 0.75-0.94; methanol-water: r ) 0.68-0.94; NaOH: r ) 0.59-0.87). The average biota-to-soil accumulation factors (BSAF) were 10100 times higher for PCBs (0.71-70) than for PAHs (0.130.41). The BSAFs of the PAHs were independent of KOW; those of the PCBs decreased with increasing KOW (-0.72 e r e -0.97).
Introduction Earthworms (Lumbricidae) comprise the largest part of the soil fauna biomass and contribute substantially to the cycling of nutrients in terrestrial ecosystems. Earthworms are also an important food source for many vertebrate species (1). Therefore, contaminants taken up by earthworms from soil may enter the food chain (2). Hydrophobic organic contaminants (HOCs) are taken up by earthworms (i) via passive diffusion from the soil solution through the outer membrane or (ii) via resorption of the compounds from soil material passing the gut (3). The relative importance of the two pathways for different species and compounds is still not fully understood (4). If resorption in the gut is small, the uptake of HOCs by earthworms from soil can be described by the equilibrium partitioning theory (5, 6). According to this theory, the accumulation of HOCs does not closely correlate with the octanol-water partition coefficient (KOW) because the pronounced bioaccumulation potential of hydrophobic com* Corresponding author phone: ++49 921 55 2178; fax: ++49 921 55 2246; e-mail:
[email protected]. 10.1021/es001137s CCC: $19.00 Published on Web 09/08/2000
2000 American Chemical Society
pounds is compensated by their strong sorption to soil organic matter (5). However, this theory suffers from some shortcomings because the uptake of HOCs in earthworms depends on behavior of the earthworms, on soil organic matter composition, and on the residence time of HOCs in soils (4). A significant uptake via the gut may cause higher concentrations in earthworms than expected from the equilibrium partitioning theory (4). In recent years evidence was presented that only part of the total HOC concentration in soils and sediments is available for the partitioning between soil or sediment solid phase and solution and a subsequent uptake by biota or microbial degradation (7, 8). The size of this pool depends on soil chemical and physical properties (9, 10). It decreases with increasing residence time (“aging”) of the compounds in soil or sediment (11-13) and differs between compound classes. In particular, PAHs sorb more strongly to soils and sediments than PCBs (8, 14, 15). A direct method to assess bioavailable portions is the determination of HOC concentrations in soil pore water. However, HOC concentrations in solution are often not detectable. Therefore, nonexhaustive extractions with mild solvents were suggested to assess bioavailable pools (13, 16). These studies demonstrated that it is possible to predict the bioavailability of differently aged PAHs and pesticides to earthworms and bacteria by organic solvent-water mixtures. Up to now, studies on bioavailable portions of HOCs have been conducted with laboratory-spiked and subsequently aged soils (12, 13, 16). Information on biovailability in fieldcontaminated soils is lacking. Therefore, we selected 25 urban soils with a wide range of properties, which were not additionally spiked with PAHs and PCBs, to conduct an uptake experiment with the earthworm species Lumbricus terrestris L. in the laboratory. The objectives of our study were (i) to evaluate the relationship between different PAH and PCB pools in soils and PAH and PCB concentrations in earthworms and (ii) to assess whether the equilibrium partitioning model may be applied to describe the uptake of PAHs and PCBs from fieldcontaminated soils by earthworms.
Experimental Section Soils. We selected 25 urban or peri-urban topsoils with a wide range of PAH and PCB concentrations. Twenty-four sampled sites are located in the city of Bayreuth (northern Bavaria, Germany, 76,000 inhabitants) or in the surrounding rural area up to 7 km away from the city center. One site (R1) was sampled at a distance of 10 m to a main road near Rosenheim in southern Bavaria. Soil samples were taken from 0 to 5 cm depth, air-dried, sieved
