114-7 Assessing the Role of Earthworms (Lumbricus terrestris) on Lead Transport and Accumulation in Lead Contaminated Urban Garden Soils and Potential for Bioremediation

Session: Environmental Geochemistry and Health (Part I)

Presenting Author:

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Abstract:

Anthropogenic activities including use of lead-based paint and lead-gasoline have resulted in elevated legacy lead levels in most older cities in the US. Urban community gardens provide benefits to their communities but may also provide a ready source of contaminated soil. Our previous studies show that raised bed soils can become contaminated with Pb in a few years and lead transport by earthworms is a primary cause.

To evaluate Pb redistribution via earthworms, microcosms designed to mimic urban gardens in NW Ohio were constructed using a sandy loam with 5 wt% sphagnum moss, 0.1 wt% CaCO₃, and ~20% water. Lead as PbCO₃, the form found in lead paint, was homogenously mixed into the soils. Lead concentrations of 100, 200 and 400 ppm were used corresponding to EPA action levels.

The microcosms were filled with 28 cm of soil mixture, 24 adult L. terrestris were introduced, and a 2-3 cm thick layer of sphagnum moss was sprinkled on top to maintain moisture and to promote feeding and anecic burrowing behavior.

Experiments were run 2, 4, and 8 weeks. Afterwards, earthworm casts at the surface were collected and the top mulch layer was carefully removed. The soil was removed in 1 cm layers down to 5 cm from the soil-mulch interface. The remaining soil was removed and homogenized. Soil, depurated and non-depurated earthworms, egesta, and casts were collected and analyzed using FP-XRF or ICP-OES following standard EPA protocols.

Results show that L. terrestris can facilitate both Pb redistribution and accumulation in garden soils. For an 8-week experiment with 400 ppm soil Pb, casts and top mulch layer had 313 and 233 ppm Pb, respectively. Bioaccumulation factors for depurated worms ranged from 0.14 to 0.36 with a general increase with duration and soil Pb concentration. Egesta and casts have similar Pb concentrations. Overall, Pb concentrations in the underlying soil decreased (up to 17.6%) with transport of the lead to the surface and deposition in the mulch layer.

The overall reduction in lead within the underlying soils and deposition at the surface, where it could be easily collected, indicates that earthworms may provide an inexpensive bioremediation mechanism for contaminated urban gardens. However, surface deposition and accumulation of lead in earthworm tissues increases potential for human exposure and up-trophic transfer.

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