THE ROLE OF EARTHWORMS ON LEAD TRANSPORT IN COMMUNITY GARDEN SOILS

Legacy lead (Pb) remains a significant health issue especially in older urban centers. While community gardens provide numerous benefits, they can also provide ready access to potentially contaminated soils. While raised beds can greatly decrease exposure, results of our prior studies have shown elevated Pb in community garden raised beds even in no-till settings. Earthworms can provide increased nutrient availability and cycling in soils while also regulating water flow and soil structure. However, through their burrowing, they may also act to transport lead.

To test the role of earthworms on Pb transport in soils, microcosms were constructed using 18 gal plastic bins. The soil was a locally sourced (Sandman #1) sandy loam with 5 wt% dry sphagnum moss for food, 0.5 wt% CaCO₃ in the form of oyster shells to buffer pH, and ~25 wt% water. Lead was added as PbCO₃ (white lead) the primary form of lead in leaded paint. Two microcosm designs were used, one had Pb-spiked soil at the top of the bin and the other had the Pb-spiked soil sandwiched in the middle 15 cm from the top and 15 cm from the bottom.

Anecic common nightcrawlers (Lumbricus terrestris) were sourced locally. After deprecating for 2 days, 40 worms were added to each bin. Soil temperature, pH, and moisture were monitored daily and adjusted to optimize worm growth. The experiments were run for 28 days at which point the soil was removed in 2.5 cm thick layers trying to minimize mechanical mixing between layers. A control bin without earthworms was used to test mechanical lead transport. Soil layers were homogenized and 3 subsamples were removed, dried and then analyzed in triplicate using a field portable Xray Fluorescence (FP-XRF) instrument following USEPA Method 6200.

In the bin with Pb-spiked soil at the top, elevated Pb levels were found all the way to the deepest layer (32.5-35 cm). In the bins with Pb-spiked soil sandwiched in the middle, elevated Pb level were found both above in the top soil sample (0-2.5 cm) and the bottom soil sample (32.5-35 cm). The control bin showed measurable mechanical mixing up to 5-7.5 cm from Pb-spiked layer, but not at surface or bottom layers. Therefore, we conclude the Pb transported to the surface and bottom layers was due to earthworm activity. As such, earthworms could transport Pb from underlying original contaminated soil up into raised bed soils.