Earthworms affect soil chemistry, including the bioavailability of heavy metals, but these types of earthworm effects vary greatly with soil properties such as pH. With over 300 priority hard rock abandoned mines across the state (and ~5700 lower-priority sites), Montana is a natural laboratory for vermiremediation research clarifying whether worms can improve remediation outcomes. We measured Lumbricus rubellus effects on lead levels in acid soils (pH 4.5, total soil lead: ~1000 mg kg-1 [XRF]) from the Silver Bow/Butte Parrot Mine tailings in Butte, Montana. Two soil amendments were used to improve earthworm survival and activity: 1. lime was added to quantify the worms’ pH tolerance (at rates equivalent to 0.5, 0.7, and 1 ton acre-1) and 2. organic matter (OM) was added to ameliorate physical and chemical conditions. Both water and weak acid (ammonium acetate) were used to extract soils before and after a 9-week incubation; samples were also extracted one day after lime additions but before the worm additions. Extracts were used to quantify lead (Pb) mobility across four treatments beyond the control: worms and OM, worms and three lime levels (low, medium, high).
Results/Conclusions
Compared to total levels of lead, the control soils showed low initial levels of leachable lead (1.2 mg L-1), though this value is still 80 times the federal drinking water action level. Surprisingly, these levels were further reduced by nearly half after the incubation period. Disturbance associated with soil collection and homogenization may have transiently increased Pb mobility. Lime amendments were expected to decrease lead mobility, and our expectation was that lime and worm amendments together would lead to an exponential reduction in Pb mobility. As expected, the initial addition of lime led to a sharp, 24-hour drop in leachable lead (0.37 mg L-1), but lead levels then significantly increased following the addition of earthworms to values greater than that of the control soils after nine weeks (~1.5 mg L-1). We expected that more lime would lead to higher worm activity, and we were able to confirm this prediction. Nonetheless, this increased earthworm activity did not translate into reduced lead levels. While follow-on studies are underway to clarify this unexpected result, our findings highlight the importance of this type of research aimed at cost-effective rehabilitation approaches to the legacy of ~500,000 abandoned mines across the United States.