Submitted to: Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: October 20, 2006
Publication Date: November 9, 2006
Citation: Chaney, R.L. 2006. In Situ Remediation and Ecosystem Restoration on Toxic Mine and Smelter Contaminated Soils Using Soil Amendments. Meeting Proceedings. On disk.
At many locations, dispersal of mine wastes or smelter emissions caused extensive contamination of soils with Zn, Cd, Pb, Ni, or Cu and associated elements. When contaminated soils are acidic (from pyrite in ores, or SO2 emissions, or native acidic soils), highly phytoavailable Zn or Ni caused severe phytotoxicity. Such locations became barren due to metal phytotoxicity and remained barren until humans took actions to reduce soil metal phytoavailability.
Zn-Pb ores and smelter emissions have caused severe Zn phytotoxicity at numerous locations where earlier smelter technologies were used. Sites were barren for 30-50 years before modern understandings of remediation of metal phytotoxicity were applied. At some sites with strong slopes, native forest vegetation was killed and the organic layer rich in nutrients and metals was eroded leaving phytotoxic and severely infertile soil materials with shallow depth.
For the Zn-phytotoxic soils which have simultaneous Cd and Pb contamination, one must remediate the potential for future Zn phytotoxicity, potential excessive food-chain transfer of Cd, and potential ingestion of soil which can cause Pb risk. We have shown that by applying a mixture of alkalinity (limestone or alkaline byproducts) and high quality biosolids, we can increase soil binding of the toxic metals and improve soil fertility such that a diverse plant cover can be readily established. Previous research had suggested that one could use ecotypic metal tolerant grasses to revegetate such soils, but there are no metal-tolerant legumes, so such revegetation plans would require annual N fertilization to achieve persistent cover. By converting the metals into adsorbed or precipitated forms which are not phytotoxic or bioavailable, we achieve a remediated soil on which legumes, grasses and other herbs thrive.
In the case of Pb contaminated soils, two methods have been shown to markedly reduce the bioavailability of soil Pb to animals which ingest soil. Pb uptake by plants is not an important source of soil Pb risk. Addition of phosphate can induce formation of chloropyromorphite, a mineral with low solubility and low bioavailability to animals. The application of high Fe biosolids products can cause adsorption of Pb on the Fe oxides with P coatings. Both methods reduce bioavailability as shown by field tests of remediation mixtures.
We believe that this approach is applicable to most contaminated soils depending on the potential for young children to ingest the soil. The most sensitive receptor for excessive soil Pb is young children who inadvertently ingest soil and dust by hand-to-mouth play. Present guidance in the US is to remediate soils with >400 mg Pb/kg if the soil is bare, or >1200 mg Pb/kg if the soil has plant cover. Research has shown that phosphate or Fe-rich biosolids compost treatment allows double these levels while still achieving protection of children.
A similar approach was applied to Ni contaminated soils at several locations in Canada. At one location, soils were very low in Mn, and adding limestone to reduce Ni phytoavailability induced Mn deficiency. Thus, Mn fertilizers were needed to achieve effective remediation of highly Ni phytotoxic soils. This finding stresses that a thorough evaluation of native soil fertility should be made as part of the assessment of how to remediate any particular metal toxic soil. Revegetation and remediation can achieve “revitalization” of highly phytotoxic soils, allowing generation of a new ecosystem safe for wildlife. Costs of this approach are very much less than soil replacement, and the method can achieve persistent remediation of these contaminated soils.