The reaction of copper ions and hypochlorite with minesite soils in relation to fungicidal activity

Abstract

Phytophthora cinnamomi is a soil-borne pathogenic fungus and is the primary cause of dieback disease in the jarrah forests of south-west Western Australia. Treatments are needed to eliminate the fungus from infected soils. Compounds containing cupric ions (Cu2+) or hypochlorite (ClO-) have a known fungicidal activity against P. cinnamomi, but their efficacy is affected by soil factors. This study explores the possibility of containing P. cinnamomi by treatment of surface materials for haul roads at minesites in infected areas with these compounds. Solution concentrations of Cu2+ between 50 and 100 mg/L are considered to be fungicidally effective against P. cinnamomi. In samples from the lateritic regolith at the Huntly minesite of Alcoa in Western Australia, concentrations in this range are obtained after adding about 0-5 g (range = 0.3-1 g) of Cu2+ per kg of regolith material. Soil materials from mineral sand areas on the Swan Coastal Plain in Western Australia adsorb Cu2+ less strongly than the regolith materials from the Huntly minesite. Addition of about 0.1-0.2 g Cu2+ per kg is sufficient to reach a fungicidal concentration in solution in these soil materials. Movement of Cu2+ from treated surfaces of haul roads to groundwater and surface water can present a problem. For a single application, travel times of Cu2+ in both lateritic regolith and sandy soils were calculated to be = 200 years per metre. A single treatment of materials from the lateritic regolith would be effective for a period of 2-10 years, depending on the selected material. Sandy soils would have to be treated once or twice a year. Colloidal particles in both surface runoff and throughflow can contribute significantly to the spread of copper in the lateritic regolith, but much less in sandy soils. Hypochlorite is rapidly reduced to chloride by organic matter in soils. At additions of about 0.5 g ClO- per gram of organic carbon, however, hypochlorite can be an effective fungicide against P. cinnamomi.