AbstractSignificant structural failure rates in creosoted electricity distribution poles of Scots pine (Pinus sylvestris) are caused by decay of uncreosoted sapwood and heartwood of the interior groundline region. To eradicate the basidiomycetes responsible for this decay a waterborne chromated fluoride preservative paste, Rentex, has been proposed for in-situ remedial groundline injection treatment of creosoted distribution poles. The Rentex formulation has been investigated with a view to firstly establishing its temporal effectiveness in protecting distribution poles from further decay, and secondly, examining its effects on the adjacent environment.
Laboratory toxicity studies established the fluoride concentrations in Scots pine heartwood and sapwood required to provide protection against decay by strains of Neolentinus lepideus, the basidiomycete most commonly associated with internal decay of distribution poles. Field studies of Rentex treated aged pole sections showed that fluoride readily migrated through the cross-section of treated timbers till at twelve to eighteen months after treatment, toxic fluoride concentrations were generally found throughout the groundline region. The results of a microbiological isolation study of 'on-line' poles could not be used to directly corroborate these laboratory and field results as basidiomycetes were infrequently isolated regardless of treatment. However, field pole isolations of the commonly found mould Cladosporium resinae were significantly reduced for up to sixteen months after remedial treatment and C resinae displayed a greater resistance to fluoride than N. lepideus in laboratory studies.
Field studies of remedially treated creosoted pole sections showed that the chromium component of the preservative, intended to inhibit leaching of fluoride from the timber, did not migrate and was restricted to the sites of preservative injection. At positions remote from the injection sites therefore, fluoride remained mobile. Consequently, at twenty months after remedial treatment, fluoride concentrations within the susceptible groundline area had generally fallen below toxic levels. Chemical analysis of field soils adjacent to creosoted remedially treated pole sections and 'on-line' poles confirmed that falls in timber fluoride concentrations were due to leaching to the surrounding soil. Leaching of chromium to soil was also established, probably facilitated by its remaining at the injection site which provided a path of little resistance to the movement of this element from the timber.
Field studies of the environmental effects of this characteristic soil contamination were impractical due to economic and seasonal constraints. Therefore to facilitate the measurement of physical and biological indicators of any environmental impact associated with the preservative treatment, a novel physical field model was designed to simulate severe field exposure of remedially treated timber. Each of three models constructed consisted of a layered and drained microbiologically active soilbed supporting an aged creosoted Scots pine pole section. In two models the pole sections had received Rentex treatment. Above each model unit was a source of artificial rainfall to encourage leaching conditions and a source of phtosynthetically active radiation for consecutive crops of Perennial ryegrass (Lolium perenne) and Rye (Secale cereale).
Soil concentrations of fluoride and chromium adjacent to remedially treated timbers within the model units were found to be significantly higher than background levels in the control model unit and similar to those previously found around remedially treated field poles. The drainage waters of the control unit contained fluoride and total chromium concentrations which were frequently significantly lower than those concentrations found in leachates collected adjacent to treated timbers but compatible with values from uncontaminated field sites. These results indicated that the physical model provided an accurate picture of the effects of remedially treated timber on the immediate physical field environment.
However, the total quantities of fluoride and chromium found in drainage waters from the contaminated model units did not indicate that treated timber in the field would represent a serious contamination risk to groundwater supplies. Similarly, though elevated soil concentrations of fluoride and chromium resulted in reduced dry matter yield and bio-accumulation of both elements in swards of L. perenne, and reduced soil microbial activity (dehydrogenase levels), these effects were restricted to within ten centimetres of the treated timber. No long term phytotoxic effects over larger areas around treated pole sections were indicated by studies of S. cereale crops. These model results showed that the environmental impact of remedially treated field poles was likely to be minimal.
The physical field model was successful in allowing the accurate measurement of a number of physical and biological indicators of the environmental impact of remedially treated timber. The potential for further development of the physical model for inclusion in other environmental studies of chemicals and chemically treated structures is clear.
|Date of Award||Oct 1995|