Soil Leaching from Copper Naphthenate-Treated Utility Poles

by

J. A. Brient¹

Abstract

Leaching of wood preservatives into the soil surrounding wooden structures is an area of concern to utilities as well as to informed consumers and the treated wood industry. States have established residential soil cleanup standards for sites contaminated with hazardous substances, including heavy metals and organic contaminants introduced into soil from treated wood. Copper naphthenate is an effective preservative for utility poles, posts, piles, and other wooden structures that contains no arsenic, chromium or chlorinated dioxins/dibenzofurans.

Copper naphthenate-treated utility poles installed in 1987 in the Puget Sound, WA area were monitored for copper migration and depletion over a six-year period after initial installation, and soil samples were also assayed for copper leaching. This paper reports on a follow-up study on those poles and soil near the poles for copper content after 14 years of service, and expands the original study to include soil assays for organic constituents. Soil samples from more recently installed poles (1993) and background soil samples from both sites were also measured for copper and organics content.

Introduction

Copper naphthenate is a proven effective wood preservative for protection against decay fungi and wood-destroying insects.^1-4 Standards were established within the American Wood-Preservers' Association (AWPA) for specific commodities for wood treated with copper naphthenate in the mid-1980s⁵, and major commercial uses include utility poles (both pressure treatment and remedial treatment), fence posts and piles. Copper naphthenate is diluted in organic solvents such as diesel fuel or mineral spirits to facilitate penetration into the interior of the wood. Recommended AWPA minimum retention levels for coastal Douglas fir poles treated with copper naphthenate include 0.075, 0.095, and 0.150 pounds per cubic foot (pcf, as Cu metal), depending on the deterioration zone and local natural sub-environment. The Puget Sound area of Washington is in the AWPA Deterioration Zone 4 (high potential for deterioration).

Harp and Grove⁶ studied the retention of copper in poles and leaching of copper into soil from copper naphthenate-treated coastal Douglas fir poles at five utilities across the country. Poles installed in the Puget Sound area in 1987 originally had an average copper naphthenate retention of 0.142 pcf (as Cu). Pole cores were taken near the groundline and near the brand ~5 feet above groundline over 4 years to monitor copper migration within the pole. Retention data from the study (Figure 1) indicated copper depletion from the outermost core assay zone over four years but no significant downward migration toward the groundline. The lack of any consistent migration of copper into deeper (interior) assay zones suggests possible radial migration of copper out into the surrounding soil. Analyses of soil samples taken within 4 inches of the poles confirmed migration of copper into the soil, with average soil copper content increasing from 80 ppm to 800 ppm after 4 years. This 1993 study did not address any associated migration of diluent oil/carrier (#2 diesel) into soil surrounding the poles.

A follow-up study to the 1993 Harp and Grove study was recently done to reevaluate soil around these copper naphthenate treated poles. Specifically, the previous study was expanded to include analyses of soil for total petroleum hydrocarbons (TPH) and other regulated hydrocarbon contaminants such as polynuclear aromatic hydrocarbons (PAH). Soil samples and pole core samples were again taken for copper assay. Pole cores and soil samples were also collected at a second site in the Puget Sound area where copper naphthenate-treated poles were put in service in 1993. An environmental auditing firm supervised sample collection, and contract analytical labs performed all sample analyses.

Although all soil samples were assayed for copper, cost considerations precluded a comprehensive survey of organic constituents in all samples. Soil from selected poles at Site #1 were fully analyzed for TPH, volatile petroleum hydrocarbons (VPH) and PAH. Unfortunately, information came to light after the study was completed showing that creosote treated poles had been in service at site #1 since the 1930's, and were replaced with the existing copper naphthenate poles in 1987. The potential for soil contamination from the previously installed creosote poles adds an element of uncertainty to the results of soil from that site, particularly on PAH results.

Background

EPRI^7,8 conducted a similar project on soil leaching from utility poles treated with pentachlorophenol (PCP) and creosote. Samples collected at several distances and depths proximate to 202 in-service utility poles (180 PCP and 22 creosote) in 28 states were analyzed for several physical and chemical parameters, including pH, total organic carbon, TPH, chlorinated phenols, and PAH. The data revealed a rapid decline of PCP concentrations with increasing distance (up to 4 feet) from the wood pole. TPH concentrations exhibited a trend similar to PCP in that attenuation of TPH occurred rapidly with increasing distance from the pole. Maximum TPH values ranged over three orders of magnitude, ranging from <60 to >50,000 mg/kg, with an average of 5000 mg/kg. Total PAH concentrations in soil near creosote poles decreased by as much as five orders of magnitude over the same distance, with an overall average attenuation of two orders of magnitude. Overall, migration was highly dependent on localized factors such as soil type, soil chemistry, local weather and topography, initial level of pole treatment, age of pole, and other factors.

Laboratory studies on leaching of copper naphthenate in various solvents from treated southern pine wafers showed an increasing tendency to leach with increasing boiling point range of the solvent.¹⁸ No differences in Cu leaching rates were seen between copper naphthenate in mineral spirits and waterborne chromated copper arsenic (CCA), and copper leaching into soil was greater than into water. Hein¹⁹ showed that copper naphthenate was essentially immobile and non-leachable by water after application to the top of a column of soil. Non-detectable levels of copper were found in neutral water leachate over a one-year leaching study. Buchanan²⁰ found that copper naphthenate treated utility poles exposed to simulated acid rain leached measurable amounts of copper to the environment, but the copper was immobile and remained largely fixed in the soil.

The Model Toxic Control Act (MTCA) is a Washington State extension of the federal Superfund Act that established procedures to clean up sites contaminated with hazardous substances, including total petroleum hydrocarbons. Washington State promulgated maximum water and soil levels for various chemicals, including wood preservatives and components of their diluent oils. Action levels requiring cleanup based on the amended MTCA Method A include 2000 mg/kg for TPH in soil, including diesel-range organics.⁹ Copper is included in the cleanup levels and risk calculations (CLARC) under MTCA risk-based Method B (unrestricted/residential land uses) and Method C (industrial/commercial land uses). The soil cleanup levels based on direct ingestion of copper are 2960 mg/kg and 130,000 mg/kg for Methods B and C, respectively.¹⁰

Petroleum distillate fractions such as diesel or #2 fuel oil are predominantly composed of alkanes, cycloalkanes (naphthenes), and monocyclic aromatic compounds. The small quantities of PAH present in #2 diesel are typically lower molecular weight species by virtue of their lower boiling points, mostly 2-ring compounds such as naphthalenes.¹¹ Creosote is a higher boiling coal tar fraction that contains significantly higher levels of 3-, 4- and 5-ring PAH isomers (Table I). The carcinogenic PAH isomers of concern are 4- and 5-ring compounds such as benzo[a]pyrene and chrysene. Testing for carcinogenic PAH content is not required for #2 diesel because data exist to indicate their absence.¹²

1. Manager, Naphthenic Acid Technology, Merichem, Houston, TX

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