Published in J. Environ. Qual. 32:2095-2099 (2003).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Heavy Metals in the Environment
Copper, Zinc, and Arsenic in Soil Surrounding Douglas-Fir Poles Treated with Ammoniacal Copper Zinc Arsenate (ACZA)
J. J. Morrell*,a,
Donn Keefeb and
Randall T. Baileysc
a Department of Wood Science and Engineering, Oregon State University, Corvallis, OR 97331
b Wood Products, Inc., Stone Mountain, GA 30083
c J.H. Baxter & Co., 85 North Baxter Street, Eugene, OR 97402
* Corresponding author (jeff.morrell{at}orst.edu).
Received for publication November 26, 2002.
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ABSTRACT
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The levels of copper, zinc, and arsenic in soil surrounding Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] utility poles treated with ammoniacal copper zinc arsenate (ACZA) were investigated at sites in Florida, Virginia, and New York. Copper levels were elevated near the poles and declined with both horizontal distance away from the pole and depth beneath the soil surface. Zinc levels were also elevated next to the poles, but the levels declined more slowly than did those of copper. Arsenic levels were elevated in soil immediately next to the poles but declined to near background levels farther away. The results indicate that metals can leach from ACZA-treated poles, but do not migrate far in the soil, and thus the levels decline sharply with distance from the poles.
Abbreviations: ACZA, ammoniacal copper zinc arsenate • CCA, chromated copper arsenate
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INTRODUCTION
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THERE IS LITTLE DOUBT that preservative treatment using pressure processes markedly improves the service life of a variety of wood products, including poles used to support electric distribution and transmission lines. Although oil-type (organic) preservatives such as pentachlorophenol and creosote have been the mainstay of many utilities, some utilities also use water-borne inorganic arsenicals such as chromated copper arsenate (CCA) or ACZA. Chromated copper arsenate has typically been used for southern pine poles, while ACZA has historically been used to treat more refractory species such as Douglas-fir. The latter system has also found some application for treatment of poles prone to woodpecker attack (Bruccato, 1994). Ammoniacal copper zinc arsenate treating solutions contain copper oxide, zinc oxide, and arsenic pentoxide at a ratio of 2:1:1 (weight basis) in an aqueous ammonia solution. The resulting treated wood contains the same elemental level (American Wood Preservers' Association, 1999a).
Both CCA and ACZA have been shown to "fix" in the wood cells to some degree following treatment (Dahlgren, 1974; Dahlgren and Hartford, 1972a, 1972b, 1972c; Hartford, 1997; Pizzi, 1982; Ruddick, 1996), in a process whereby the metal components become less soluble either through reduction or precipitation, thereby reducing the risk of leaching in service. However, questions concerning the safety of some applications of arsenical-based systems have recently emerged in a number of states, most notably California and Florida (Solo Gabriele et al., 1999; Townsend et al., 2001). Although many of these concerns have been raised about the use of treated wood in playgrounds (Matus, 2001), one outcome of the publicity has been the realization that there is relatively little data on the long-term movement of the components of these preservatives into the surrounding environment, particularly for utility poles. Tests of poles treated with pentachlorophenol in P9 Type-A oil showed little evidence of preservative migration more than 300 mm from the poles (Muraka et al., 1996). Mortimer (1991) found little evidence of movement of metals from poles treated with CCA amended with polyethylene glycol, while other work noted substantial metal levels at the groundline immediately adjacent to CCA-treated poles and treated stakes (Cooper et al., 2001; Cooper and Ung, 1997; DeGroot et al., 1979; Zagury et al., 2003); however, there is little information on the movement of metal components from ACZA-treated wood.
Brooks (1997) reviewed the aquatic risks associated with the use of ACZA and developed models to evaluate the risk of using treated wood in aquatic applications. This work also led to the development of best management practices (BMPs) for wood treatment in these environments (Western Wood Preservers' Institute, 1996). In addition, various industrial technical groups, such as the American Wood Preservers' Association, have examined preservative migration, immobilization, and fixation, but have yet to promulgate specific limitations or standards.
The components of both CCA and ACZA, although generally referred to as "fixed," have been shown to move from the wood at low levels (Jin et al., 1993). The low levels of metal in solution within the wood cell lumen probably play a role in biocide efficacy; however, the exact levels of migration that occur have not been accurately determined. Cooper (1993) showed that loss of CCA components in service was a function of rainfall, pH, and wood species. Ammoniacal copper zinc arsenate, which was developed in the mid-1980s as a replacement for ammoniacal copper arsenate (Gordon, 1947; Morgan, 1989), has received the least attention of the arsenicals, owing to its relatively recent development and specific use for treating Douglas-fir and other refractory species.
There is a tremendous debate about the future use of arsenical-based preservatives for wood protection. By voluntary agreement with the USEPA, CCA will not be used for treatment of wood used in residential applications after 31 Dec. 2003; however, there are many applications where this preservative will continue to be used, including utility poles. This decision has also produced increased scrutiny for other arsenic-containing preservatives including ACZA. Future uses of many preservative systems will depend on the development of data describing the potential for migration into the surrounding environment under varying conditions. In this report, we describe tests to assess metal levels in the soil around ACZA-treated Douglas-fir poles that have been in service in Florida, Virginia, and New York.
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MATERIALS AND METHODS
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Nineteen transmission poles (>18.9 m long) were selected at random from within utility systems in Florida, Virginia, and New York. The poles had been installed between 1991 and 1997, and were all large-size transmission poles (Table 1) treated with ACZA according to AWPA Standard C4 (American Wood Preservers' Association, 1999b)
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Table 1. Characteristics of Douglas-fir poles treated with ammoniacal copper zinc arsenate (ACZA) around which soil samples were obtained.
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Soil samples were collected at two depths from the surface, 0 to 150 and 300 to 450 mm (0 to 6 and 12 to 18 in), using a hand auger equipped with a 50-mm-diameter corer. Individual samples were taken at three equidistant locations around each pole: immediately adjacent to the pole (0 mm) and 150 and 300 mm away from the pole. Zones corresponding to 0 to 150 mm (0 to 6 in) and 300 to 450 mm (12 to 18 in) from the surface were collected from an auger sample, placed in sealable plastic bags, and stored in a cooler. Limited initial sampling indicated that metal levels around CCA-treated poles fell off sharply below 450 mm. Therefore, we elected to confine our sampling to horizons nearer the surface. The bit was washed with distilled water between sampling to minimize the risk of cross-contamination. In addition to the samples around the poles, two separate soil samples were obtained at these same depths at least 6 m away from the pole to provide a control (background-level) sample for comparison. A total of 20 soil samples (18 from around the pole and 2 background samples) were collected from most poles, although the presence of large rocks sometimes prevented us from obtaining the deeper sample. Additional soil samples were obtained from similar soil at least 10 m from any pole to serve as controls.
In addition to soil samples, wood was obtained from the outer 6 mm of each pole at either 150 mm below or 300 mm above the groundline. These wood samples were dried, ground to pass a 20-mesh screen, and extracted as described below. This assay zone does not correspond to the zone used for assessing treatment quality of new poles (American Wood Preservers' Association, 1999b), but its primary purpose was to determine whether the outer surfaces of particular poles contained either excessively high or low chemical levels.
The soil samples (10 g dry wt. basis) were extracted in 20 mL of 0.025 M diethylenetriaminepentaacetic acid (DTPA) for 2 h on a mechanical shaker (Horneck et al., 1989). The extract was filtered through Whatman (Maidstone, UK) no. 42 filter paper and the filtered extract was analyzed using inductively coupled plasma (ICP) spectroscopy. Soil metal content results were compared with prepared standards, as well as with blank samples containing only DTPA. These procedures are used for standard analyses of agricultural soils to determine fertilization rates.
Wood samples were microwave-digested and analyzed according to previously described procedures (Gavlak et al., 1994). Briefly, 500 mg of material was placed in a 120-mL Teflon digestion vessel, then 0.5 mL of trace metal–grade concentrated nitric acid and 2 mL of 30% hydrogen peroxide were added. The samples were then predigested for 30 min, capped, and microwaved for 4 min at 296 W, then 8 min at 565 W. The digested samples were transferred to a centrifuge tube and the volume was adjusted to 15 mL with deionized water before ICP analysis, as described above.
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RESULTS AND DISCUSSION
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Preservative Levels in Poles
Metal levels in the outer zones of wood 300 mm above the ground in most poles were far higher than those typically found in the traditional assay zone 6 to 25 mm beneath the pole surface (Table 2). Metal levels below the ground in the Florida poles did not differ markedly from the aboveground zone. These results also ranged from 2.4 to 3.1 times the minimum specified ground-contact retention level of 9.6 kg m-3 (all metal-oxide basis) (American Wood Preservers' Association, 1999b) (Table 2), but were expected, since there is normally a steep preservative retention gradient inward from the wood surface. Levels of copper, zinc, and arsenic generally followed the normal proportions in an ACZA treating solution (2:1:1), although there were slight differences (American Wood Preservers' Association, 1999a). Overall, however, there was no evidence of extreme over- or under-treatment, nor was there evidence of excessive loss of one or more components from the wood in service. Poles at the New York site had the lowest average surface retention, although the levels were still far above those necessary for effective protection against fungal or insect attack.
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Table 2. Mean concentrations of copper, zinc, and arsenic in the outer 6 mm of Douglas-fir poles treated with ammoniacal copper zinc arsenate (ACZA).
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Background Metal Levels
Background metal levels in soil 6 m away from the poles varied widely (Tables 3–5). Levels of copper averaged 26.8 mg kg-1 in the 0- to 150-mm sampling zone and 1.4 mg kg-1 in the deeper zone, 300 to 450 mm from surface (Table 3). However, copper levels near the surface at several Florida locations were well above those found in the New York and Virginia soils, a finding that contradicts the sandy nature of the typical Florida soils. Sandy soils should have lower cation exchange capacity and, lacking any significant volume of decomposed organic material at the surface, should be less capable of binding metals (Wang et al., 1998). The reasons for the elevated copper levels away from the Florida poles are unclear. It is possible that the elevated arsenic was contamination from prior agricultural treatments (Adriano, 2001), although it was not possible to confirm this possibility. Zinc levels were generally lower, averaging 7.6 mg kg-1 near the surface and 1.9 mg kg-1 deeper below ground. Arsenic levels were also generally low, except for one sample from Florida that contained 75 mg kg-1. As a result, the average background arsenic levels (10.3 mg kg-1) at the Florida sites must be viewed with some caution, since they were skewed by this one sample and were far higher than those found in other tests run in our laboratory. All other background analyses from all three sites showed arsenic levels of <1 mg kg-1.
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Table 3. Mean copper levels in soil at selected distances away and depths below ground from Douglas-fir poles treated with ammoniacal copper zinc arsenate (ACZA).
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Table 5. Arsenic levels in soil at selected distances away and depths below ground from Douglas-fir poles treated with ammoniacal copper zinc arsenate (ACZA).
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Table 4. Zinc levels in soil at selected distances away and depths below ground from Douglas-fir poles treated with ammoniacal copper zinc arsenate (ACZA).
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Metal Levels around Poles
Metal levels in the soil matrix at all sites were higher close to the poles and nearer to the soil surface. This tendency may indicate some downward washing of surface preservative deposits from the aboveground portion of the poles during rainfall events, coupled with a dilution effect with distance away from the pole and downward from the soil surface. It is also possible that some of the metals originated from wood fibers washed from the poles, although the low arsenic levels would argue against this premise.
Copper levels were generally elevated in samples taken immediately adjacent to the poles at both the surface and 300 mm belowground, then declined four- to fivefold at a distance of 150 mm away from the pole (Table 3). At an additional 150-mm distance, copper levels at the soil surface declined approximately 50%, although these results were still nearly 10 times higher than the background control levels from samples taken at the same depth. Extrapolating the relationship from the outer (300 mm) sampling zone suggested that copper levels should reach background levels within 450 mm of the pole at the surface, as well as 300 mm below groundline.
Zinc levels tended to be much lower than those found for copper, a finding that reflects, in part, the 2:1 ratio of copper to zinc in ACZA (Table 4). Zinc levels, however, were still lower than would be expected, suggesting that this component was more tightly bound to the wood. Previous studies have shown that zinc enhances the overall fixation of ACZA compared with the older formulations containing only arsenic and copper (Lebow and Morrell, 1995), and this enhanced fixation may contribute to the proportionally lower zinc migration. As with copper, zinc levels adjacent to the pole surface were three to seven times the background control samples, but declined to nearly half that amount 150 mm away. The extrapolated decline 450 mm away from the pole was less substantial than seen with copper, suggesting that the reduction to background levels might occur somewhat farther away with zinc. In some cases, the extrapolated zone of elevated zinc readings extended up to 1 m or more away from the structure.
Arsenic levels were generally low around all structures, averaging <1 to 10.3 mg kg-1 (Table 5). Interestingly, the highest average arsenic levels were found near the surface immediately adjacent to poles in Florida. There is substantial debate concerning the relative risks posed by arsenic in treated wood (Matus, 2001; Parris, 2000; Townsend et al., 2001; Conklin, 2001). Some sites have exceptionally low native arsenic levels, and, as a result, the impact levels are correspondingly lower. For example, Florida cites an impact level of 0.8 mg kg-1 (Townsend et al., 2001). Our test results indicated that all arsenic readings were below this limit for a given sample location (depth/distance from pole) in 8 of 18 samples and were less than or equal to 2 mg kg-1 in six other zones. Ironically, the highest arsenic levels were found in soil adjacent to poles in service in Florida. Arsenic migration has received tremendous recent attention as a result of its use in CCA, but analyses of soil from 150 or 300 mm away from these test poles indicated that arsenic levels approached the normal, low background levels.
Implications
Mean arsenic levels in various soils ranged from 1.5 to 3.0 with spikes up to 120 mg kg-1 (Adriano, 2001). Mean copper soil concentrations ranged from 6 to 33 mg kg-1 and individual values ranged to 100 mg kg-1, while those for zinc ranged from 20 to 97 mg kg-1 and up to 1500 mg kg-1 depending on soil type (Adriano, 2001). Our background levels generally fell within the ranges for the various metals. In many cases levels 150 and 300 mm away from the poles also fell within the reported background range. The mobility of metal components from ACZA in the surrounding soil might be expected to be in decreasing order of arsenic, zinc, and copper. Both copper and zinc can sorb to organic matter, although their bioavailability is unknown. Arsenic tends to be more mobile but can complex with iron and aluminum oxides. Unlike many applications of treated wood, utility poles are engineered structures that are rarely placed close together. As a result, the potential risk of broad-based or extended contamination is minimal. Our results indicate that elevated metal levels are limited to a zone within 1 m of the pole for zinc and much smaller zones for arsenic and copper; therefore, the risk of contamination beyond the narrow zone surrounding an individual structure is minimal. It is also important to note that the pole material examined for this study was treated before the promulgation of more stringent standards for treatment and post-treatment fixation of western wood species, which are designed to minimize the risk of over-treatment (Western Wood Preservers' Institute, 1996). The use of these best management practices should further reduce the risk of post-treatment chemical loss from wood treated with ACZA.
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CONCLUSIONS
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Levels of copper, zinc, and arsenic were elevated in soils immediately adjacent to ACZA-treated poles; however, the affected zone was generally confined to within about 1 m of the structure, and levels declined rapidly within 0.3 m of the pole and in the deeper soil zones sampled. In most cases, these levels were within the broader background levels reported elsewhere. Levels of both copper and zinc in soil were generally higher than those of arsenic, although in the original ACZA treatment solution and in the treated wood analyzed, the proportions of arsenic and zinc were equal.
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NOTES
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This is paper 3571 of the Forest Research Laboratory, Oregon State University, Corvallis, OR 97331.
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REFERENCES
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ABSTRACT
INTRODUCTION
