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Preservative-Treated Wood and Alternative Products in the Forest Service

Leaching and Its Effects on the Environment

All wood-preservative treatments contain active ingredients that protect the wood from insects and fungi. Preservatives intended for use outdoors (figure 5) have chemical properties that are intended to keep the active ingredients in the wood and minimize leaching. Past studies indicate that a small percentage of the active ingredients of all types of wood preservatives leach out of the wood.

Photo of wooden stairs constructed in a switchback pattern on a steep hill in a forested area.
Figure 5—Stairs on the Falls of Hills Creek Trail
in the Monongahela National Forest, WV.

The amount of leaching from a particular product used in a specific way depends on factors such as fixation conditions, the preservative's retention in the wood, the product's size and shape, the type of exposure, and the years in service. Some ingredients in all preservatives are toxic at high concentrations to a variety of organisms. Laboratory studies indicate that the levels of preservatives leached from treated wood generally are too low to create a biological hazard.

In recent years, several studies have been conducted on preservative releases from structures and on the environmental consequences of those releases. For instance, the Forest Service, the Bureau of Land Management, and industry partners cooperated to study the environmental impacts of waterborne preservatives that leached from wood used to construct a wetland boardwalk (USDA Forest Service Forest Products Laboratory 2000). The construction project was considered a worst case because a lot of treated wood was used and the site had high rainfall.

Separate boardwalk test sections were constructed using untreated wood and wood treated with ACQ-B, ACZA, CCA-C, or CDDC. Surface soil, sediment, and water samples were removed before construction and at intervals after construction to determine the concentrations and movement of preservative elements that leached from the boardwalk. Aquatic insect populations in the vegetation, in sediments, and on artificial substrates were monitored.

During the first year, each of the preservatives evaluated released measurable amounts of copper, chromium, zinc, or arsenic into rainwater collected from the wood. Each preservative also appeared to elevate soil and sediment levels of the elements used in the preservative. In some cases, levels appeared to peak soon after construction. In other cases, levels appeared to increase during the course of the year.

With few exceptions, the elevated concentrations were confined to areas near the boardwalk. These levels of environmental accumulation did not appear to have any measurable biological impact. Although seasonal fluctuations in insect populations were noted, none of the invertebrate taxa evaluated were significantly reduced in the wetlands surrounding any of the treated wood.

Brooks (2000) evaluated the environmental effects of timber bridges treated with either CCA-C, pentachlorophenol, or creosote. In that study, bridges that had been in service for several years were evaluated by comparing upstream and downstream levels of preservative concentrations in sediments and populations of aquatic insects at the same sampling locations.

The two bridges treated with pentachlorophenol were in forested areas in Washington and Oregon. The Washington site appeared to contain low levels of pentachlorophenol, although the concentrations detected were approaching the lower detection limit of the instrumentation. No biological effects would be expected at those levels, and none were detected.

At the bridge treated with pentachlorophenol in Oregon, sediment samples were collected underneath the bridge and 3 feet (0.9 meter) downstream from the bridge. These samples contained slightly elevated levels of pentachlorophenol. Small decreases in several biological indices were noted directly under the bridge, but these decreases appeared to be related to differences in stream bottom habitat. No adverse effects on biological organisms were noted when a laboratory bioassay was conducted on sediments collected under the bridge (Brooks 2000).

Two CCA-treated bridges in Florida were also evaluated, one over a saline bay and the other over a freshwater marsh (Brooks 2000). The bridge over the bay was in the final stages of construction, while the bridge over the marsh had been built 2 years before. Some samples of sediments removed within 10 feet (3 meters) of the newly constructed bridge contained elevated levels of copper, chromium, and arsenic. The patchy nature of the samples with elevated levels and the observation of wood chips in the sediments led Brooks to suspect that at least a portion of the elevated samples contained treated wood sawdust. Despite the elevated levels of CCA detected in the sediments, no adverse biological effects were observed.

Very slightly elevated copper, chromium, and arsenic levels also were noted in sediments within 10 to 20 feet (3 to 6 meters) of the 2-year-old bridge, but again, no adverse biological effects were observed. In this case, the population and diversity of aquatic insects actually appeared to increase closer to the bridge.

Brooks also evaluated two creosote-treated bridges in agricultural areas in Indiana. One had been in service for about 2 years and the other for about 17 years. In each case, elevated levels of polycyclic aromatic hydrocarbons were detected in sediments 6 to 10 feet (1.8 to 3 meters) downstream from the bridges. Levels of polycyclic aromatic hydrocarbons at the newer bridge approached levels of concern. No significant effect on insect populations was noted downstream from the newer bridge. The population and diversity of aquatic insects appeared to be reduced within 20 feet (6 meters) downstream from the older creosote-treated bridge.

The author postulated that this trend was caused by the deposition of maple leaves in this area and was not a response to the polycyclic aromatic hydrocarbons that had been released (Brooks 2000). Sediments from that area did not adversely affect aquatic invertebrates in a laboratory bioassay, supporting Brooks' hypothesis.

The release and biological impacts of creosote also have been evaluated for newly installed six-piling dolphins (clusters of pilings used as moorings or bumpers) installed in the waters of Sooke Basin on Vancouver Island, BC, Canada (Goyette and Brooks 1998). Polycyclic aromatic hydrocarbon contamination was detected within 25 feet (7.5 meters) downstream from the piling, and significant biological effects were noted within 2.1 feet (0.65 meters) of the perimeter of the structure. Slight biological effects were noted in laboratory bioassays of sediments from up to 6.6 feet (2.0 meters) downstream from the pilings, but not in samples of organisms collected there (Goyette and Brooks 1998).

These recent studies of the environmental impact of treated wood reveal several key points:

  • All types of treated wood evaluated release small amounts of preservative components into the environment. These components can be detected in soil or sediment samples.

  • Shortly after construction, elevated levels of preservative components sometimes can be detected in the water column. Detectable increases in soil and sediment concentrations of preservative components generally are limited to areas close to the structure.

  • The leached preservative components either have low water solubility or react with components of the soil or sediment, limiting their mobility and the range of environmental contamination.

  • The levels of these components in the soil immediately adjacent to treated structures can increase gradually over the years.

Although elevated preservative levels have been detected in sediments adjacent to treated wood in aquatic environments, Brooks (Brooks 2000, USDA Forest Products Laboratory 2000) did not find any measurable impact on the abundance or diversity of aquatic invertebrates associated with those sediments. In most cases, levels of preservative components were below levels that might be expected to affect aquatic life. Samples with elevated levels of preservative components tended to be limited to fine sediments beneath stagnant or slow-moving water where the invertebrate community is somewhat tolerant of pollutants.

All construction materials, including the alternatives to treated wood, have some type of environmental impact. Leaching from plastic and wood-plastic composites has not been studied as thoroughly as that from treated wood, but one study found that over 70 different contaminants were released from one type of recycled plastic lumber (Weis and others 1992). Releases from recycled plastic may depend on the types of chemicals that were stored in the containers originally. Production of concrete and steel requires mining (Mehta 2001), consumes energy, and contributes to the production of greenhouse gases.

Conditions with a high potential for leaching and a high potential for metals to accumulate are the most likely to affect the environment. For typical Forest Service applications, these conditions are most likely to be found in boggy or marshy areas with little water exchange. Water at these sites has low pH and high organic acid content, increasing the likelihood that preservatives will be leached from the wood. In addition, the stagnant water prevents dispersal of any leached components of preservatives, allowing them to accumulate in soil, sediments, and organisms near the treated wood.

Riparian zones, wetlands, and meadows may provide essential habitat for key species during critical periods of their life cycles. Boardwalks and fishing platforms are commonly used in these areas. The challenge is to use the most durable, esthetically pleasing, cost-effective materials available, while still protecting sensitive ecosystems.