Treated timber has been a mainstay in aquatic construction projects for many years and remains so today. It has been used successfully in a variety of structures such as docks, retaining walls, bridges, boardwalks and building foundations. The strength, ease of installation, low cost and natural appearance make timber a practical choice for many applications.
The durability of timber is a key consideration in its selection for aquatic settings such as oceans, rivers and wetlands, which can be harsh environments. To ensure durability and a long service life, timber is pressure treated with preservatives that inhibit biological attack. The preservatives in most common use today are waterborne preservatives, chromated copper arsenate (CCA), ammonical copper zinc arsenate (ACZA), alkaline copper quat (ACQ) and copper azole (CA-B & C). These preservatives do an excellent job of extending the service life of timber so it can provide decades of use.
Preservatives consist of chemical components that discourage or prohibit biological attack. These chemicals, like the chemicals in any number of consumer products, are beneficial when properly used in appropriate quantities. The preservatives used to treat timber are subject to approval by the Environmental Protection Agency. They have been rigorously studied and approved for their intended use.
When treated timber is immersed in water, preservatives can leach out in small amounts. Most preservative loss occurs within the first few days of immersion. Rates of preservative loss decline quickly and are often undetectable after a week or two.
Preservative components that have leached from treated wood can accumulate in sediments surrounding a structure and these accumulations are typically minimal and do not pose any environmental risk, as evidenced by the long history and large number of timber structures constructed along waterways surrounded by abundant plant and animal life.
The use of treated timber in or over water will not create a significant risk unless a large quantity of material is installed in an area with little to no water movement or at a site that had been previously contaminated from other sources.
Every structure placed in an aquatic environment has an environmental impact. This includes both the structure and the construction process. Minimizing environmental impact is an important part of project planning.
Despite the negligible environmental impact posed by treated timber, it further benefits everyone to minimize any potential release of preservatives into the environment. There are several simple, basic steps that can ensure that the use of treated timber is as efficient as possible.
The first step is to select the proper preservative for the species of wood being utilized.
The American Wood Protection Association (AWPA) has developed standards for the appropriate use of wood preservatives for many wood species to meet a variety of service conditions. Through a system of Use Categories, the AWPA specifies the amount of preservative needed to accomplish a given task based on exposure conditions. The primary Use Categories that apply to treated timber in aquatic applications are UC3B, UC4A C and UC5A C. UC3B addresses above ground timber used in proximity to fresh water, UC4A C address timber and piles used in fresh water applications while UC5A C address timber and piles used in marine conditions.
Like all products that rely on chemical components, the objective is to use no more preservative than is necessary to accomplish the specified task. Using the minimum amount of preservative needed leaves no surplus to be absorbed by the surrounding environment.
In general, higher rates of preservative retention in treated timber increase the potential for the leaching of chemicals into the environment. Using a higher preservative retention than prescribed by AWPA standards "just to be sure" increases the environmental impact without providing additional protection.
Other means of introducing excess preservative into the environment are utilizing timber that has been pressure treated more than once or using treated timber with visible surface deposits of preservative. Both should be avoided in aquatic areas.
The second step is to allow sufficient time for fixation of the preservative. Fixation is the chemical process that binds the preservative to the wood thereby increasing its resistance to leaching. Most fixation occurs relatively quickly after pressure treatment, but the rate of fixation soon slows. Incremental increases in the amount of fixation take progressively longer periods of time. The amount of time required for the fixation process to be completed can last from days to weeks, depending on the temperature. Warm weather enhances the fixation process while cold conditions slow it down. The more time that can be allowed for the fixation process to bind preservative to the wood, the better the wood will be protected and the less preservative will be available for leaching.
A third step to minimize environmental impact is to minimize the amount of field fabrication required for construction. To the fullest extent possible, the timber should be prefabricated prior to treatment. Prefabrication not only allows the pressure treatment process to provide the best protection possible, it speeds installation on the site and eliminates or minimizes the amount of debris and retreatment that results from field fabrication.
Field fabrication, when required, should be accomplished in upland areas away from water when practical. Sawing, drilling and fitting in the field results in treated timber debris in the form of sawdust and scraps. When this debris is allowed to fall into water the amount of preservative leached is often at a much higher rate than the primary structure due to the relatively larger amount of surface area. Whether field fabrication occurs on land or over water this debris should always be collected for proper disposal.
Field fabrication also damages the protective preservative envelope created by the initial pressure treatment. To protect the wood, all cuts and holes must be retreated with the AWPA recommended copper naphthenate preservative containing a minimum of two percent copper. Care should be taken to prevent the liquid field preservative from entering water and all excess solution should be removed or allowed to dry before the material is placed into service. If field retreatment is required, take all appropriate steps to prevent the liquid preservative from entering the environment.
For a more comprehensive review of this topic see "Treated Wood in Aquatic Environments." Copies can be downloaded from the Timber Piling Council website, www.timberpilingcouncil.com. t
The Timber Piling Council serves as a resource for the proper design and use of treated timber piles.