Soil and Groundwater

International Research
Australian Research
Vegetables
Factors Influencing Rate of Leaching
Age of Timber
References

A number of international studies, and at least one Australian study (Kennedy, 2004) have documented the impacts of heavy metals from CCA-treated timber leaching into surrounding soil and groundwater. The amounts leached vary for each study, depending on climatic and geological conditions, UV exposure and acid levels. However, the studies listed here all found results that proved arsenic levels were raised through leaching to above acceptable standards.

Solo-Gabriele et al (2003b), found that the soil below and around CCA-treated timber decks contained an average arsenic concentration of 28.5 mg/kg, well above average background soil arsenic concentrations of 1.5 mg/kg. Runoff collected from the decks was found to contain over 1 mg/L arsenic and chromium. In another study, by the same researchers, soil below the CCA-treated timber decks contained an average of 34 mg/kg chromium and 40 mg/kg of copper, in contrast to an average background level of 10 mg/kg for both metals (Townsend et al, 2001).

The main impacts of leaching into soil are localized. Both Lebow (1996) and Townsend et al (2001) found that the highest concentrations of arsenic, chromium, and copper were found within five centimetres (laterally) of the CCA-treated timber, with the soil metal levels decreasing with distance. The highest median concentrations were found in the upper 20 centimetres of soil.

Australian Research

Similar results have been found in the wine-growing region of Marlborough in Australia. Robinson et al (2004), found that a quarter of soil samples exceeded the Australian National Environment Protection Council’s ‘Guidelines on the Investigation Levels for Soil and Groundwater’ for arsenic, set at 100 mg/kg, and 10 percent of the soil samples exceeded these guidelines for chromium (set at 100 mg/kg). Note that these guidelines set 20 mg/kg as the level of ecological concern in urban areas (National Environment Protection Council, 1999).

Vegetables

These elevated heavy metal levels have been found by some studies to have been taken up by plants. For example, Aziz Shiralipour (2004) from the Department of Agronomy, at the University of Florida, found that different vegetables absorb inorganic arsenic at different rates. For carrots and turnips the arsenic is absorbed by the root more than the leaves but in lettuce it is absorbed by the leaves and in higher concentrations than in root crops. The closer the vegetables are to CCA-treated timber the more inorganic arsenic they absorb. However, television and radio programmes in Australia continue to recommend the reuse of CCA-treated timber for garden bed borders.

Factors Influencing Rate of Leaching

The amount of leaching from CCA-treated timber depends on the local conditions and occasionally age of the timber. Ultraviolet exposure increases the amount of arsenic removed through rainfall by five times (Lebow et al, 2003). Weathered wood leaches more of the toxic trivalent arsenic than unweathered wood (Solo-Gabriele et al, 2003a). Acid levels can also increase leaching. The CCA chemical itself is acidic, and if this chemical is not properly fixed into the wood, can raise the levels of chromium in surrounding soils. Humic acid in mulch also poses an increased risk of leaching. Research cited by Enviros Consulting et al (2004) found that ‘metal concentrations in humic matter can be up to one thousand times greater than those not containing humic matter, and that copper is most affected’. Farm soils with applied fertilizer containing calcium, magnesium, potassium and phosphorous also increases the chance of leaching from CCA-treated timber.

Age of Timber

It is unknown just how long arsenic leaches out of timber but the studies that have been done have found that older timber is just as likely to leach arsenic as freshly treated timber. A 2002 study by the Washington DC-based Environmental Working Group (EWG) found that arsenic levels on CCA-treated wood remained high for 20 years and sealants are only effective at reducing arsenic levels on the surface of the wood for about six months (Gray and Houlihan, 2002: 4-6).

Similarly, a study by David Stilwell (1999) of the Department of Analytical Chemistry, Connecticut Agricultural Experiment Station involved a study of treated decks that were between 4 months and 15 years old. He took 45 wipe samples from horizontal deck plank surfaces and 12 from vertical poles holding up decks and found arsenic in all cases. There was no correlation between the amount of arsenic and the age of the deck although there was large variability between decks and also places on the deck.

Stilwell (1999) also found that there were elevated levels of copper, chrome and arsenic in the soils below the decks and that the amounts increased with the age of the deck. The average amount of arsenic was 76 mg/kg (138 mg/kg after 8 years) compared with a state safety limit of 10 mg/kg in residential soils.The EWG study (Gray and Houlihan, 2002: 7) found that in ‘two of five backyards and parks, the soil tested had enough arsenic to qualify as a Superfund site.’ (The Superfund Program in the US was set up to locate and clean up the most contaminated sites in the country.) This means that if these backyards and parks were industrial sites they would be considered hazardous and have to be cleaned up.

Stilwell et al (2003) describe an unusual pattern of leaching over the years. During the first year they observed a steady decline. Then erosion and weathering ‘rejuvenated’ the surface and could increase leaching. These phases were observed for several years. The resultant heavy metal levels in the soil can remain for many years. In Washington, US, the Everett Smelter hazardous waste site still contains high levels of arsenic trioxide, 85 years after production was discontinued. 9000 years is one cited estimate of the residence time for arsenic in soil (OEHAS, 1999).