A mass balance approach for evaluating leachable arsenic and chromium from an in-service CCA-treated wood structure

Abstract

Many existing residential wood structures, such as playsets and decks, have been treated with chromated copper arsenate (CCA). This preservative chemical can be released from these structures incrementally over time through contact with rainfall. The objective of this study was to evaluate the levels of arsenic and chromium leached from an in-service CCA-treated deck exposed to rainfall, as well as their possible impacts on soils and shallow groundwater. Two monitoring stations, one containing a CCA-treated deck and the other containing an untreated deck as a control, were constructed outside for this study. Rainfall, runoff water from the decks, soils below the decks, and infiltrated water through 0.7 m depth of soil were monitored for arsenic and chromium over a period of 3 years. The concentration of the CCA-treated deck runoff for arsenic (0.114 -4.66 mg/L) and chromium (0.008 -0.470 mg/L) were significantly (p < 0.001) higher than the untreated deck runoff (≤ 0.002 mg/L for both). During the 3 year monitoring period, 13% of the arsenic and 1.4 % of the chromium were leached from the amount initially present in the CCA-treated wood. Arsenic levels (< 0.1 -46 mg/kg) in soils under the CCA-treated deck were significantly (p < 0.001) higher than under the untreated deck (< 0.1 -2.7 mg/kg), while chromium levels were statistically the same below the two decks (2.4 -9.6 mg/kg). Approximately 94% of the arsenic from the runoff was absorbed in the soils below the CCA-treated deck; the upper 2.5 cm of the soils captured 42% of the total. The infiltrated water concentrations for arsenic (< 0.001 -0.085 mg/L) and chromium (< 0.001 -0.010 mg/L) below the CCA-treated deck were both significantly (p < 0.001) higher than below the untreated deck (≤ 0.006 mg/L). The amounts of arsenic found in the infiltrated water below the CCA-treated deck represented 6% of total arsenic leached and less than 0.7% of the initial mass in the wood. The study demonstrated that exposure of a CCA-treated deck to rainfall resulted in elevated arsenic concentrations in both runoff and soil. Although only a relatively small fraction of the initial arsenic from the wood was found to infiltrate through the soil, these impacts were significant and caused the infiltrated water to exceed drinking water standards. The study suggests that potential

Leaching and decay resistance of alder and pine wood treated with copper based wood preservatives

“…Chromium is present as hexavalent chromium but it is reduced to trivalent chromium in the treated wood while arsenic is pentavalent and copper is curic oxide. Inorganic pentavalent arsenic and hexavalent chromium (Cr VI ) are classified hazardous to the environment and to humans (Cui 2004, Hingston et al 2001, Shibata et al 2007, Moghaddam and Mulligan 2008, Lin et al 2009). …”

Field-scale leaching of arsenic, chromium and copper from weathered treated wood

“…In general rainfall leachate samples were mostly in the acidic range (pH<7) with the pH of the leached water higher than that of the background levels of rain and inversely proportional to the volume of leachated water (R 2 of 0.42–0.5), supporting that wood buffers the pH of the rainwater (Shibata et al, 2007). …”

“…Arsenic and chromium are considered human carcinogens and Cu can be toxic to aquatic organisms (Flemming and Trevors, 1989; Weis and Weis 1999; Dubey et al, 2007). Metals release from treated wood has been reported by many researchers during the wood service life (Khan et al, 2006a; Shibata et al, 2007), during disposal (Khan et al, 2006b; Jambeck et al, 2006; Moghaddam and Mulligan, 2007) and from recycled forms such as mulch (Jacobi et al, 2007; Shibata et al, 2006). Shibata et al 2007 summarized other researchers’ results of bulk metal loss rates ranging from an average 25% lost after 20 to 43 years of exposure in temperate Sweden (Evans and Edlund, 1993) and 22% after 44 months in tropical Hawaii (Jin et al, 1992) and 5% in 1 year in southern Florida (Khan et al, 2006a).…”

Variation of arsenic concentration on surfaces of in-service CCA-treated wood planks in a park and its influencing field factors

“…The main agents of CCA including Cr, Cu and As are listed as priority pollutants by the US Environmental Protection Agency (Weis et al 1992). Leaching of As from CCA-treated wood has been widely observed in the field (Stilwell and Gorny 1997;Zagury et al 2003;Khan et al 2006;Shibata et al 2007) and in the laboratory (Shibata et al 2006;Mercer and Frostick 2012) as well as after disposal (Jambeck et al 2006;Moghaddam and Mulligan 2008). The toxicity of As to human health has been widely recognised, and the potential As-related risk to children playing on CCA-treated wood has been well documented (Dang and Chen 2003).…”