• Introduction
• Arsenic Gets into the Soil
• Arsenic and Plants
• Plant Uptake of Arsenic
• References

Excerpts on Uptake of Arsenic by Plants Grown Near CCA Preserved Wood
By David E. Stilwell
Department of Analytical Chemistry
The Connecticut Agricultural Experiment Station
PO Box 1106
New Haven, CT 06504
March 2002

Introduction
Chromated Copper Arsenate (CCA) is the most popular water-born wood preservative used today. It is commonly used for outdoor construction of raised beds in gardens, decks, porches, picnic tables, children's playscapes, docks, and sound barriers. However, this diverse use of CCA has raised concerns regarding the dispersal of arsenic into the environment (Kelsall et al., 1999; Stilwell and Gorny, 1997; Weis and Weis, 1996). Indeed, the production of CCA wood accounts for about 90% of the arsenic used each year in the United States (Reese, 2000). The health hazards of acute arsenic poisoning are well documented (NRC, 2000), and arsenic is a known human carcinogen, but the risk assessment at low levels is a matter of controversy (Guo and Valberg, 1997; NRC, 2000). Nonetheless, the US EPA has recently proposed lowering the drinking water standard for arsenic from 50 µg/L to 10 µg/L, and recommended a non-enforceable goal of zero (US EPA, 2000). And it has now proposed a phase out of this material. Nonetheless it will take many years for this wood to be taken out of service.

The potential environmental effects in manufacture and use of CCA wood include: the translocation of arsenic to soil and water by leaching of the CCA from the wood by water; dispersal of CCA laden sawdust (generated during construction) in the soil; physical wearing of the wood; and runoff at lumber yards and at treatment facilities. Soil contamination can also result from improper disposal of CCA wood in unlined landfills or mixing it with other wood that is chipped and subsequently sold as mulch. The potential pathways for human exposure to arsenic include physical contact with treated wood surfaces, followed by ingestion via hand to mouth, exposure during construction involving inhalation of sawdust and ingestion, consumption of plants grown in the vicinity of CCA wood (such as raised beds or around decks), and by exposure to contaminated soil (inhalation of dust and ingestion).

The wide use of CCA in garden environments may be attributed to its comparatively low capital costs, wide availability, and lack of alternative products, coupled with the undisputed ability of the preservative to protect the wood from termites and to retard the decay of the wood by fungi, particularly in the damp environment associated with productive gardens. Treated wood is used in gardens for borders, raised beds, posts and stakes. Typically, plants are also grown around the perimeter of decks, patios and porches, all of which may contain CCA wood. Because of its widespread use, many people are concerned about the potential harmful effects caused by leaching of CCA into the soil where the arsenic could be taken up by edible plants. Although the amounts of inorganic arsenic that is normally consumed in the diet has been estimated (Kabata-Pendias and Pendias,1992; Dabeka et al., 1993; NRC, 2000), not much is known about the additional amounts that a gardener would be exposed to by growing plants in raised beds or around decks built using CCA treated wood.

Arsenic Gets into the Soil

The potential of the CCA preservatives in wood to leach into the soil in significant amounts is becoming well established. In the early to mid 1990's laboratory studies demonstrated that high percentages of CCA could be released from the wood by aqueous solutions. Aceto and Fedele (1994) used simulated rainwater (pH 3.0-6.1) to study the release of CCA in wood ground to a coarse powder. Between pH 4.5 to 6.1, 21-24% of the Cu, 7% of the Cr, and 6% of the As in the CCA wood were released after a 72 hour extraction, while at pH 3.0 100% of the Cu, 14% of the Cr, and 18% of the As were extracted. Warner and Solomon (1990) also concluded that acidity was a major factor in the leaching mechanism. In their study, the percentages of the CCA components that were leached after 40-day immersions in buffered solutions between pH 3.5 and 5.5 ranged from 92-100% for Cu, 12-53% for Cr, and 32-68% for As. Thus, considerable amounts of Cu, Cr and As could leach from outdoor use of CCA treated wood since rainwater is acidic.

To determine the extent of preservative leached into the soils we have carried out a number of studies. In one study the Cu, Cr, and As content in soils under a total of 7 decks built using CCA were compared to control soils (Stilwell and Gorny, 1997). In all cases, the preserving elements were elevated in the soils under the decks compared to the controls. The average amounts of Cu, Cr, and As in the soils under the decks were 75, 43, and 76 mg/kg, respectively, as compared to 17, 20, and 3.7 mg/kg, respectively, in control soils. Recently, a group in Florida reported finding larger amounts of these elements in soils under decks (Townsend et al., 2001).

We also conducted a survey of soils near traffic sound barriers built using CCA wood (Stilwell and Graetz, 2001). In this study we found that the Cu, Cr, and As contents in the soils at a small lateral distance from the structure were statistically indistinguishable from background soil samples. Concentrations of Cu, Cr, and As in the soils directly under the barrier averaged 80, 38, and 67 mg/kg, respectively, while the background soil and soil samples taken 80 cm lateral from the barriers were 16 and 20 (Cu), 15 and 15 (Cr), and 2.1 and 1.4 (As). These data suggest that since the elements leached from the wood do not migrate laterally, the contamination could be confined to small areas. Localized confinement of CCA, leached from wooden stakes, was also noted by DeGroot (1979).

While all of these studies have shown elevated amounts of Cu, Cr, and As in soils near CCA wood, only the As levels reached values that exceeded the regulatory standards. The State of Connecticut regulatory standard for As, Cu, and Cr in soils is 10, 2500, and 3900 mg/kg, respectively (State of Connecticut, 1996). Thus, the element of concern from a regulatory standpoint and hence, from a potential health concern, is arsenic.

Arsenic and Plants

Given the evidence that this widely used preservative leaches into the soil, and that the amounts of arsenic found in the soil can exceed regulatory limits, what are the implications to the homeowner, farmer, and gardener who grow plants near CCA wood? Unfortunately, very few, and somewhat conflicting, studies have been conducted on the analysis of arsenic in plants grown near CCA structures. In an early study (Levi et al., 1974), analysis of grapes grown adjacent to stakes showed no increase in arsenic, chromium or copper. In a later study (Hickson, 1992), carrots, okra, peppers, cucumbers and tomatoes were grown in raised beds made with and without CCA wood, and the results were compared to store purchased vegetables. In this study the chromium and copper were all in the same range as controls. However, the arsenic (mg/kg, dry weight basis) was higher in carrot (2.2 and 2.9) and tomato (0.5 and 2.1) samples compared to the vegetables grown in the control raised bed (<0.8 mg/kg dry weight basis), but not higher than store purchased samples (2.7 -3.7 mg/kg dry weight basis). No differences in arsenic levels were found in okra, pepper and cucumber samples. In another study, Speir et al. (1992) grew three indicator plants (beetroot, white clover, and cos lettuce) in soils to which CCA sawdust was added at a 10% (v/v) level (45, 136, and 63 mg/kg Cu, Cr, and As, respectively) to assess the feasibility of using CCA sawdust as a soil amendment. They found that although the plant roots concentrated Cu, Cr, and As in high levels, the above ground portions of the plants did not. For example, As (mg/kg, dry weight basis) in lettuce root grown in the CCA amended soil was about 150 compared to less than 10 in the controls, while the amounts in the leaf portions were about 6-9 regardless of growth media. These workers stressed the need to conduct tests on a wider range of edible plants.

Plant uptake of arsenic in vegetables grown in garden soils contaminated with arsenic from mining activity in south-west England has also been reported (Thornton, 1994). In this study, 32 home garden sites were examined. The soil arsenic at these sites ranged from 144-892 mg/kg and averaged 322. In normal agricultural soils in England the range is 2-53 mg/kg As with an average of 10 mg/kg As. The arsenic content was determined in six garden crops: lettuce, onion, beetroot, carrot, pea and bean. The arsenic uptake (mg/kg dry weight) was species dependant with the highest amount in lettuce (average 0.85, range 0.15-3.9, n=28) and lowest in beans (average 0.04, range 0.02-0.09, n=7). The effects of soil constituents such as iron, phosphorus, and calcium on arsenic uptake were examined. In lettuce, the uptake increased with increasing phosphorus in the soil and decreased with increasing iron content, presumably due to competitive sorption reactions between phosphorus and arsenic in the soil and with precipitation reactions with the iron to form insoluble iron arsenates. The UK statutory limit of 1 mg/kg As (fresh weight) was not exceeded in any of the vegetable samples.

Conversely, the limit for arsenic in vegetables in plants grown in soil in the Antofagasta region in northern Chili exceeded the statutory limits (Queirolo et al., 2000). In this survey, various vegetables were analyzed and the amounts of arsenic, on a fresh weight basis, were reported. The amounts in corn (average, 1.8 mg/kg) and in potatoes (0.86 mg/kg) exceeded the 0.5 mg/kg Chilean limit for arsenic in food. The elevated arsenic in the vegetables was attributed to soil and water contamination caused by proximity to active volcanoes. The water was reported to contain 50-250 µg/L arsenic. The soil As levels was not reported.
Studies have also been carried out on plant uptake of arsenic under laboratory conditions, primarily hydroponic (Burlo et al., 1999; Carbonell-Barrachina et al., 1999; Cox et al., 1996; Pickering et al., 2000), or in soils spiked with arsenic (Onken and Hossner, 1995). The major parameters affecting the arsenic level in the plant tissue were found to be dependent on the type of plant, the part of the plant (root vs. shoot), the concentration and form of arsenic in the solution, the soil and in the soil solution, the amount of iron oxides in the soil, and finally, the amounts of phosphorus added to solution or to soil.

Literature review thus suggests that the arsenic concentrations in soils near structures built with CCA wood are likely to be sufficiently elevated to distinguish arsenic levels in plants grown in such soils from controls, provided sensitive present day equipment was employed. Initially then, we gathered leaves from four different plant species (maple, berry, lily, woody nightshade) growing around a deck perimeter and compared them to control plants of the same species growing 10-30 feet away. The amounts of arsenic in these leaves ranged from 0.2 to 0.9 mg/kg (dry weight), depending on species. The amounts in the control leaves were <0.2 mg/kg. Encouraged by these results, we started on growth studies using edible plants. For these preliminary studies we grew romaine lettuce and Indian mustard greens in two types of growth media (a sandy loam soil and a potting mix). In some trials various amounts of organic matter (compost) and iron oxide were added to the soil. The sources of arsenic were from either CCA wood blocks, CCA wood sawdust, or from a liquid spike of sodium arsenate. The plants were grown in pots and the aerial plant parts were harvested after 3-5 weeks of growth, cleaned, dried, and ground, followed by digestion and analysis using atomic spectroscopy, as previously described (Stilwell, 1993). The results are tabulated below.

Plant Uptake of Arsenic

* Based on 93±2 % Moisture (Lettuce) and 93±0.7 % Moisture (Mustard Greens).
** ProMix - 50% Peat Moss, 25% Perlite, 25% Vermiculite, lime to pH 5.6.

These preliminary data show that the amount of arsenic in the mustard was about 8 times greater than the lettuce under equivalent conditions. Also the arsenic levels tended to increase in the plant tissue with increasing amounts of arsenic in the soil, but in many instances reached a plateau or saturation region. The relationship between soil arsenic and plant arsenic in the spiked trials closely followed the observed plant uptake in the pots where CCA boards were added, but not in those to which CCA powder was added. Somewhat surprisingly, the amounts of arsenic in the lettuce were similar when grown in the promix and the soil. Another unexpected result was the lack of differences in plant uptake of arsenic as a function of compost or added iron oxide, which we now attribute to the spike level (100 mg/kg As) being in the saturation region (Onken and Hossner, 1995). We plan to test this hypothesis by growing the plants under lower arsenic spike concentrations. Finally, we noted that the use of CCA powder in the growth studies was problematic due to the difficulty in growing plants at high powder loading, and as such any further trials using CCA powder will be avoided.

Comparison of our preliminary results to the earlier work (Speir et al., 1992; Thornton, 1994) shows some discrepancies. The amounts of arsenic in the lettuce shown in the table above is not only somewhat less than that reported by Speir, where the lettuce was grown in CCA spiked soil, but is also much less than Speir reported in the control lettuce tops (6-9 mg/kg, dry weight). The amounts of arsenic in the lettuce grown in the spiked soils were much greater than expected based on the English garden study reported by Thornton (1994). For example, from the first row in the table it can be seen that we found 2.6-12.3 mg/kg arsenic (dry weight) in the lettuce leaves when grown in soil containing 25-100 mg/kg arsenic, while, as given earlier, the arsenic in the English garden soil averaged 322 mg/kg and the leaf arsenic in the lettuce ranged 0.15-3.9 mg/kg dry weight. In all cases, however, the arsenic in the lettuce did not exceed the English fresh weight limit of 1 mg/kg arsenic, but in some cases the more stringent limit of 0.5 mg/kg in Chili (Queirolo et al., 2000) or 0.2 mg/kg in Germany (Arnold, 1988) were exceeded in lettuce. All of the limits for arsenic were exceeded in the mustard greens, even when grown at the lowest spike level of 25 mg/kg arsenic in the soil. The amounts found in the mustard greens are similar to those given earlier for corn (1.8 mg/kg, fresh weight) and potatoes (0.9 mg/kg fresh weight) when grown in arsenic contaminated soils in Chile (Queirolo et al., 2000).

These discrepancies in uptake may be partly explained by the use of differing soil conditions, which may dramatically alter plant uptake of arsenic. In order for a plant to take in inorganic constituents, including arsenic, the material needs to be in solution form, not bound to the soil. On the other hand, through a combination of material and soil properties, for continued uptake, the constituent needs to be insoluble enough that it does not rapidly wash away from the root zone. The form of arsenic that initially leaches from the wood is the arsenate anion (AsO4-3). Some of the factors that tend to decrease the solubility of arsenate in soil include sorption and precipitation reactions with clay and iron oxides, while factors that tend to increase the solubility are increasing amounts of sand and organic matter in soils (Nriagu, 1994). Phosphates from fertilizing can release arsenate from the soil by replacement reactions (Peryea, 1998; Woolson, 1973). Moreover, plant uptake of arsenic can also be affected by plant-induced reactions. An example is the formation of oxidizing environments near the roots (rhizosphere) of wetland plants, leading to the precipitation of iron oxyhydroxides. The precipitate, also termed ironplaque, binds with arsenic, which results in a net accumulation of arsenic near the plant roots (Otte et al., 1995).

Further complicating the picture is that arsenic can exist in many forms. After leaching from the wood, and depending on the conditions, the arsenic, in the form of arsenate, may undergo numerous transformations. Such transformations include reduction to arsenite and even arsine, and through biological mechanisms transformations to organo-arsenicals (Nriagu, 1994). Any arsenic species thus transformed could be expected to differ in its sorption properties in the soil as well as with in its uptake by plants (Burlo et al., 1999; Carbonell-Barrachina, 1999). Under the conditions of normal garden soils it is believed that such transformations are increased by the presence of organic matter.

In summary, arsenic uptake by plants is affected not only by plant species and arsenic type and concentration in soils, but by soil properties such as the amounts of phosphorus, iron oxide and organic matter. The interaction of these factors on arsenic uptake by plants are not well understood and need to be investigated because of the potential for adverse health effects by arsenic in the food supply.

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