Arsenic in Raised Bed Gardens with CCA

Research was conducted to determine if significant or harmful amounts of arsenic were leaching from CCA-treated landscape timbers in raised bed gardens. Two types of raised beds were sampled. The first type were seven years old and tilled every year. The second type were two years old and left untilled.

The sampling procedure was replicated four times from each bed to make the study statistically valid. Control samples that had not been exposed to landscape timers were also collected from supplies of soil media similar to that placed in the raised beds. Total arsenic was not measured because it does not correlate with harmful effects.

The results showed arsenic was not accumulating in the raised beds to harmful levels. Average arsenic levels at different locations in the bed ranged from < 0.01-0.39 ppm and were not significantly different from control samples. The 7-year-old tilled beds showed a trend of increasing arsenic near the timbers but the differences were not statistically significant. The 2-year old untilled beds showed the same trend, but in this case significantly more arsenic was present next to the timbers than within the beds. The arsenic can only move a few inches in the soil before it is tied up by soil particles. The research indicates arsenic is not a concern when using CCA-treated landscape timbers in raised bed gardens.

Extractable arsenic levels in raised beds

Location	Arsenic Concentration (ppm)
	7-year Tilled Bed	2-year Untilled Bed
Edge of Beds	0.19	0.39
Halfway to Center	0.14	0.03
Center of Beds	0.09	< 0.01
Control Samples	0.83	0.30

Source: Earl Allen, Soil Extension Specialist, Okalahoma State University

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