Size and Edge Effects of Concrete Plots on Chlorpyrifos Degradation in Sub-Slab Sand

Chlorpyrifos degradation in sub-slab sand was studied using concrete plots of small or large (16X) size. Degradation half-life (k) for chlorpyrifos in sub-slab sand from small plots was faster (approximate to 4.5 mo) than that measured from any of the 3 distance zones of large plots (approximate to 15.9-21.6 mo). There was a tendency for more rapid degradation toward the slab edge of a large plot or edge effects. Some chlorpyrifos at the slab edge may have been released into the atmosphere or may have degraded more rapidly than inner zones because of more exposure to environmental factors such as water immersion or temperature fluctuation. Chlorpyrifos beneath a small plot degraded faster (k = 4.5 mo) than that of the outer zone of a large plot (k = 15.9 mo) because the edge effects affected small plots from 4 sides, whereas only 1 or 2 sides of outer zones of large plots contained a slab edge. Because a typical house may occupy a much larger area, results from simulated held evaluation studies using miniature plots may substantially underestimate chlorpyrifos longevity.