The r2 value of the linear model in the CAFO
zone is small because lithogenic materials remain the predominant
source of arsenic in the soils. As discussed earlier, the mean content
of soil arsenic (16.0 mg kg1) was not too much higher than
that (12.2 mg kg1) in the agricultural soils in the Pearl River Delta,
thus TOC in the surface soils could not be the only predictor for soil
arsenic content, even if the soil organic matter came completely
from the animal wastes.