Background Air Component of Measured Vegetation/Soil Bioconcentration Ratios

Soil-to-plant biotransfer plays a critical role in assessing both human and ecological risks at phytoremediation sites and in establishing risk-based screening levels at hydrocarbon impacted exploration and production sites. The plant/soil bioconcentration ratios (BCRs) that are used to estimate exposure concentrations in crops and feed are based on empirical relationships developed from measured chemical concentrations in plants and soil. These measurements often neglect the contribution of background air to observed concentrations in vegetation. As a result, plant/soil BCRs and the models that use BCRs may overstate the contribution of soil to contaminant concentrations in vegetation. To explore this issue we combine two separate but related studies of polycyclic aromatic hydrocarbon (PAH) uptake into vegetation. Results from a controlled exposure chamber study provide details on direct (soil-plant) and indirect (soil-air-plant) uptake pathways while results from a cooperative phytoremediation study yield relevant field measurements with a range of soil concentrations. Mass transfer at the air/plant interface is fast relative to uptake from soil, so atmospheric pollutants can contribute to above ground vegetation concentrations. Atmospheric pollutant concentrations place a limit on the concentrations in soil below which soil-to-plant transfers can no longer be reliably determined. We find that even low background atmospheric concentrations can influence soil-to-plant BCR measurements, particularly for the more lipophilic chemicals. In addition to the field data and laboratory study, we use a multimedia mass balance approach with illustrative atmospheric PAH concentrations to estimate the limit on soil concentrations where transfer to above ground vegetation can be observed. By screening out lower soil concentration data we provide an alternate regression to estimate soil-to-vegetation BCRs for PAHs.