Chemicals-plant

Chemicals-plant
The major pathway of exposure of terrestrial wildlife to contaminants in soil is through food ingestion. The prediction or estimation of risks to wildlife requires knowledge of their diets, body weights, habitats, and concentrations of contaminants in all ingested media (food, soil, and water). The direct measurement of chemical concentrations in wildlife food is advisable to minimize uncertainty in ecological risk assessments. However, site-specific data on the bioaccumulation of contaminants in vegetation and other biota that comprise wildlife diets are often not available because of constraints in
funding or time. At a minimum, concentrations of inorganic and organic chemicals in soils are measured at contaminated sites prior to a risk assessment. The challenge is to develop models that estimate concentrations of chemicals in plants from these concentrations in soil. The simplest linear model for estimating the concentrations of chemicals in vascular plants is the soil-plant uptake factor, the ratio of the concentration of a chemical in a plant or portion of a plant to that in soil. The concentration of a contaminant in plants at a particular location is estimated by multiplying the measured concentration in soil by the soil-plant uptake factor. Chemical concentrations in plants and soil are assumed to be at equilibrium; thus, exposure time is not considered. The usefulness of uptake factors lies in the ease by which distributions can be developed and conservative (e.g., 90th percentile) values chosen. However, the evidence below suggests that uncertainty in uptake model predictions may be minimized if:
(1) nonlinear models are employed and
(2) environmental factors and other sources of variability are
incorporated in the model.
Uptake factors would be most useful if they did not vary with soil concentration. Although the uptake relationship between soil and plants is probably valid for narrow ranges of chemical concentration in the relatively nontoxic range (e.g., Jiang and Singh 1994, Carlson and Bazzaz 1977), some evidence demonstrates that uptake factors are dependent on soil concentration. Baes et al. (1984), who developed soil-vegetative tissue uptake factors that are often used in human health and ecological risk assessments,
found that the uptake factors for copper and zinc were inversely correlated with soil concentration. These metal contaminants are also nutrients, and it is not surprising that they would be regulated by plants. Alsop et al. (1996) showed that the use of Baes factors underpredicted the uptake of lead and zinc by oats at concentrations within background ranges in soil and overpredicted metal concentrations in the plants at concentrations exceeding background levels. Clearly, nonlinear models would sometimes be
more useful for risk assessments than the Baes factors. Both Neuhauser et al. (1995) and Sample et al. (1998a) have obtained significant regressions for the uptake of inorganic elements by earthworms using log-transformed concentrations, so it is reasonable to assume that log-transforming soil and plant concentrations could result in a statistically significant relationship. Inorganic chemicals are passively taken up by plants from soil water, with the additional possibility of active uptake in the case of required nutrients, such as copper and zinc. Soil properties such as pH, clay content, and organic matter strongly affect the concentrations of inorganic chemicals in soil solution. For example, the amount of zinc in soil water and plant tissues is generally observed to increase with
decreasing pH and cation exchange capacity (Bysshe 1988). Cadmium uptake by plants has been shown in numerous studies to decrease with increasing pH (He and Singh 1994, Miller et al. 1976). Sims and Kline (1991) found significant multiple regression models between nickel, copper, and zinc in wheat and soybean and soil metal concentrations and pH, but not with soil metal concentrations alone. The type of soil is significant for accumulation of chemicals by plants, with arsenic uptake in crops dependent on soil type (Jiang and Singh 1994) and cadmium uptake by soybeans related to the sorptive capacity of soil
(Miller et al. 1976)