# Human and Ecological Risk Office

# CalTOX Frequently Asked Scientific Questions

## 1. What is the basis of produce uptake of chemical from soil?

**PLANT-SOIL PARTITION COEFFICIENT FOR EDIBLE PORTION ABOVE GROUND**

The plant-soil partition coefficient, Kps, is the concentration of chemical in edible portions of plant growing above the soil surface divided by the concentration of chemical in the soil in which the plant is growing at equilibrium. Travis and Arms (1988) reviewed the literature and found dry weight Kps values for 29 persistent organochlorine chemicals. They also obtained octanol-water partition coefficients for those same 29 chemicals. The equation that best fits a regression through those points is:

Log Kps =1.588 –0.58 log Kow | eq.1 |

Taking the anti-log of both sides of equation 1 yields:

Kps = 38.7 x Kow^{-0.58} |
eq. 2 |

People eat much less dried produce than fresh produce. Since produce on average is about 80% water, this equation was multiplied by 0.2 to yield:

Fresh wt. Kps =7.7 x Kow^{-0.58} |
eq. 3 |

The scatter in the data around the regression line was used to compute the coefficient of variation associated with this estimate of Kps. Based on the mean square error of the estimator for the regression equation shown as equation 1, the error term is 0.73. Transforming this error term into linear space:

10^{0.73} = 5.37 |
eq. 4 |

An error term of 5.37 has a CV of 4 for these 29 data points.

**PLANT-SOIL PARTITION COEFFICIENT FOR EDIBLE PORTION BELOW GROUND**

Topp et al. (1986) developed a correlation between root-soil partition coefficient and Koc for several organic chemicals with roots of barley and cress. These Kps(root) measurements were made on a fresh plant weight basis. Substituting Kow = Koc/0.411, Karrikhoff (1981), into the equation developed by Topp yields:

Kps(roots) = 273 Kow ^{–0.622} |
eq. 5 |

The exponents for Kow in equations 3 and 5 are nearly identical. Dividing the coefficient of equation 5 by the coefficient of equation 3 allow us to express Kps(roots) as a function of Kps:

Kps(roots) = 35 Kps | eq. 6 |

For Kow values between 10^{3} to 10^{6} , the Kps computed by equation 6 is less than 43% different from that estimated from equation 5. Therefore, CalTOX uses 35Kps to estimate the concentration of plants with the edible portion below ground.

**References:**

Karickhoff, S.W. (1981) Semi-Empirical Estimation of Sorption of Hydrophobic Pollutants on Natural Sediments and Soils, Chemosphere 10, 833-846.

Travis, C.C. and A.D. Arms (1988) Bioconcentration of Organics in Beef, Milk and Vegetation. Environ. Sci. Technol. 22, 271-274.

Topp et al. (1986) Factors Affecting Uptake of C14-Labeled Chemicals by Plants from Soil. Ecotoxicol. Environ. Saf. 11. 219-228.

## 2. Can CalTOX be used for inorganic chemicals?

CalTOX can be used for inorganic chemicals other than mercury only if site-specific data is provided for the following parameters:

- Soil-water partition coefficients for sediment, root, and vadose soils
- Plant-soil partition coefficient
- Biotransfer factor, plant/air
- Biotransfer factor; cattle-diet/milk
- Biotransfer factor; cattle-diet/meat
- Biotransfer factor; hen-diet/eggs
- Biotransfer factor; breast milk/mother
- Bioconcentration factor; fish/water
- Skin permeability coefficient
- Fraction dermal uptake of metal from soil

The value of these parameters will likely differ for different salts of the chemical as well as different valance states of the inorganic chemical. The partition coefficients for soil-water and plant-soil are highly dependent on the soil. For many inorganics, the difference in all parameter values is enormous. Unless these values are measured or accurately estimated, CalTOX should not be used for an inorganic chemical.

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