Uptake of Organic Contaminants from Soil into Vegetables and Fruits - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 373

where C Plant is the concentration in plant tissues and C Soil is the concentration in
soil (ideally at steady state, but practically at harvest). This BCF will only apply to
the specific contaminant and soil type used for the determination.
Care must be taken in cases where a measurable background concentration
in plants is present. Because then, for low soil concentration ( C Soil

→ 0), the

concentration ratio BCF can be very high ( C Plant / C Soil →∞ ). For higher soil con-

centrations, however, the BCF decreases and approaches a constant value. This
pattern was occasionally interpreted as a variable BCF with soil concentration, i.e.
a decreasing BCF with increasing soil concentration. A real-world example is the
ratio between the measured concentration of p,p -DDT in radishes and in soil. The
concentration ratio is high at low soil concentrations, and decreases for higher soil
concentrations. A plausible explanation for this pattern is that plants have a limited
sorption capacity for organic contaminants, which becomes saturated at higher soil
concentrations. However, a more likely interpretation is that the uptake into plants is
from two different and independent sources, namely from soil and from air. When
soil concentrations are very low there still is a background contamination of the
plant tissue originating from air (Mikes et al. 2009 ).
Instead of simply calculating the concentration ratio of plant to soil, the relation-
ship between concentrations in plant and soil can be quantified by a linear regression
between both if measurements at different concentration levels are available. The
slope of the regression between soil concentration as predictor variable and plant
concentration as estimated variable can be interpreted as the BCF plant to soil, while
the y -axis-intercept can be interpreted as the constant background concentration due
to uptake from air.

C Plant = BCF × C Soil + C Background (9.2)

where C Background is the constant concentration due to uptake from air.
This method has several advantages:

• all measured values contribute to the calculated BCF ;

• variations in the measured concentrations are adequately considered;

• the y-axis gives the concentration in plants due to the (constant) concentration

in air;

the square of the correlation coefficient ( r 2 ) describes how much of the vari-
ance in the measured concentration in plants is explained by the variance of the
concentrations in soil.

•

An example is shown in Fig. 9.2 .Itshowsthe BCF s for p,p -DDT in radishes
which decrease with increasing soil concentration (Mikes et al. 2009 ), but with
C plant plotted as a function of C Soil . The slope of the regression curve, i.e. the value
0.17, is the BCF derived from all measured values minus the background concen-
tration in air. The BCF is statistically highly significant. The explained variance r 2
is 0.98, which means that the increase of concentrations in plants can be explained
almost completely by the increase of concentration in soil.