Toxicity measurements in concentrated water samples - Rivm

this purpose, a concentration according a certain level of protection is derived from the distribution (i.e.
the hazardous concentration for 5 or 50 percent of the species, called HC5 or HC50). This is known as
the ‘inverse use’ of an SSD (Van Straalen and Denneman, 1989). ‘Forward use’ of an SSD is useful for
ecological risk assessment, as it allows the fraction of the species in the community that is at risk to be
estimated (Fraction Affected, or FA). Estimating FA is shown in Figure 5-2.
Different input data can be used to derive an SSD. The method presented in this report uses the results
from a set of toxicity measurements. These measurements give an idea of toxic pressure of a certain
water sample for the test species in the test battery. Using these measurements to derive an SSD, the
fraction affected can be calculated indicating the toxic pressure for the test battery as a whole. This
application is presented in section 5.3.2.
Metals are not concentrated by the XAD-technique, so the measured toxicity does not express the
toxicity from different metals in the original sample. Fortunately, chemical analysis of metals can be
performed quite effectively using a multi-element analysis technique like ICP-MS. This information
can be used to gain an impression on the toxicity of metals, using knowledge of chemical speciation of
metals and interactions of metals with biota (see Appendix III for proposed methods).
5.3.2 Calculating toxic pressure
De Zwart and Sterkenburg (2002) developed a calculating method for toxic pressure from measured
toxicity in unknown mixtures. In this method, the endpoints of the toxicity measurements are used as a
basis for calculating the toxic pressure of a water sample. The measurements are used to estimate the
parameters of the SSD. The toxic pressure (‘pT’) of the untreated water sample is estimated from the
SSD as the percentage of species exposed above their chronic NOEC. The observed acutely effective
concentration factors (EC f 50 or LC f 50) are extrapolated to represent chronic no-effect concentration
factors (NEC f ) by assuming a constant acute-chronic ratio (ACR) of 10 (see also section 5.1):
f
f
f EC 50
f LC 50
NEC = and NEC =
Equation 2
10
10
Afterwards, the NEC f values are log transformed and fitted according to the chosen distribution
function. Originally, De Zwart and Sterkenburg (2002) used the integrated log-logistic distribution
function to fit the data. However, the log-logistic distribution can hardly be distinguished from the lognormal
distribution for practical purposes. Because of the availability of standard functions in
Microsoft Excel and the possibility to calculate confidence limits it is easier to use the log-normal
distribution. Differences between these distributions will be discussed in section 5.4.4.
When we consider the toxicity measurements as a sample from a normal distribution, the mean and
sample standard deviation are reasonable estimates of the parameters from a normal SSD. The
cumulative distribution function can be estimated using mean and standard deviation. The fraction
affected can be calculated using the Excel function NORMDIST(x, mean, standard_dev, 1), that returns
the normal cumulative distribution for the specified mean and standard deviation:
( ( ); ; ; 1)
10
f
pT = Fraction Affected = NORMDIST LOG C AVG STDEV Equation 3
64 RIVM Report 607013010