Illinois Administrative Code › Title 35 - ENVIRONMENTAL PROTECTION › Part 302 - WATER QUALITY STANDARDS › Subpart F - PROCEDURES FOR DETERMINING WATER QUALITY CRITERIA › Section 302.618 - Determining the Acute Aquatic Toxicity Criterion - Toxicity Dependent on Water Chemistry

Current through Register Vol. 48, No. 28, July 12, 2024
If data are available to show that a relationship exists
				  between a water quality characteristic (WQC) and acute toxicity to two or more
				  species, an Acute Aquatic Toxicity Criterion (AATC) may be calculated. The
				  best-documented relationship is between the water quality characteristic,
				  hardness, and acute toxicity of metals. Although this relationship between
				  hardness and acute toxicity is typically non-linear, it can be linearized by a
				  logarithmic transformation (i.e., for any variable, K, f(K) = logarithm of K)
				  of the variables and plotting the logarithm of hardness against the logarithm
				  of acute toxicity. Similarly, relationships between acute toxicity and other
				  water quality characteristics, such as pH or temperature, may require a
				  transformation, including no transformation (i.e., for any variable, K, f(K) =
				  K), for one or both variables to obtain a least squares linear regression of
				  the transformed acute toxicity values on the transformed values of the water
				  quality characteristic. An AATC is calculated using the following
				  procedures:
a) For each species for
				which acute toxicity values are available at two or more different values of
				the water quality characteristic, a least squares linear regression of the
				transformed acute toxicity (TAT) values on the transformed water quality
				characteristic (TWQC) values is performed to obtain the slope of the line
				describing the relationship.
b)
				Each of the slopes determined pursuant to subsection (a) is evaluated as to
				whether or not it is statistically valid, taking into account the range and
				number of tested values of the water quality characteristic and the degree of
				agreement within and between species. If slopes are not available for at least
				one fish and one invertebrate species, if the available slopes are too
				dissimilar, or if too few data are available to define the relationship between
				acute toxicity and the water quality characteristic, then the AATC must be
				calculated using the procedures in Section
				302.615.
c) Normalize the TAT values for each species,
				by subtracting W, the arithmetic mean of the TAT values of a species from each
				of the TAT values used in the determination of the mean, such that the
				arithmetic mean of the normalized TAT values for each species individually or
				for any combination of species is zero (0.0).
d) Normalize the TWQC values for each species
				using X, the arithmetic mean of the TWQC values of a species, in the same
				manner as in subsection (c).
e)
				Group all the normalized data by treating them as if they were from a single
				species and perform at least squares linear regression of all the normalized
				TAT values on the corresponding normalized TWQC values to obtain the pooled
				acute slope, V.
f) For each
				species, the graphical intercept representing the species TAT intercept, f(Y),
				at a specific selected value, Z, of the WQC is calculated using the equation:
f(Y) = W - V( X - g(Z))
Where:
f() is the transformation used to convert acute toxicity
				  values to TAT values;
Y is the species acute toxicity intercept or species acute
				  intercept;
W is the arithmetic mean of the TAT values as specified in
				  subsection (c);
V is the pooled acute slope as specified in subsection
				  (e);
X is the arithmetic mean of the TWQC values as specified in
				  subsection (d);
g() is the transformation used to convert the WQC values to
				  TWQC values; and
Z is a selected value of the WQC.
g) For each species, determine the species
				acute intercept, Y, by carrying out an inverse transformation of the species
				TAT value, f(Y). For example, in the case of a logarithmic transformation, Y =
				antilogarithm of (f(Y)); or in the case where no transformation is used, Y =
				f(Y).
h) The Final Acute Intercept
				(FAI) is derived by using the species acute intercepts, obtained from
				subsection (g), in compliance with the procedures described in Section
				302.615(b) through
				(g), with the word "value" replaced by the
				word "intercept". Note that in this procedure, geometric means and natural
				logarithms are always used.
i) The
				Aquatic Acute Intercept (AAI) is obtained by dividing the FAI by two.
j) The AATC at any value of the WQC, denoted
				by WQCx, is calculated using the terms defined in subsection (f) and the
				equation:
AATC = exp[V (g(WQCx) - g(Z)) + f (AAI)].