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In some cases, development of a biologically-based reference curve is not possible 
due to lack of data describing the health of the relevant species or biological commu¬ 
nities. Such cases require a different approach. The EPA recommends use of a 
default reference curve in situations for which a biologically based reference curve 
remains unavailable. This default reference curve is defined as a hyperbolic curve 
that encompasses no more than 10 percent of the area of the CFD graph (percent of 
space multiplied by percent of time) (see page 174 in U.S. EPA 2003a) (Figure II-4). 
The default reference curve has the following important properties: 1) the plot is 
symmetric about the 1:1 line; 2) the plot is hyperbolic; 3) the total area under the 
Percent of Area/Volume Exceeding the Criterion 
Figure 11-4. Default reference curve for application in the attainment assessment of 
Chesapeake Bay water quality criteria for which biologically based reference curves 
cannot be derived. 
curve equals 10 percent; and 4) the ends of the curve pass through x- and y-axis 
intercepts (100, 0) and (0, 100), respectively. 
Figure II-4 is defined by the equation: 
(x + b)(y + b) = a Equation 1 
where: b = 0.0429945 and a = b : + b. 
chapter ii 
Refinements to the Chesapeake Bay Water Quality Criteria Assessment Methodology 
