12 
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 
Percent of Area Exceeding the Criterion 
Figure 11-3. Graphical representation of the CFD from the above theoretical example 
assessment curve (blue) with a hypothetical reference curve (black). 
CFD REFERENCE CURVES 
Two approaches are feasible in defining the reference curves proposed for use in the 
CFD assessment methodology. One is biologically based and identifies appropriate 
regions of the Bay, its tidal tributaries, and its embayments that have healthy biolog¬ 
ical indicators and are in attainment of their designated use (U.S. EPA 2003a). The 
CFDs are developed for these areas in the same way that assessment CFDs would be 
developed elsewhere. Curves generated for biologically healthy tidal areas are 
considered “reference” curves. 
For example, healthy benthic indices of biotic integrity (1BI) scores might be used as 
indicators of adequate bottom dissolved oxygen (Weisberg et al. 1997; U.S. EPA 
2003a). Thus, after stratifying by salinity zone and perhaps other factors, a series of 
dissolved oxygen reference CFD curves could be developed from the existing moni¬ 
toring database. One advantage of this approach is that each biological reference 
curve could be tailored to each designated-use-based criteria component. This tech¬ 
nique tailors the pattern of criteria exceedance that the part of the Bay ecosystem 
could tolerate and remain healthy to the protected species and biological communi¬ 
ties and the specific criterion component. Thus, each reference curve may have a 
somewhat different shape (see pages 168-177 in U.S. EPA 2003a). 
I 
chapter ii 
Refinements to the Chesapeake Bay Water Quality Criteria Assessment Methodology 
