16 
understanding that reference curves typically have positive curvature and that this 
curvature can dip below the line between consecutive points on the assessment 
curve, causing a spurious, non-allowable exceedance. 
To address this problem, the EPA recommends that reference curves be evaluated 
only at the temporal axis points in the assessment curve as illustrated in Figure II-7. 
For non-continuous biological reference curves, the points should be interpolated 
from neighboring points. Appendix B provides a detailed description of the complete 
Chesapeake Bay water quality criteria attainment assessment methodology. 
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Figure 11-7. The graph on the left (A) shows spurious non-attainment as the reference curve passes below the 
assessment curve between points. The graph on the right (B) shows attainment as the reference and assessment 
curves are evaluated at the same temporal axis points. 
DEVELOPMENT OF A STATISTICAL DECISION-MAKING FRAMEWORK 
A statistical framework for making decisions on water quality criteria attainment 
based on the CFD methodology would yield additional information on the certainty 
of the attainment decisions. It would also help direct appropriate monitoring strate¬ 
gies to reduce uncertainties. However, many theoretical obstacles remain in 
developing such a framework. The CFD methodology is a new and innovative 
approach to water quality criteria assessment. The relatively recent application of 
this methodology to criteria assessment suggests that conducting further evaluations 
and making improvements should prove constructive. The following section 
discusses the steps in applying the CFD methodology. 
Step 1—Data Collection 
One of the advantages of the CFD approach is that it can accommodate a variety of 
input data and still arrive at the same assessment endpoint. Data collection methods 
currently in place include: fixed-station data, cruise track data, continuous moni¬ 
toring data, aircraft flight path data, and satellite imagery data. Because of the 
chapter ii 
Refinements to the Chesapeake Bay Water Quality Criteria Assessment Methodology 
