provided for in EPA national guidance for assessing criteria attainment (U.S. EPA 
1997). Ten percent of the samples collected at a point are allowed to reflect nonat¬ 
tainment of water quality criteria without indicating nonattainment of designated 
uses. These criteria exceedances are considered ‘allowable exceedances’ that had 
limited impact on the designated use. The 10-percent rule is not directly applicable 
in the context of the CFD methodology for defining criteria attainment because it 
was designed for samples collected at one location and, therefore, is only reflective 
of time. 
A more appropriate approach for defining ‘allowable exceedances’ in the CFD 
context is to develop a reference curve (described below) that identifies the amount of 
spatial and temporal criteria exceedance that can occur without causing significant 
ecological degradation. Such curves can be based on biological indicators of ecolog¬ 
ical health that are separate from the criteria measures themselves. Biological 
indicators can be used to identify areas of the Chesapeake Bay and its tidal tributaries 
that have healthy ecological conditions and supportive water quality conditions. CFDs 
can be developed for those areas as well. Since healthy ecological conditions exist in 
the selected areas, CFDs developed for the area would reflect an extent and pattern of 
criteria exceedance that did not have significant ecological impact. Thus, the refer¬ 
ence curve approach takes the development of criteria levels beyond those developed 
in a laboratory setting and provides actual environmental context. Small incidents of 
spatial and temporal criteria exceedance that do not have ecological impacts are iden¬ 
tified and allowed in the assessment of criteria attainment. A description of the 
application of the reference curve is provided in this section, with more details on 
reference curves in the section titled “Defining the Reference Curve.” 
Figure VI-9 illustrates the use of the reference curve and the interpretation of criteria 
attainment using the CFD. The light blue line illustrates a possible reference curve, 
below which a certain amount of spatial or temporal exceedance is allowed. An 
actual reference curve could be asymmetrical, indicating that the system could with¬ 
stand either short-term excursions in time or chronic exceedances in small portions 
of space, but not both. 
Development of the reference curve is intended to identify such specifics to more 
accurately reflect what the ecological system needs to thrive. It also is intended to be 
developed as a benchmark that is not changed on a regular basis, recognizing the 
potential for updates as new information is gathered. By contrast, the attainment 
curve is developed over every assessment period during which monitoring data are 
collected. 
The attainment curve is the assessment of the condition in the segment during the 
assessment period and is compared to the reference curve. The area above the refer¬ 
ence curve and below the attainment curve reflects criteria attainment and is referred 
to as “non-allowable exceedances.” It is recommended that separate attainment 
curves be developed for each criteria component, for subsequent application in every 
spatial assessment unit (Chesapeake Bay Program segment/designated use) and for 
at least one full assessment period of three years. 
chapter vi • Recommended Implementation Procedures 
