6 
Figure 11-1. Example of interpolation of Chesapeake Bay water quality data. 
temporal dimension. Water quality in a large area of a segment may exceed the 
criteria’s threshold for a short time. If this degradation proves infrequent and short¬ 
lived with the segment quickly returning to a healthy state, this situation does not 
represent an impairment of the ecologically defined designated use of the segment. 
Recognition that ephemeral exceedances of the criterion’s threshold in time or space 
do not represent persistent impairment of the segment’s designated use ultimately 
led to the development of a criteria assessment methodology that deems such 
exceedances as acceptable. Persistent, widespread criteria exceedance, however, is 
considered an impairment of the segment’s designated use (U.S. EPA 2003a). 
The criteria assessment methodology determines how much of the spatial assess¬ 
ment unit is not in compliance with the criteria (percent of space) for each moment 
in time. In the second step of the methodology, a determination is made of how often 
(percent of time) a segment is out of attainment by more than a fixed percent of 
b 
chapter ii 
Refinements to the Chesapeake Bay Water Quality Criteria Assessment Methodology 
