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scatter of points in the plot for segment MAGMH (Magothy River), indicating large 
between-segment differences in variability and predictability. 
The plots in Figure V-3 illustrate the differences among segments in their patterns of 
criteria non-achievement. The four quadrants bounded by the reference lines in the 
plots represent the four possible results from a two-criteria achievement assessment. 
Let the quadrants be numbered clockwise 1 through 4, beginning with the upper 
right hand quadrant. Any data points in quadrant 1 achieve both the 30-day mean and 
instantaneous minimum criteria. Data points in quadrant 2 achieve the 30-day mean 
criterion, but do not achieve the instantaneous minimum criterion. Data points in 
quadrant 3 do not achieve both the 30-day mean and instantaneous minimum 
criteria. Data points in quadrant 4 achieve the instantaneous minimum criterion, but 
do not achieve the 30-day mean criterion. In a fully restored Chesapeake Bay, one 
would expect that most data points would fall in quadrant 1. In impaired segments, 
where low dissolved oxygen conditions are frequent or chronic, one would expect 
most data points to fall in quadrant 3. In segments where low dissolved oxygen 
events are episodic, ranging from occasional to frequent, one would expect a dense 
population of data points in quadrant 2. And, where dissolved oxygen concentrations 
are chronically reduced, but really low dissolved oxygen concentrations are rare, 
then one would expect some data points in quadrant 4. 
Providing plots such as those presented in Figure V-3 for each designated use for 
every segment is impractical for this document. Instead, Table V-8 shows the number 
of points in a representative data set that would be in each quadrant, if the data were 
plotted as in Figure V-3 using the summer only data from a recent 10-year period: 
June-September, 1993-2002. 
There are 66 segments that have only open-water designated uses. A total of 28 of 
these segments achieve both the 30-day mean and instantaneous minimum criteria, 
i.e., which have all their data points in quadrant 1 and none or only one data point in 
the other quadrants. These segments are marked with a single asterisk in Table V-8. 
In these open-water only segments, assessment of attainment of the instantaneous 
minimum criterion can be directly based on assessment of attainment of the 30-day 
mean criterion (Table V-9). 
A total of 18 segments with only open-water designated uses had the vast majority 
(greater than two-thirds) of their data points in either quadrant 1 or quadrant 3. These 
segments are marked with double asterisks in Table V-8. The assessment of attain¬ 
ment of the instantaneous minimum criterion can be directly based on assessment of 
attainment of the 30-day mean criterion in these segments (Table V-9). 
In five segments with only open-water designated uses there were sufficient data 
points in quadrant 2 indicating a much higher occurrence where the 30-day mean 
criterion was achieved yet the instantaneous minimum criterion was not achieved. 
These segments are marked with a single dash in Table V-8. These five segments 
were: upper Chesapeake Bay (CB20H), Magothy River (MAGMH), Severn River 
(SEVMH), Mobjack Bay (MOBPH) and Little Choptank River (LCHMH). Users 
chapter v 
Guidance for Attainment Assessment of Instantaneous Minimum and 7-Day Mean Dissolved Oxygen Criteria 
