64 
Ten of these thirteen segments with deep-water or deep-water and deep-channel 
designated uses also showed evidence of a strong relationship between achieved/not 
achieved in the assessment of the instantaneous minimum using monthly mean data 
for the deep-water and/or deep channel designated uses (Table V-8). These segments 
were: middle central Chesapeake Bay (CB4MH), western lower Chesapeake Bay 
(CB6PH), eastern lower Chesapeake Bay (CB7PH), Patapsco River (PATMH), lower 
Potomac River (POTMH) [deep-channel use only], lower Rappahannock River 
(RPPMH) [deep-channel use only], lower York River (YRKPH), Southern Branch 
Elizabeth River (SBEMH), lower Chester River (CSHMH), and Eastern Bay 
(EASMH) [deep channel use only] (Table V-9). 
In the cases of the upper central Chesapeake Bay (CB3MH), lower central Chesa¬ 
peake Bay (CB5MH), lower Patuxent River (PAXMH), lower Potomac River 
(POTMH) [deep-water use only], lower Rappahannock River (RPPMH) [deep-water 
use only] and Eastern Bay (EASMH) [deep-water use only] there are sufficient data 
points in quadrant 2 indicating a higher occurrence where the 30-day mean criteria 
were achieved yet the instantaneous minimum criteria were not achieved in deep¬ 
water and/or deep-channel designated use habitats (Table V-8). Users assessing 
attainment of 30-day mean and instantaneous minimum dissolved oxygen criteria 
within these seven segments and their respective deep-water/deep channel desig¬ 
nated uses are cautioned not to automatically assume that attainment of the 30-day 
mean criterion reflects attainment of the instantaneous minimum dissolved oxygen 
criterion (Table V-9). Site-specific buoy deployments may be necessary either to 
better quantity a relationship or assess attainment using both low- and high- 
frequency data sources. 
ASSESSMENT OF 7-DAY MEAN CRITERIA ATTAINMENT 
FROM MONTHLY MEAN DATA 
The open-water designated use habitats are also subject to a 7-day mean criterion. 
The continuous buoy data were examined to look for relationships between the 30- 
day mean and the 7-day mean values. Buoy data records with durations over 14 days 
(at least two 7-day periods) were examined. Figure V-4 shows plots of the sequen¬ 
tial as opposed to a rolling series of 7-day means versus the 30-day mean for the 
more limited number of data records that were available. There is more scatter in 
these relationships than in the 30-day mean versus instantaneous minimum relation¬ 
ships. However, a significant majority of the data points are found in the first and 
third quadrants, where the data points both achieve (quadrant 1) or both do not 
achieve (quadrant 3) the 30-day mean and 7-day mean criteria. There is clearly a 
strong relationship between achieving/not achieving of the 30-day mean and 7-day 
mean criteria. The remaining data points tended to be in the second quadrant where 
the data points do not achieve the 30-day mean criterion but achieve the 7-day mean 
criterion. Only 3 data points were located in the fourth quadrant. 
chapter v • Guidance for Attainment Assessment of Instantaneous Minimum and 7-Day Mean Dissolved Oxygen Criteria 
