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chapter \/ 
Guidance for Attainment 
Assessment of Instantaneous 
Minimum and 7-Day Mean 
Dissolved Oxygen Criteria 
BACKGROUND 
As published in the Ambient Water Quality Criteria for Dissolved Oxygen, Water 
Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries (U.S. 
EPA 2003), it is accepted that concentration minima need to be defined, which if 
exceeded for some defined (short) duration result in lethal or other adverse effects. 
Instantaneous minimum criteria have been derived and published for protection of 
each of the five tidal water designated uses. A 1 -day mean dissolved oxygen crite¬ 
rion was also determined to be necessary for the protection of the deep-water 
designated use. In addition, a 7-day mean criterion has been derived for protection 
of the open-water designated use (U.S. EPA 2003). 
However, it is also acknowledged that assessing the attainment status of these criteria 
requires data collections at temporal and spatial scales that are simply not practicable 
nor sustainable across all Chesapeake Bay and tidal tributary waters. To address this 
issue, there are ongoing efforts to develop statistical methods to estimate attainment 
of these dissolved oxygen criteria using a synthesis of: 1) seasonal and inter-annual 
patterns found in the long term, low-frequency, spatially-limited monitoring data; 2) 
the short-term patterns of temporal variability found in high-frequency, spatially 
uneven ‘buoy’ data; and 3) the small-interval patterns of variability observed in data 
records generated through the ‘data-flow’ and ‘scan-fish’ sampling devices. 
CURRENT STATUS 
These methods are in the exploratory and trial application phases. However, we can 
still address the question of how best to assess attainment of these criteria given the 
almost two-decade record of dissolved oxygen concentrations for Chesapeake Bay 
tidal waters. First, there are some Chesapeake Bay Program segments, such as the 
deep-channel mid-Chesapeake Bay mainstem segments and the lower Potomac 
chapter v 
Guidance for Attainment Assessment of Instantaneous Minimum and 7-Day Mean Dissolved Oxygen Criteria 
