39 
instantaneous minimum deep-channel criterion were also determined using the 
fixed-station data. 
The rationale behind the assessment of the instantaneous minimum deep-channel 
criterion was based on the long-term fixed station data record in the deep-channel 
locations which shows that dissolved oxygen does not vary strongly through time in 
the deep channel during the summer months because of the physical isolation from 
the atmosphere and the photic zone. Dissolved oxygen concentrations remain rela¬ 
tively constant; therefore, a 30-day mean should be similar to any instantaneous 
measure (see section below). 
No assessments were made of the 7-day and 1-day mean dissolved oxygen criteria 
because the data were considered inadequate (as described above). In most cases, 
this situation did not affect listing decisions because many spatial assessment units 
did not attain the 30-day mean criterion (see Figure IV-1) and all criteria components 
need to be attained to justify removal from the impaired list (part 5). The 30-day 
mean criterion was attained in some cases. These spatial assessment units, if not 
previously listed on part 5. were placed in part 3 of the states’ lists for waters with 
insufficient data (see Chapter 8 for further details). As water quality conditions 
improve in Chesapeake Bay, a method to assess higher frequency dissolved oxygen 
criteria will be needed so that spatial assessment units in attainment with all appli¬ 
cable dissolved oxygen criteria components can be removed from the state’s 
impaired waters list (see Appendix E). 
Until the EPA publishes methodologies for assessing the 7-day mean, 1-day mean 
and instantaneous minimum open-water and deep-water dissolved oxygen criteria 
components, the agency recommends that the states rely strictly on the assessment 
of the 30-day mean open-water and deep-water dissolved oxygen criteria for listing 
decisions. For those open-water and deep-water designated-use segments in which 
the 30-day mean criteria are not in attainment, the jurisdictions should list the desig- 
nated-use-segment on part 5 as impaired in the absence of data and/or methodologies 
for assessing the remaining criteria components. For those designated-use segments 
in which the 30-day mean criteria are in attainment, the jurisdictions should generate 
additional data and apply the criteria assessment procedures to assess attainment of 
the 7-day mean, 1-day mean, and instantaneous minimum criteria components. 
DISSOLVED OXYGEN REFERENCE CURVES 
SUMMER OPEN-WATER AND DEEP-WATER DISSOLVED OXYGEN 
CRITERIA REFERENCE CURVES 
Reference curves for both the 30-day mean open-water (June 1-September 30 only) 
and 30-day mean deep-water dissolved oxygen criteria were based on criteria levels 
that would not impair biological communities (U.S. EPA 2003a). Reference areas for 
chapter iv 
Refinements to the Chesapeake Bay Dissolved Oxygen Criteria Assessment Procedures 
