representing approximately 10 percent exceedance is appropriate in this case to account for 
anticipated natural criteria exceedances” (U.S. EPA 2003; p. 173). 
Rationale for Acceptable Exceedances of the Deep Channel Instantaneous Minimum 
Dissolved Oxygen Criterion 
EPA determined that there are allowable criteria exceedances that would not adversely effect 
protection of the designated use. As documented on p. 168 in U.S. EPA 2003: 
“The recommended criteria attainment assessment approach is designed to 
protect the living resources as defined by the designated uses. The criteria levels 
themselves were largely based on scientific studies performed in laboratory 
settings or under controlled field conditions. The criteria establish the level of a 
given habitat condition that living resources need for survival. They do not 
account for many other environmental factors that could affect survival. 
Reference curves were developed to provide a scientific-based, direct measure 
of the ‘allowable' criteria exceedances. These exceedances are defined to be 
those that last a short enough time or cover a small enough area to have no 
adverse affects on the designated use. It is assumed that the designated uses can 
be attained even with some limited level of criteria exceedances and thus, the 
reference curves define those criteria exceedances deemed to be allowable— 
chronic in time but over small areas, or infrequent occurrences over large areas. 
Exceedances that occur over large areas of space and time would be expected to 
have significant detrimental effects on biological communities, which would 
imply nonattainment of designated uses.” 
As reported in a recent paper on the Chesapeake Bay dissolved oxygen criteria by the key 
members of the original Chesapeake Bay dissolved oxygen (DO) criteria team (Batiuk et al. 
2009): 
“Unlike chemical contaminants or other more conventional pollutants, there 
were no clear, well established guidelines for deriving criteria for DO, 
particularly for estuarine waters inhabited by fresh-water and marine species. 
The goal in setting Chesapeake DO criteria was to use the best science possible 
to define conditions that would improve or sustain the suitability of Chesapeake 
Bay habitats for finfish and invertebrates, with the states ultimately factoring in 
consideration of attainability in adopting the criteria as water quality standards. 
Thus, we developed criteria that would greatly increase the spatial and temporal 
extent of Bay waters in which oxygen concentrations were not major limitations 
to growth and survival of organisms dependent on particular Bay habitats. We 
did not, however, derive criteria that would require oxygen concentrations high 
enough at all times and in all locations such that no organism would be 
negatively affected in any location in the Bay. The states and U.S. 
Environmental Protection Agency (EPA) determined that such conditions would 
not be achievable either economically nor technologically (U.S. EPA, 2003d) 
and may not, in fact, reflect pre-historical conditions of Chesapeake Bay, which 
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