Appendix D. 
History of EPA Guidance Regarding the Deep-Channel 
Reference Curve 
In April 2003, the EPA published the guidance document. Ambient Water Quality Criteria for 
Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal 
Tributaries (U.S. EPA 2003). In this publication, EPA documented the derivation of the 
dissolved oxygen criterion protective of the seasonal deep channel designated use. For seasonal 
deep-channel designated use, an instantaneous minimum criterion of 1 mg/L was determined to 
protect benthic organisms residing in the: 
“deep water-column and adjacent bottom surficial sediment habitats located 
principally in the river channel at the lower reaches of the major rivers and 
along the spine of the middle mainstem Chesapeake Bay at depths below 
which seasonal anoxic (< 0.2 mg/L dissolved oxygen) to severe hypoxic 
conditions (< 1 mg/L dissolved oxygen) routinely set in and persist for 
extended periods of time under current conditions” (p. 60 in U.S. EPA 2003). 
In support of the instantaneous minimum criterion of 1 mg/L, U.S. EPA (2003) summarized 
findings published in peer-reviewed literature sources indicating that several keystone benthic 
species “are resistant to dissolved oxygen concentrations as low as 0.6 mg/L,” and that 
“extensive mortality is likely only under persistent exposure to very low dissolved oxygen 
concentrations at high summer temperatures” (p. 61). 
U.S. EPA (2003) also reported that in the mesohaline Chesapeake Bay (the primary location of 
the seasonal deep-channel designated use), “dissolved oxygen concentrations of less than 1 mg/L 
lead to mortality for even tolerant species (p. 61) and that “when dissolved oxygen drops 
significantly below 1 mg/L for even short periods of time (on the order of hours) mortality 
increases, even for tolerant species” (p. 65). Furthermore, it was stated that “States and other 
users must recognize that the deep-channel dissolved oxygen criterion is stated as an 
instantaneous minimum, thus any exceedance is assumed to have direct consequences to the 
survival of the bottom-dwelling community” (p. 151). 
Regarding the definition of a water quality standard, it is explained in U.S. EPA 2003 and in 
Chapter 3 of the U.S. EPA’s Water Quality Standards Handbook, 2 nd Edition (U.S. EPA 1994) 
that water quality criteria definition and assessment comprises not just the magnitude of a water 
quality criterion (i.e. “the quantifiable condition,” in this case the concentration of dissolved 
oxygen), but also the duration and frequency of that condition. 
In this context, duration is addressed by restricting the applicability of the criterion to the 
summer period (June - September) when stratification and severe hypoxia occur in deep-channel 
regions of the Chesapeake Bay, and by defining the assessment period as “the most recent three 
consecutive years for which relevant monitoring data are available” (U.S. EPA 2003; p. 150-1). 
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