The frequency component of the criterion “is directly addressed through comparison of the 
generated cumulative frequency distribution (CFD) with the applicable criterion reference curve” 
(U.S. EPA 2003;p. 151). 
In summary, statements were made in U.S. EPA 2003 suggesting that the benthic community can 
tolerate small violations of the deep-channel instantaneous minimum criterion, but statements 
were also made suggesting that any violation of this criterion has negative effects on the survival 
of deep-channel benthic species. However, as also described in U.S. EPA 2003, national 
guidelines define a water quality standard as comprising not only the magnitude of a given 
condition, but also the duration over which that condition is assessed and the frequency of 
violation allowed within the given assessment duration. For the case of the Chesapeake Bay, the 
frequency of allowable violation is defined by the location of a reference CFD, more commonly 
called a “reference curve" (both the rationale for use of a biological reference curve and the 
development of the 10 percent reference curve are also well documented in U.S. EPA 2003). 
LITERATURE CITED 
U.S. Environmental Protection Agency. 1994. Water Quality Standards Handbook, 2 nd Edition. 
August 1994. EPA 823-8-94-005a. Washington D.C. 
U.S. Environmental Protection Agency. 2003. Ambient Water Quality Criteria for Dissolved 
Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Day and Its Tidal Tributaries 
(Regional Criteria Guidance). April 2003. EPA 903-R-03-002. Region III Chesapeake Bay 
Program Office, Annapolis, MD. 
53 
