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details with illustrations of the CFD development and comparisons procedure, refer 
to Chapter vi. Recommended Implementation Procedures in Ambient Water Quality 
Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake 
Bay and Its Tidal Tributaries (Regional Criteria Guidance) (U.S.EPA 2003). A 
review of the procedure is provided below. 
PLOTTING A BIOREFERENCE CURVE 
A biological reference curve of acceptable violation rates is generated using a cumu¬ 
lative frequency distribution (CFD) of violation rates for “healthy” designated uses. 
The violation rates are sorted in ascending order, ranked in descending order, and 
graphed on a quantile plot: 
• Violation rates are plotted on the x axis, with plotting position on the y axis. 
• Plotting position represents the probability, i/n, of being less than or equal to a 
given violation rate, or x, and is plotted on the y axis as a function of rank, or 
“i”, and sample size, or “n”. 
• The x axis is labeled “space” because the violation rate represents the fraction 
of volume that is in violation. 
• The y axis is labeled as “time” because “probability” represents the probable 
amount of time that a given violation rate will be observed. 
• The Chesapeake Bay Program currently uses the Wiebull plotting position to 
plot the cumulative distribution function. The Wiebull equation for calculating 
probability, y, for each violation rate with rank “i” is: 
• y = i/(n+l); i = rank 
In order to generate a graph of the CFD: 
• Xj , x 2 , x 3 ,...x n = violation rates provided herein, sorted in ascending order, 
with rank (i) assigned in descending order 
• yj = i/(n+l) 
• After plotting the data’s violation rates and probabilities, two additional points 
should be added to the distribution in order to complete the CFD curve: 
- Insert (x 0 ,y 0 ) = (0,1) before the first data point 
- Insert (x n+ i,y n+l ) = (1,0) after the last data point 
LITERATURE CITED 
Bahner, L. 2001. The Chesapeake Bay and Tidal Tributary Volumetric Interpolator, VOL3D, 
Version 4.0. National Oceanic and Atmospheric Administration, Chesapeake Bay Office. 
http://www.chesapeakebay.net/cims/interpolator.htm 
U. S. Environmental Protection Agency. 2003. Ambient Water Quality Criteria for Dissolved 
Oxygen, Water Clarity and Chlorophyll afar the Chesapeake Bay and Its Tidal Tributaries. 
April 2003. EPA 903-R-03-002. Region III Chesapeake Bay Program Office, Annapolis, MD. 
U.S. Environmental Protection Agency. 2007. Ambient Water Quality Criteria for Dissolved 
Oxygen, Wlater Clarity and Chlorophyll afar the Chesapeake Bay and Its Tidal Tributaries - 
2007 Addendum, July 2007. EPA 903-R-07-003 Region III Chesapeake Bay Program Office, 
Annapolis, MD. 21403. 
appendix a 
Procedure for Assessing Dissolved Oxygen Criteria Attainment 
