time) is produced containing interpolated values for a set of cells representing the 
bathymetry of Chesapeake Bay (with depths in 1-meter increments). 
STEP 2.3. 30-Day Average Interpolations by Month 
A 30-day average is then calculated for each grid cell, for each parameter-cell 
combination. The output is a set of files for each parameter. Each set of files 
includes an individual file for each month (e.g., 30-day average interpolation 
output per month) of the three-year assessment period. 
STEP 2.4. Apportioning Results by Designated Use 
Another program uses, in this case, the 30-day average interpolated pycnocline 
and salinity files (i.e., salinity data that were originally, coincidently measured at 
the same time of the dissolved oxygen measurements) to first divide the interpo¬ 
lated dissolved oxygen data into separate files for each designated use. Second, 
the program then applies the appropriate water quality criterion based on the envi¬ 
ronmental parameter and designated use to calculate violation rates for each 
Chesapeake Bay Program assessment segment. The result is a file for each Chesa¬ 
peake Bay Program segment-designated use combination. (Note: This procedure 
of implementing different criteria over space for a segment that bridges more than 
one salinity zone reflects previous documentation in U.S. EPA 2007, Chapter IE 
Refinements to Chesapeake Bay Water Quality Criteria Assessment Methodology , 
“Step-4 - Pointwise Compliance” (pp. 17-18) and that “the only requirement (of 
the assessment) is that the final attainment determination be “yes” or “no” for 
each interpolator cell.” This procedure assures that salinity-variable criteria 
(e.g., 30-day mean = 5.5 mg 0 2 /L where salinity 0-0.5 ppt, and = 5.0 mg 0 2 /L 
where salinity > 0.5 ppt in Open Water Designated Use) are appropriately applied 
based on measured salinities during the assessment period. The Chesapeake 
Bay Program segmentation boundaries (e.g., XXXTF= “Tidal Fresh”, 
XXOH=“oligohaIine”) are not used as the salinity determinant because they are 
based on historical salinity patterns and would not accurately depict salinity 
conditions present during individual assessment periods. 
STEP 2.5. Water Quality Criteria Assessment, Attainment 
and Violations 
Output files contain a row for each month of the assessment period (2004 - 2006), 
and each row contains the following columns: 
“failed volume,” “assessed volume,” “total volume,” and 
(calculated as failed volume/assessed volume). 
“fraction failed” 
A final program takes the accumulated violation rates for each segment-desig¬ 
nated use assessment and creates a cumulative frequency distribution (CFD) 
curve. 
Criteria violation results of the assessment CFD (i.e., non-attainment) are compared 
with a standard reference or “bioreference” CFD curve, which represents an “allow¬ 
able” amount of criteria violation that can still represent a healthy habitat. For further 
appendix a 
Procedure for Assessing Dissolved Oxygen Criteria Attainment 
