B-3 
Criteria assessments are based on each component criterion’s specific averaging 
period. Assessments of attainment of the instantaneous minimum criteria are directly 
evaluated using the individual cruise interpolations. All 30-day mean criteria assess¬ 
ments rely on monthly averages of interpolated data sets. To calculate these 
averages, each interpolated cruise within a month is averaged on a point-by-point 
basis in matching interpolator grid cells. Generally, two cruises per month run 
through the warm season with one cruise per month during the cooler period. Spatial 
violation rates are calculated for each temporally aggregated interpolation in an 
assessment period. For example, the 12 monthly average interpolations representing 
the four summer months (June, July, August, September) over three years were used 
for a three-year summer open-water dissolved oxygen assessment. 
Cumulative frequency diagrams (CFD) are generated from the spatial violation rates 
for each assessed designated use, water quality parameter, criterion, and averaging 
period using the Weibull plotting position (rank/(n+l)). 
The assessment CFD is compared to a reference CFD to determine if unallowable 
exceedances of the criterion occur. The diagrams of both CFDs show three areas: 
non-exceedance (above the assessment curve), allowable exceedance (below both 
curves), and unallowable exceedance (below the assessment curve and above the 
reference curve). If the assessment CFD surpasses the reference CFD at any point, 
an unallowable exceedance exists. 
Reference CFDs are continuous or generally have many more points than assessment 
CFDs. This situation can lead to spurious unallowable exceedances even without 
individual points in the assessment CFD topping reference CFD levels. To address 
this problem, reference curves are evaluated only at the temporal axis points in the 
assessment curve (see Figure II-7 in Chapter 2). For non-continuous biological refer¬ 
ence curves, these points are interpolated from neighboring points. 
The trapezoidal rule is used to calculate the areas. This rule is a method of approxi¬ 
mate integration, which calculates the areas of discrete trapezoids that make up the 
area below a curve when summed. Since both the assessment and reference curves 
are piecewise linear, repeated application of the trapezoidal rule results in an exact, 
rather than approximate, value. 
For dissolved oxygen criteria assessed without reference curves, the assessment 
space is divided in two—non-exceedance and unallowable exceedance. 
LITERATURE CITED 
U.S. Environmental Protection Agency (U.S. EPA). 2003a. Ambient Water Quality Criteria for 
Dissolved Oxygen, Water Clarity' and Chlorophyll a for the Chesapeake Bay and its Tidal Trib¬ 
utaries. EPA 903-R-03-002. Region III Chesapeake Bay Program Office, Annapolis, MD. 
U.S. Environmental Protection Agency. 2003b. Technical Support Document for Chesapeake 
Bay Designated Uses and Attainability. EPA 903-R-03-004. Region III Chesapeake Bay 
Program Office Annapolis. MD. 
appendix b 
Detailed Chesapeake Bay Water Quality Criteria Assessment Methodology 
