does move in and out of degradation within a single spring or summer season. Thus, the P-IB1 
does identify instances of high quality conditions, but currently does not provide the 
characteristics of a season-long “healthy” condition in terms of allowable exceedances that could 
be used to support derivation of a biologically-based reference for chlorophyll a criteria 
assessments. 
Further work is needed to specify a metric that can provide a priori identification of an 
unimpaired system on the relevant timescale, from which allowable exceedance of the 
chlorophyll a criteria can then be inferred. EPA, therefore, recommends a default 10% reference 
curve for assessing the chlorophyll a criteria (U.S. EPA 2007a, Figure 11-4, and Equation 1). 
REVIEW OF THE CURRENT CHLOROPHYLL A CRITERIA ATTAINMENT 
ASSESSMENT PROCEDURE: METHOD AND ASSUMPTIONS 
In Table IV-1, the current Chesapeake Bay chlorophyll a attainment assessment procedure is 
outlined for developing a seasonal mean for a Chesapeake Bay management segment to compare 
with published numerical criteria (e.g., Virginia tidal James River and the District of Columbia's 
tidal waters, U.S. EPA 2007a, Appendix C). 
Table IV-1. Outline of the previously published Chesapeake Bay chlorophyll a criteria 
attainment assessment methodology. __ 
Outline of chlorophyll a attainment assessement steps 
Comments 
Chlorophyll a data used for scenario assessments comprise all 
chlorophyll a values in the CIMS water quality database with layer 
flagged “S” for surface. 
For Virginia chlorophyll a assessments, 
use all publically available and 
appropriate surface data, i.e., CIMS data 
plus VIMS/HRSD DATAFLOW data. 
(U.S. EPA 2008 p.30). 
Data are organized into individual “cruise” files for interpolation. 
Individual cruise files are interpolated using the Chesapeake Bay 
Interpolator (version 4.61), with the “In-transform” and the “2-D 
Inverse-Distance Squared” options selected. 
The Interpolator automatically back- 
transforms chlorophyll a values in its 
output Files. (U.S. EPA 2008 p.30). 
Interpolated chlorophyll a surfaces are averaged for an entire season (on 
a cell-by-cell basis). 
The current methodology calculates an 
arithmetic mean on the back-transformed 
chlorophyll a values. 
Seasonal means are assessed (cell-by-cell) against the criterion for the 
relevant river segment-season. Assessment curves were compared 
against a default reference curve. Non-attainment is calculated by 
subtracting the area of the reference curve from the area under the 
chlorophyll a criteria assessment curve*. 
* If the assessment curve exceeds, at any 
point, the reference CFD, then the given 
segment is considered “impaired”. 
Source: U.S. EPA 2008 
To review the method details, U.S. EPA (2007a, p. 62) first states in the assessment procedure: 
“Assessments of seasonal mean chlorophyll a criteria should be based on 
seasonal averages of interpolated data sets. To calculate seasonal averages, 
each interpolated cruise within a season should be averaged on a point-by- 
point basis in matching interpolator grid cells. Spatial violation rates should be 
calculated for each seasonally aggregated interpolation in an assessment 
period. For example summer open water seasonal chlorophyll a criteria 
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