A-20 
primarily for the dissolved oxygen (DO) open- and deep-water monthly mean 
criteria because there were insufficient data collected to assess the higher-frequency 
DO criteria components. The clarity criteria were not assessed based on the CFD 
because there were few systems in which there was sufficient data for an assessment. 
Chlorophyll criteria were not available from the Chlorophyll criteria team in time to 
implement a chlorophyll assessment. 
In general, the CFD analysis indicated that most of the Bay waters failed one or more 
of the open-water or deep-water DO criteria components. However, there were also 
many tributaries in which all of the DO criteria assessed indicated attainment. Thus 
in this initial application, the CFD method did appear to distinguish between 
impaired and unimpaired systems in a manner that is consistent with the expectations 
of the many stakeholders in the CAP workgroup. 
In the 2006 application of the assessment methodology, there were many details that 
required resolution in order to fully implement the methodology. Procedures gener¬ 
ally followed the theoretical description as described in Section 2.1, but some details 
were modified to address unforeseen complications. The following describes some 
of those details. 
In general, data were obtained from the CBP CIMS data base and parameters 
included date, location, depth, salinity, temperature and the water quality parameter 
being assessed. Some State data were also incorporated and those data were obtained 
directly from the relevant State. Once all the data were compiled, they were assigned 
to a time period based on the sample date. Fixed-station data are normally collected 
during a monitoring cruise that covers the entire tidal Chesapeake Bay over several 
days. However, in order to provide a “snapshot” in water quality, the data collected 
within a cruise are assumed to be contemporaneous in order to perform a single 
spatial interpolation. For any data not associated with a cruise, a cruise number is 
assigned representing the closest cruise in time to the collection of each datum. Co- 
located data points in the same cruise were averaged. 
The assessment procedure requires assessment over large areas rather than at points 
in space. Spatial interpolation using the CBP IDW interpolator was performed for 
each water-quality criteria parameter for each cruise. Clarity and surface chlorophyll 
were interpolated in the two horizontal dimensions using inverse distance squared 
weighting. Dissolved oxygen was first linearly interpolated in the vertical dimension 
within each column of data beginning at 0.5 meters and continuing at one meter 
intervals, not to exceed the deepest observation in that column. Each depth was then 
interpolated horizontally using inverse distance squared weighting. Data regions 
were specified for each segment in order to prevent the interpolation algorithm from 
using data points in neighboring tributaries. 
Designated uses in the Chesapeake Bay are defined vertically in order separate stable 
water layers that have differing criteria levels for dissolved oxygen. The surface layer 
(open water) is that layer defined to be above the pycnocline and thus exposed to the 
atmosphere. The middle layer (deep water) is defined to be the layer between the 
upper and lower pycnocline. And the lower layer (deep channel) is defined to be the 
layer below the pycnocline. Given that the pycnocline is dynamic and moves up and 
appendix a 
The Cumulative Frequency Diagram Method for Determining Water Quality Attainment 
