abilities are spatially interpolated to estimate probabilities for all interpolator cells 
that represent the bathymetry of the Bay, its tidal tributaries and embayments. The 
interpolator cells that are contained within the designated use where the criterion 
applies are parsed out by segment and the probabilities calculated for each cell are 
evaluated cell-by-cell against a threshold of probability which indicates an unac¬ 
ceptably high risk that the dissolved oxygen criterion was exceeded (Jordan et al 
1992). The volume of water represented by the interpolator cells exceeding the 
threshold as a percentage of the total volume in the designated use is tallied for each 
segment, for each month in the assessment period. 
There are several elements of the logistic regression approach which should be eval¬ 
uated as part of the attainment assessment procedure. Each of the station-specific 
logistic models has its own goodness-of-fit measure. Each station will have a result 
from the predictive model, i.e., the probability of exceeding the instantaneous 
minimum over the assessment unit. Each segment will have an estimate of the 
percent volume exceeding the criteria, based on spatial interpolation of the station 
probabilities. As with other components of the dissolved oxygen criteria, these 
results can also be assessed and visualized using the CFD methodology, although 
this is not mandatory. 
The limitations of this methodology have been noted earlier, particularly the 
temporal frequency on which the models are based. In addition, the lack of good 
spatial representation in the tidal tributaries and embayments is a concern. Most of 
the fixed-stations are situated more or less longitudinally in mid-channel and there 
is insufficient lateral coverage of the flanks, where different oxygen conditions and 
different model relationships may exist. Data now being collected through the 
Chesapeake Bay Shallow Monitoring Program will help answer where and to what 
extent this is true. 
LITERATURE CITED 
Jordan, J., C. Stenger, M. Olson, R. Batiuk and K. Mountford. 1992. Chesapeake Bay 
Dissolved Oxygen Goal for Restoration of Living Resource Habitats. CBP/TRS 88/93. 
Chesapeake Bay Program, Annapolis, Maryland. 
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. 
EPA 903-R-03-002. Region III Chesapeake Bay Program Office, Annapolis, Maryland. 
Potential Methods for Assessing Shorter Duration Dissolved Oxygen Criteria 
