well as high-resolution spatial and temporal monitoring in selected locations. 
However, these new components are only in the planning and early implementation 
stages at this point, and because of financial constraints or limitations to current 
technology, direct monitoring at the scales of the criteria may not be possible in the 
foreseeable future. Therefore, the direct assessment of attainment for some 
geographic regions and for some short-term criteria elements (e.g., instantaneous 
minimum, 1-day mean and 7-day mean) must be waived for the time being or based 
on statistical methods that estimate probable attainment. Several approaches to 
addressing the duration issue are described below. 
Thirty-Day Mean Attainment Procedure. This duration appears to be within the 
temporal scale of the current Chesapeake Bay water quality monitoring programs. 
The simplest assessment approach is to use the one value or average of two values 
collected within a month as the best estimate of the true 30-day mean. At present, 
this is the approach recommended for assessing attainment of criteria with this dura¬ 
tion. However, it is debatable how well one or two samples per month represent what 
is intended as protective by the 30-day mean. 
These procedures assume the existence of a baywide tidal-water monitoring program 
with a fixed-station sampling design and sampling frequency at least once per month 
during the seasons defined by the criteria. The procedures assume that horizontal and 
vertical measurements of dissolved oxygen will be sufficiently dense that the inter¬ 
polator can create an accurate three-dimensional representation of dissolved oxygen 
in the defined designated uses. It also assumes that data are sufficient to define the 
boundaries of the designated uses where boundaries are variable, depending on 
pycnocline depth. 
To simplify computations, if there is more than one observation per month, then the 
monthly average is calculated prior to input to the volumetric interpolator. Prior to 
averaging for the month, each station’s dissolved oxygen profile is interpolated verti¬ 
cally to obtain a value at each half-meter interval from surface to bottom. The 
monthly average concentrations at each fixed station at each half-meter are then 
interpolated horizontally by the Chesapeake Bay interpolator to yield a basinwide 
grid of concentrations for each month. A comparable reference grid or a table of grid 
coordinates and depths can be used to relate the monthly cell concentrations to be 
evaluated with the correct designated use and corresponding criteria concentrations. 
The cell is scored as meeting or not meeting the criterion level and cell volume is 
accumulated in the pool of passing or failing totals for each designated use in each 
Chesapeake Bay Program segment. From this, the spatial extent of nonattainment, 
i.e., the percentage of the total volume exceeding the criterion in each designated use 
in each Chesapeake Bay Program segment is tallied for each month in the assess¬ 
ment period (most recent three years). 
Dissolved oxygen criteria attainment is reported seasonally (see Table VI-1). To 
assess, for example, attainment of the summer season 30-day mean criterion for the 
deep-water seasonal fish and shellfish designated use, the percent exceedance data 
chapter vi • Recommended Implementation Procedures 
