As described previously in this chapter, the interpolator cells can be associated with 
the proper Chesapeake Bay Program segment and salinity zone so that each cell’s 
PLW value can be compared to the proper salinity regime-based water clarity crite¬ 
rion value. The cell area is then accumulated in the ‘fail’ or ‘pass’ tally for each 
Chesapeake Bay Program segment for each month. The cumulative frequency distri¬ 
bution curve resulting from the monthly percent attainment measures over the 
respective underwater bay grass growing season (see Table VI-2) and three-year 
attainment period is then compared statistically to the reference curve for the appro¬ 
priate salinity zone to determine the degree of attainment or nonattainment. If the 
curves are differ significantly, then the segment/designated use is considered out of 
attainment and fails by the amount defined by the area between the two curves. 
Assessing Water Clarity Criteria Attainment throughout a Defined 
Shallow-Water Habitat Acreage. Restoring underwater bay grasses within a 
segment requires that the particular shallow-water habitat meets the Chesapeake Bay 
water clarity criteria across acreages much greater than those actually covered by 
underwater bay grasses. The ratio of underwater bay grass acreage to the required 
shallow-water habitat acreage achieving the necessary level of water clarity to 
support return of those underwater bay grasses varies, based upon the different 
species of bay grasses inhabiting the Chesapeake Bay’s four salinity regimes. The 
average baywide ratio of underwater bay grass acreage to suitable shallow-water 
habitat acreage is approximately one acre of underwater bay grasses for every three 
acres of shallow-water habitat achieving the Chesapeake Bay water clarity criteria 
(U.S. EPA 2003). 
The salinity regime and, therefore, bay grass community-specific underwater bay 
grass acreage to shallow-water habitat acreage ratios have been derived through an 
evaluation of extensive underwater bay grass distribution data within tidal-fresh, 
oligohaline, mesohaline and polyhaline salinity regimes (reflecting different levels 
of coverage by different underwater bay grass communities). The Technical Support 
Document for the Identification of Chesapeake Bay Designated Uses and Attain¬ 
ability documents the methodology followed and the resulting underwater bay 
grasses acreage to shallow-water habitat acreage ratios derived for each of the four 
salinity regimes (U.S. EPA 2003). 
The same procedures as described above in “Assessing Water Clarity Criteria Attain¬ 
ment at an Established Application Depth” are followed for determining attainment 
of the water clarity criteria across the total required shallow-water habitat acreage for 
a specific Chesapeake Bay Program segment. The only difference is that a segment- 
specific water clarity criteria application depth is not applied. Instead, the depth of 
attainment of the water clarity criteria is determined for each interpolator cell. The 
area in each interpolator cell from the intertidal zone out to the water-column depth 
at which the water clarity criteria was attained is combined along with other similar 
areas determined for the other interpolator cells comprising the shallow-water areas 
in a specific segment. 
chapter vi 
Recommended Implementation Procedures 
