27 
make them coincident with the boundaries of the 
larger Chesapeake Bay Program segments. This 
way, the smaller assessment units nest within the 
larger ones and the larger-scale assessment results 
can be attributed to each of the smaller units within. 
The approach allows states to remain consistent 
with previous listing decisions while accounting for 
the broader designated-use-segment-assessment 
results on their 303(d) lists. 
In some cases, adoption of the new Chesapeake Bay 
spatial assessment units represented a less detailed 
and possibly less precise assessment of water-quality 
criteria attainment. For example. Figure III-1 illus¬ 
trates Chesapeake Bay Program segment CB7PH, 
which covers the southeastern portion of the main- 
stem Chesapeake within Virginia. As is typical in 
most of the Bay, the shoreline is extremely complex 
with many small tidal rivers, creeks, and embay- 
ments. These smaller tidal habitats may have different 
water quality than the mainstem Bay section of the 
segment due to different circulation patterns or land 
uses or pollution sources that dominate local water 
quality conditions. These smaller tidal habitats may 
even have monitoring information that demonstrates 
the differences in water quality conditions. In such a 
case, it may make sense to separate the smaller tidal 
river, creek, or embayment from the main assess¬ 
ment unit by subdividing it to create a new smaller 
spatial unit for separate assessment. Thus, the states ha 
larger units to characterize conditions in specific parts 
and embayments more precisely. 
Figure 111-1. Segment CB7PH covering the southeastern 
portion of the Chesapeake Bay in Virginia. 
Source: U.S. EPA 2004b. 
re the option to “sub-segment” 
of the Bay, its tidal tributaries, 
Allowing jurisdictions to subdivide the larger segments is consistent with national 
EPA guidance and with EPA-published Chesapeake Bay water quality criteria assess¬ 
ment guidance, which both provide specific considerations for sub-segmenting water 
bodies for criteria assessment and listing decisions (U.S. EPA 2003a, 2005b). 
Published EPA guidance states that waters can be partitioned “to represent homo¬ 
geneity in physical, biological or chemical conditions.” The EPA recommends that 
jurisdictions use similar principles in deciding to subdivide the larger Chesapeake 
Bay assessment units. A state’s decision to sub-segment an existing segment should 
be based on: 1) clear physical, biological, or chemical differences that can be docu¬ 
mented; 2) homogeneity of water quality in the water body under consideration; and 
3) confirmed future availability of monitoring data in the new sub-segment to provide 
the capability to assess conditions and allow a determination regarding its 303(d) list 
status. In all cases, there should be a priori knowledge of the conditions that support 
a decision to subdivide, and preferably specific data that demonstrate how conditions 
chapter iii • Application of Chesapeake Bay Water Quality Criteria Assessment Procedures 
