26 
ASSESSMENT UNITS, SEGMENTATION, 
AND SUB-SEGMENTATION 
To assess attainment of the Chesapeake Bay water quality criteria, the spatial and 
temporal extent over which they apply must be defined. The temporal extent is 
defined implicitly for each component of the states’ Chesapeake Bay water quality 
standards. Described on page 150 in the 2003 EPA Chesapeake Bay water quality 
criteria document (U.S. EPA 2003a) and adopted into the jurisdictions’ water quality 
standards regulations, the spatial extent is defined by the intersection of a Chesa¬ 
peake Bay Program segment (U.S. EPA 2004b, 2005a) and each tidal water 
designated use (U.S. EPA 2003b, 2004c). The spatial units defined by this intersec¬ 
tion are referred to as “spatial assessment units.” The intent is for each unit to be 
assessed and listed independently on each jurisdiction’s 303(d) list (part 1 through 
part 5) (see Chapter 8 for further details). 
The scale of the Chesapeake Bay spatial assessment units is large, with selection 
based specifically on conditions in the Bay and on the factors affecting these condi¬ 
tions. The Chesapeake Bay Program segments themselves were based on salinity 
regimes, circulation patterns, and other natural physical features, but are generally 
reflective of variations in water quality conditions and living resource communities 
(U.S. EPA 2004b, 2005a). Thus, these segments serve as appropriate spatial units for 
measuring the scope of water quality impairments in the Chesapeake Bay, its tidal 
tributaries, and its embayments. They also work at a logical scale for developing 
necessary management plans (TMDLs). Many of the water quality impairments 
currently extend over large areas of the Bay and its tidal tributaries, so performing 
assessments and reporting on these impairments at the segment scale are both appro¬ 
priate. Developing management plans at this scale is also appropriate since multiple 
jurisdictions often contribute to impairments. 
Even though the scale of the spatial assessment units is suitable, in many cases it 
varied from the scale of past tidal water quality criteria attainment assessments. The 
change in scale introduced several challenges to the states as they implemented the 
new Chesapeake Bay water quality criteria and tidal water designated uses. Bound¬ 
aries of some previously established state assessment units were moved or shrunk to 
address the spatial variability in some state water quality standards assessment meas¬ 
ures. Furthermore, management decisions (e.g., listing certain waters as impaired, 
developing TMDLs) had already been made based on the previously established 
assessment units and were being implemented at the time the new Chesapeake Bay 
water quality standards were adopted into state regulation. Thus, it was necessary to 
establish procedures for transitioning to new spatial assessment units and relating 
prior management decisions to new assessments that were sometimes defined at a 
different spatial scale. 
In general, the states could address the differences in boundary locations by making 
small adjustments to state-defined spatial units. Primarily, adjustments consisted of 
small changes in the boundaries of the previously state-defined assessment units to 
chapter iii • Application of Chesapeake Bay Water Quality Criteria Assessment Procedures 
