CHAPTER 2 
Designated Use Boundaries: Episodic Pycnocline 
Application and Expanded Designated Uses 
BACKGROUND 
In the 2003 Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and 
Chlorophyll a for Chesapeake Day and Its Tidal Tributaries, EPA defined five tidal water 
habitats as designated uses providing the context for setting protective Chesapeake Bay water 
quality criteria (U.S. EPA 2003). Detailed dissolved oxygen criteria were established for 
Chesapeake Bay and its tidal tributaries and embayments tailored to each designated use 
accounting for its variations in space and time. EPA has published and Delaware, Maryland. 
Virginia and the District of Columbia have adopted into their state's water quality standards 
regulations dissolved oxygen criteria protective of the published migratory spawning, open- 
water, deep-water and deep-channel designated uses. These dissolved oxygen criteria include 30- 
day, 7-day and 1-day means along with instantaneous minima as needed to protect various 
species and life stages within the designated uses (U.S. EPA 2003). 
Since the Chesapeake Bay dissolved oxygen criteria were published in 2003, refinements and 
updates to the criteria attainment assessment methodologies have been published. Most recently, 
the refined and expanded dissolved oxygen criteria assessment methodologies documented in 
Chapter 3 and associated appendices of the Ambient Water Quality Criteria for Dissolved 
Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and its Tidal Tributaries - 
2008 Technical Support for Criteria Protocols Addendum , replaced the methodologies 
previously published by EPA (U.S. EPA 2008). 
Critical to the dissolved oxygen criteria assessments are the pycnocline delineations defining the 
timing and vertical position of the open-water, deep-water and deep-channel designated use 
boundaries. The standardized method for calculating upper and lower boundaries of pycnoclines 
was originally published in Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity 
and Chlorophyll a for the Chesapeake Bay and its Tidal Tributaries - 2004 Addendum (U.S. 
EPA 2004a). U.S. EPA (2008), on pages 15-18 together with its Appendix A, provide a review 
of, and step by step details associated with, calculating upper and lower pycnoclines which, in 
turn, delineate the vertical boundaries for the open-water, deep-water and deep-channel 
designated uses. 
The following outline lays out the assessment protocol steps for the 30-day mean criteria (open- 
water and deep-water dissolved oxygen criteria (see U.S. EPA 2008, Appendix A for details): 
1) Compiling and formatting the data set 
2) Interpolation of water quality monitoring data 
2.1 Vertical interpolation 
2.2 Horizontal interpolation 
2.3 30-day average interpolation by month 
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