CB3MH Deep Water 1987, a segment/designated use considered having a healthy B-1BI for that 
year, whose acceptable violation rates were included in the generation of the biological reference 
curve, fails assessment by that same biological reference curve. 
Figure III-2. An example of a 30-day mean deep-water dissolved oxygen criteria and the 
violation expressed by a healthy segment (CB3MH 1987) curve used in deriving the 30-day 
mean criterion biologically-based reference curve. 
Further analyses revealed that the biological reference curves used for the deep-water and deep- 
channel dissolved oxygen criteria attainment assessments fail the majority of supposedly 
“healthy” segment-years used to construct those same curves. 
As described in U.S. EPA 2003, the preferred methodology for defining the reference curve is to 
determine levels of allowable violation based on the demonstrated tolerance of the living 
resources for whose protection the water quality criteria were designed. Benthic habitat 
assessments were conducted with the updated methodology, which is described below, for 
assessing the appropriateness of biologically-based reference curves as indicators of water 
quality conditions. 
UPDATES TO DISSOLVED OXYGEN BIOLOGICALLY-BASED REFERENCE 
CURVE DERIVATION METHODOLOGY 
Based on the findings described above, the following revisions are recommended to the 
methodology for categorizing benthic communities as “healthy” for the purposes of providing a 
reference for allowable frequency of dissolved oxygen criteria exceedance. The intent of these 
revisions is to improve the accuracy with which benthic communities are categorized as healthy. 
Revisions to the previously published methodology for developing dissolved oxygen biological 
reference curves include: 
1) Restriction of the reference dataset to data collected beginning in 1996; 
2) expansion of time period for classifying benthic community health from 1 year to 
sequential 3-year time periods; 
17 
