K-8 
and Gunpowder River (GUNOH). Except for the Severn River, these segments were 
also identified as impaired in the 2004 assessment. 
4.3. DIAGNOSTIC TOOL AND INSUFFICIENT AND EXCESSIVE 
ABUNDANCE OR BIOMASS 
The diagnostic tool and the insufficient and excessive abundance/biomass criteria 
can be used as ancillary information to determine most likely source of stress 
affecting benthic communities in segments classified as impaired. The results of this 
part of the assessment should be used only as a screening tool to identify probable 
causes of degradation and to prioritize segments for further study. 
There is always a risk of misclassifying sites as affected by toxic contamination, low 
dissolved oxygen, or nutrient enrichment, so independent measurements of sediment 
and water quality should be made whenever possible. Table 4 presents the results of 
the diagnostic tool and the insufficient and excessive abundance/biomass character¬ 
ization for sites with contaminant group posterior probabilities >=0.50, and Table 5 
presents the results for sites with contaminant group posterior probabilities >=0.90. 
A general decision tree for segment assessment and characterization is provided in 
Figure 1. Results are summarized below. 
James River 
The percentages of degraded samples with a contaminant effect ranged from 67% in 
the upper James River (JMSTFa) to 78% in the middle James River (JMSOHa) for 
P >=0.5, with average contaminant group posterior probabilities ranging from 0.64 
to 0.79. At P >=0.9 contaminant percentages ranged from 33-50% (Table 4). At the 
James River mouth (JMSPHa) no samples were classified as contaminated. In addi¬ 
tion, an examination of all samples collected indicated that only one sample had 
excessive abundance/biomass and only one had insufficient abundance/biomass. In 
the Nansemond River (JMSMHb), 90% of the degraded samples were classified as 
contaminated with an average contaminant group posterior probability of 0.87. 
Eighty percent of degraded samples had contaminant group posterior probabilities of 
at least 0.90. Only three samples were collected in the Chuckatuck River/Pagan 
River segment (JMSMHc), and three in the Warwick River (JMSMHd). Although the 
low number of samples makes reliable assessments difficult, degraded samples were 
collected in both segments and each was classified as contaminated with high poste¬ 
rior probabilities of contaminant group membership. Although only three samples 
were collected in Willoughby Bay (JMSPHd), each sample was classified as contam¬ 
inated. Contaminated samples in this segment had an average contaminant group 
posterior probability of 0.84. Additional samples are required in these segments to 
determine the extent of benthic degradation and potential sources of stress. 
In summary, results indicate that contaminants may account for a large portion of the 
degradation in the James River, except for the James River mouth. The primary 
source of degradation in the Nansemond River appears to be anthropogenic contam¬ 
ination. Sampling was not sufficient for a reliable assessment in the Chucktuck/ 
Pagan River and Warwick River segments. 
appendix k 
2006 303(d) Assessment Methods for Chesapeake Bay Benthos 
