K-9 
Elizabeth River 
Percentages ot degraded samples with a contaminant effect ranged from 50% in the 
lower Elizabeth River mainstem (ELIPHa) to nearly 91% in the Eastern Branch 
(EBEMHa). At least 80% of degraded samples were classified as contaminated in 
both the Southern Branch (SBEMHa) and the Lafayette River (LAFMHa) and 68% 
were classified as contaminated in the upper Elizabeth River mainstem (ELIMHa). 
Of the remaining degraded samples without a contaminant effect, excessive abun¬ 
dance/biomass was found in 9.1%, 12.5%, and 5.3% in the Western Branch 
(WBEMHa), Southern Branch (SBEMHa) and upper Elizabeth River mainstem 
(ELIMHa), respectively, indicating the potential of stress due to eutrophication. 
Only one sample had excessive abundance in the lower Elizabeth River mainstem 
(ELIPHa). Insufficient abundance/biomass was found in 12.5%, 5.9%, and 15.8% of 
the degraded samples without a contaminant effect in the Southern Branch 
(SBEMHa), the Lafayette River (LAFMHa) and the upper Elizabeth River 
(ELIMHa), respectively, indicating low dissolved oxygen as an additional source of 
stress to benthic communities in these segments. 
In summary, the predominant source of stress to benthic communities within the 
Elizabeth River is anthropogenic contamination. Both eutrophication and low 
dissolved oxygen appear to be additional sources of stress within the Southern 
Branch (SBEMHa) and upper Elizabeth River mainstem (ELIMHa). 
York River 
None of the upper Pamunkey River (PMKTF) samples had B-IBI scores <2.7, so 
none were assessed by the diagnostic tool. Over 57% of the lower Pamunkey River 
(PMKOH) degraded samples were classified as contaminated by the tool, but the 
average contaminant group posterior probability was low at 0.62. One additional 
sample in this last segment was not classified as contaminated and had insufficient 
abundance/biomass. Few samples were degraded in the upper Mattaponi River 
(MPNTFa), and 67% of these were classified as contaminated. However, the average 
contaminant group posterior probability was low at 0.65 and no samples collected 
had a probability of contaminant group membership >=0.90. No samples were clas¬ 
sified as having excessive or insufficient abundance/biomass within this segment. In 
the lower Mattaponi River (MPNOHa) 80% of the degraded samples were classified 
as contaminated. The average contaminant group posterior probability in this 
segment was high at 0.87 and group membership probabilities for all samples clas¬ 
sified as contaminated were >=0.90. No uncontaminated degraded samples had 
excessive or insufficient abundance/biomass. In the middle York River (YRKMHa) 
64% of the degraded samples were classified as contaminated. An additional 9.1% 
of degraded samples had excessive abundance/biomass and were not classified as 
contaminated by the tool, while 12.1% of the uncontaminated degraded samples had 
insufficient abundance/biomass. In the lower York River (YRKPHa) only 46% of the 
degraded samples were classified as contaminated. An additional 9.1% and 27.3% of 
uncontaminated degraded samples were found with excessive abundance/biomass 
and insufficient abundance/biomass, respectively, in this segment. In Mobjack Bay 
(MOBPHa), 50% of the degraded samples were classified as contaminated, all with 
contaminant group posterior probabilities >=0.90. An additional 12.5% and 25% of 
appendix k 
2006 303(d) Assessment Methods for Chesapeake Bay Benthos 
