81 
much of the time the fully saturated ambient dissolved oxygen concentrations would 
still above the 5 mg liter 1 30-day mean criterion level. However, from 13 to greater 
than 30 percent of the warm months’ monitoring-based observations fell below a 
monthly mean of 5 mg liter 1 with the magnitudes of these exceedences up to 0.7 mg 
liter 1 . These observations indicate that the segments would likely fail a summer¬ 
time application of the 5 mg liter 1 30-day mean criteria. Tested against a monthly 
mean concentration of 4 mg liter however, the percentage of observations falling 
below this concentration is less than 7 percent in most cases, and the magnitude of 
the exceedance is ~0.5 mg liter 1 (Table VI-4). 
The warm months calculated dissolved oxygen saturation concentration of 8.5 
+/-0.7 mg liter 1 directly translates into a dissolved oxygen concentration range of 
7.8 to 10.2 mg liter -1 . Similarly, the warm months average oxygen deficit of 2.6 
+/-0.8 mg liter 1 converts into a oxygen deficit concentration range of 1.6 to 3.4 mg 
liter -1 . Assuming a maximum long-term average oxygen deficit of 3.4 mg liter -1 , we 
could anticipate an ambient dissolved oxygen range of 6.8 to 4.4 mg liter -1 upon 
factoring in the oxygen deficit to a saturated water column condition. These are the 
best dissolved oxygen conditions, assuming the maximum oxygen deficit, one could 
ever hope to measure in the absence of any anthropogenic nutrient pollutant loading 
influence on ambient dissolved oxygen conditions. Even without any human 
impacts, the 5 mg liter 1 30-day mean dissolved oxygen criterion would be not 
attained all times in the warm months of the year, setting up the basis for a site- 
specific criterion based on natural conditions preventing attainment of the use (U.S. 
EPA 2003b). 
SITE-SPECIFIC DISSOLVED OXYGEN CRITERIA DERIVATION 
Factoring a natural tidal wetlands-based oxygen deficit into the oxygen saturation 
levels, based on the 18-year data record (see above), along with recognition that the 
antropogenic pollutant loads can be reduced but not eliminated (U.S. EPA 2003b), a 
site specific 4 mg liter -1 30-day mean criterion is recommended in place of the 
published 5 mg liter -1 30-day mean and 4 mg liter 1 7-day mean open-water desig¬ 
nated use criteria. The EPA-published 3.2 mg liter -1 instantaneous minimum 
dissolved oxygen criterion still applies to these waters year round (U.S. EPA 2003a). 
The 4 mg liter 1 30-day mean site-specific criterion applies only to the tidal fresh and 
oligohaline segments of the Mattaponi and Pamunkey rivers during the time period 
of June 1 through September 30. Outside of this time period, the EPA-published set 
of open-water designated use dissolved oxygen criteria apply (U.S. EPA 2003a). The 
water column temperatures during the October through May time-frame are such 
that higher levels of oxygen saturation are maintained and the biological processes 
driving the natural tidal wetland oxygen deficits do not have nearly the same level of 
influence on ambient dissolved oxygen concentrations. 
This approach assumes that the nature of the wetland effect on dissolved oxygen is 
relatively constant within season and that there are no other major stresses on 
chapter vi 
Guidance for Deriving Site Specific Dissolved Oxygen Criteria 
