78 
extensive tidal wetlands were simulated to be 3 mg liter 1 lower than model esti¬ 
mated dissolved oxygen saturated concentrations. The model estimated 3 mg liter" 1 
oxygen deficit is fully consistent with the average dissolved oxygen deficits 
observed in monitoring data collected in these segments (see text below, Tables VI- 
2 and VI-3, Figure VI-1). 
MONITORING-BASED ESTIMATES OF WETLAND-CAUSED 
OXYGEN DEFICITS 
The dissolved oxygen concentration and oxygen saturation levels were calculated 
from the 1985-2002 Chesapeake Bay Water Quality Monitoring Program data 
collected at stations in the Mattaponi and Pamunkey segments. Over the 18-year data 
record, these stations were sampled at least monthly — sometimes twice monthly — 
as part of the long-term water quality monitoring program. The almost two-decade 
data record covers years of varying climatic and hydrologic conditions in the water¬ 
shed. Continuous, high frequency dissolved oxygen concentration data were also 
available for these segments, as described previously, but in most cases the duration 
of the data records is less than one year. Based on findings presented above, 
dissolved oxygen conditions characterized by the data collected at long-term (day¬ 
time) monitoring stations were very similar to those revealed by the continuous 
dissolved oxygen recording devices: short-term temporal and spatial variations in 
dissolved oxygen concentrations were relatively small; and deep nocturnal dips in 
dissolved oxygen concentrations were not observed in these segments. 
For this analysis, the long-term water quality monitoring data were partitioned into 
surface and bottom depths and into ‘cold’ (sampling events when water column 
temperatures were less than or equal to 15° C) and ‘warm’ (greater than 15° C) 
temperature categories. Table VI-4 shows: the calculated mean dissolved oxygen 
saturation concentration over the 18 year data record; the difference between calcu¬ 
lated oxygen saturation and actual observed dissolved oxygen concentrations, i.e., 
the dissolved oxygen deficit; the number and percent of dissolved oxygen measure¬ 
ments below the 5 mg liter 1 30-day mean criterion and below a 4 mg liter" 1 
concentration value; and the average magnitude of those episodic excursions below 
the 5 and 4 mg liter 1 values. Dissolved oxygen concentrations are always well above 
the 5 mg liter 1 30-day mean criterion in the cold months in the Mattaponi and 
Pamunkey river segments, so the cold month statistics are not discussed further. 
As presented earlier and previewed in Table VI-2, the average dissolved oxygen 
deficit in the warm (>15° C) months was 2.6 +/- 0.8 mg liter” 1 (Table VI-4). This 
long-term average monitoring data-based oxygen deficit value overlaps with the 
oxygen deficit of 3 mg liter" 1 estimated through the Bay water quality model simu¬ 
lation of tidal dissolved oxygen concentrations with and without tidal wetlands. 
The calculated dissolved oxygen saturation concentration in the Mattaponi and 
Pamunkey segments in the warm months was 8.5 +/- 0.7 mg liter" 1 . That means that, 
in the absence of any anthropogenic pollutant influences on water quality conditions, 
chapter vi 
Guidance for Deriving Site Specific Dissolved Oxygen Criteria 
