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Figure 3-4 Hydrologie conditions during July-September of (a) 1963 and (b) 1988.

Summer streamflow ratio estimated for each catalog unit as a percentage of long-term mean summer streamflow (July-September, 1951-1980). Hydrologie conditions characterized as dry (0%-75%), normal (75% 150%) and wet (greater than 150%)

Figure 3-4 Hydrologie conditions during July-September of (a) 1963 and (b) 1988.

"Dry" StreanrfioN

"Dry" StreanrfioN

Summer streamflow ratio estimated for each

Figure 3-5 Persistence of dry hydrologie conditions during July-

Summer streamflow ratio estimated for each catalog unit as a percentage of long-term mean summer streamflow (July-September, 1951-1980). Hydrologie conditions characterized as dry (0%-75%), normal (75%-150%), and wet (greater than 150%).

Figure 3-5 Persistence of dry hydrologie conditions during July-

1988 are shown to illustrate the similarity of the spatial extent of drought conditions within the 48 contiguous states during the before- and after-CWA time-blocks. Using this station selection approach based on summer streamflow ratios, trends identified for "before versus after" changes in DO can then be correctly attributed to changes in pollutant loadings (under comparable "dry" streamflow conditions) rather than to differences in hydrologic conditions.

Worst-Case Conditions from a Spatial Perspective In a clean river, upstream of any wastewater inputs, DO levels are typically near saturation. Downstream of an effluent discharge, however, measurements of DO lower than saturation exhibit a characteristic spatial pattern influenced by the loss of oxygen from degradation of organic matter and nitrification and the replenishment of oxygen transferred from the atmosphere into the river (see Thomann and Mueller, 1987; Chapra, 1997). An understanding of the spatial pattern of DO in rivers was critical for the design of the screening methodology used to detect "worst-case" conditions from a spatial perspective. Using river miles from a downstream confluence as a measure of distance along the river, Figure 3-6 illustrates spatial patterns of carbon (CBOD), nitrogen (organic N, NH3- N, and NO2- N + NO3- N), and DO in zones identified as "clean water," "degradation," "active decomposition," and "recovery" that are upstream and downstream of a POTW discharge. The distributions were computed using data adapted from Thomann and Mueller (1987) to describe upstream inputs for a flow of 100 cfs and wastewater loads from a 7.5 mgd primary treatment plant (see Table 2-17) discharging into a river 1 m deep and 30 m wide, with a water temperature of 25 C.

In streams and rivers, DO levels are maintained near saturation by the continuous transfer of atmospheric oxygen into solution in a thin surface layer of the river. The rate of transfer of atmospheric oxygen into the river (i.e., mixing of oxygen as a gas from the air into solution in the water) depends on how fast the river is running, how deep the water is, how "bubbly" the river appears to be, the water temperature, and how much oxygen is already in solution in the river. The less oxygen that is in solution in the river, the faster more oxygen can be transferred from the air into the water. In the "degradation" zone, more oxygen is being consumed by decomposition than can be replenished from the atmospheric supply of oxygen, and DO levels quickly drop. In the "active decomposition" zone, more oxygen is gained by the mixing of oxygen from the air into the water than is lost by the continued decomposition of a diminishing amount of carbon (CBOD) and nitrogen (NBOD), and oxygen gradually increases. In the "recovery" zone, the rate of atmospheric replenishment of oxygen greatly exceeds the oxygen lost due to small levels of CBOD and NBOD remaining in the river, and oxygen returns to the saturation level.

Immediately downstream from the POTW, the carbon concentration (CBOD) jumps from the low upstream level to a much higher flow-weighted CBOD concentration as the effluent load is diluted with the ambient upstream load (Figure 3-6a). Bacterial decomposition of the carbon results in a steady decrease of in-stream CBOD and a steep drop in oxygen in the "degradation" zone, followed by a continued decline of CBOD with a gradual increase in oxygen in the "active decomposition" zone.

As shown in Figure 3-6b for the spatial patterns of nitrogen, organic nitrogen (organic N) and ammonia nitrogen (NH3 - N), both jump from a low upstream level fol-

Wastewater Discharge

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