Fall Creek IN (05120201006)

Upper White Basin IN (05120201)

Eagle Creek IN (05120201032)

White River IN (05120201013)

White River IN (05120201007)

White River IN (05120201009)

White Rhrar IN (05120201011)

White River IN (05120202031)

Figure 3-34 Before and after changes in worst-case DO (mg/L) for RF1 reaches of the Upper White River Basin (05120201) in Indiana.

Figure 3-35 Before and after changes in ninetieth percentile BOD5 (mg/L) for RF1 reaches of the Upper White River Basin (05120201) in Indiana.

The aggregation of worst-case before- and after-CWA station records at the reach scale produced a variety of signals. As expected, the signal linking point source discharges with downstream water quality is most pronounced in reaches located immediately below point source discharges (in the critical portion of the oxygen sag zone). The signal became weaker farther downstream; however, in most reaches it was detectable, especially in the recovery zone of the oxygen sag curve associated with the Indianapolis discharges.

Comparisons of the catalog unit and reach-scale results for both the Upper Mississippi and Upper White River Basins tend to support the conclusions reached by Smith et al. (1987a, 1987b) and Knopman and Smith (1993), where they suggest that any improvements in dissolved oxygen conditions related to upgrading wastewater treatment facilities are detectable only within relatively local spatial scales downstream of wastewater discharge locations.

The weak association between observed trends in dissolved oxygen deficit and recorded changes in municipal biochemical oxygen demand loads suggests that detectable improvements in dissolved oxygen conditions do not generally extend to the locations of the [USGS] stations studied here. Given the bias toward high biochemical oxygen demand loads at these stations in comparison to river reaches in general, one is forced to conclude that detectable effects of improved municipal treatment on dissolved oxygen are limited to a small fraction of total river miles.

The "signal" of dramatic improvements in dissolved oxygen has been clearly detected at both the catalog unit and the smaller RF1 reach scales. In contrast to the results obtained by aggregation of station records at the catalog unit scale, the "before and after" "signal" detected at the smaller RF1 reach scale appears to provide a better indicator of the long-term trends in improvements of "worst-case" dissolved oxygen. In addition to the DO and BOD5 data sets presented for the Upper White River, time series data sets of DO from the Lower Big Sioux River from Sioux Falls, South Dakota, to Sioux City, Iowa, provide another example of the difference in "signal" strength obtained at the catalog unit and RF1 reach scales.

Comparison of the time series of dissolved oxygen compiled for the Lower Big Sioux catalog unit (10170203) and a single reach in the Big Sioux River (10170203037) (Figure 3-36) shows that the trend at the RF1 reach scale is consistently increasing from 1961-1965 through 1981-1985, whereas the trend detected for the larger catalog unit scale indicates an apparent erratic "signal" of improvements from 19611965 through 1991-1995 that is then marked by declines in DO during 1976-1980 and 1986-1990.

Evaluation of time series DO data sets (Figure 3-37) for two other reaches (10170203039 and 10170203042) in the Lower Big Sioux catalog unit shows that the apparently inconsistent catalog unit trend in Figure 3-36 resulted from averaging the station tenth percentile values over each 5-year time period for stations located in reaches that were characterized by both continuously increasing (10170203037 and 10170203042) and increasing followed by decreasing (10170203039) trends. In contrast to a "before and after" improvement from 0.0 to 5.1 mg/L identified for dissolved oxygen of the Lower Big Sioux catalog unit, ranked #11 of the 246 paired catalog

0 0

Post a comment