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units (see Table 3-2), the strongest "signal," ranked #1 for all 311 paired RF1 reaches nationwide, is the "before and after" improvement from 0.0 to 7.2 mg/L identified for a reach of the Big Sioux River near Sioux Falls, South Dakota (see Table 3-1).

With an "after" improvement of 7.2 mg/L for the RF1 reach that is considerably greater than the catalog unit-based "after" improvement of 5.1 mg/L and less detected for other reaches of the Lower Big Sioux catalog unit, the highest ranked RF1 reach "signal" demonstrates the ability of the "data mining" methodology to accurately detect the influence of upgrades to municipal wastewater discharges from the city of Sioux Falls within the critical oxygen sag location in a reach downstream of the wastewater inputs. Similar comparisons of the strengths of "signals" detected for catalog units and RF1 reach scales of station aggregation have also been evaluated to identify the critical location of the impact of upgrades in wastewater discharges from Indianapolis, Indiana, and Minneapolis-St. Paul, Minnesota, on water quality trends in the Upper White River and the Upper Mississippi River.

Before and After DO at Major River Basin Scale

The stations comprising the 311 reach-aggregated worst-case DO data were pooled by the 18 major river basins of the contiguous United States for statistical analyses of the significance of changes in DO concentration before and after the CWA. These analyses were limited to the 311 evaluated reaches to improve the assurance that the data were collected from the same sample population.

Table 3-3 presents the number of observations, the results of the paired t-test (95 percent confidence level), and the mean of the pooled before and after worst-case DO data. The null hypothesis assumes that there is not a significant difference between the mean concentrations for the before and after periods. The means of the pooled worst-case DO data are presented as column graphs in Figure 3-38.

■1361-1665 (Before CWA) 1986-19S0 (After CWA)

■1361-1665 (Before CWA) 1986-19S0 (After CWA)

Figure 3-38 Before versus after trends in worst-case DO for major river basins: 1961-1965 versus 1986-1990. Source: USEPA STORET.

S 10 11 12 13 14 15 16 17 18 43 States Rivet Basin Number

Figure 3-38 Before versus after trends in worst-case DO for major river basins: 1961-1965 versus 1986-1990. Source: USEPA STORET.

TABLE 3-3 Statistical Significance of Trends in Mean Tenth Percentile (Worst-Case) DO by Major River Basin: Before Versus After CWA (1961-1965 vs. 1986-1990)

TABLE 3-3 Statistical Significance of Trends in Mean Tenth Percentile (Worst-Case) DO by Major River Basin: Before Versus After CWA (1961-1965 vs. 1986-1990)

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