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Dissolved Cbcygsn(mgfL) \

ZZI 0-2.5 !□ 25-5.0 ■ 50-7.5 H Greater than 7.5 I Data not available

Figure 3-19 Locations and worst-case DO concentrations of catalog units potentially eligible for the paired analysis for the 1961-1965 time-block (before CWA). N = 333 catalog units. Source: USEPA STORET.

Figure 3-20 Locations and worst-case DO concentrations of catalog units potentially eligible for the paired analysis for the 1986-1990 time-block (after CWA). N = 905 catalog units. Source: USEPA STORET.

In comparing these results with the historical data from the FWPCA surveillance network (see Figure 3-1 in Section A of this chapter), many of the catalog units characterized by poor DO conditions (DO less than 5 mg/L) in 1961-1965 correspond to the areas represented by many of the stations compiled by Gunnerson (1966) with minimum DO less than 0.5 and minimum DO between 0.5 and 4 mg/L in the 19571965 data set.

In the after-CWA map (Figure 3-20),

• 49 of the 905 catalog units (5 percent) have worst-case DO less than 2.5 mg/L.

• 252 of the catalog units (28 percent) have levels from 2.5 to 5 mg/L.

• 604 of the catalog units (67 percent) are characterized by worst-case DO greater than 5 mg/L.

Undesirable levels of worst-case DO (less than 5 mg/L) are still quite prevalent after the CWA in some midwestern and southeastern watersheds, with a pattern of moderately low worst-case DO (2.5 to 5 mg/L) that appears to be characteristic of the Atlantic coastal plain from Florida to New Jersey. Higher worst-case DO (5 to 7.5 mg/L) characterizes the Piedmont region and the watersheds of the Appalachian Mountains and is likely due to cooler water temperatures. The coastal plain pattern of moderately low worst-case DO most likely reflects natural factors such as warmer summer temperatures, higher decomposition rates, and relatively long residence times within sluggish rivers and tidal waters rather than municipal or industrial point source loading within these watersheds.

Overlaying the 333 eligible catalog units in the before-CWA data set with the 905 eligible units in the after-CWA data set yielded a total of 246 intersecting catalog units that had both before- and after-CWA data. Notably, these 246 evaluated catalog units represent a disproportionately high amount of urban/industrial population centers, with approximately 61.6 million people represented (32.5 percent of the total population served by POTWs in 1996). Figure 3-21 presents maps that display the locations and worst-case DO concentrations of the evaluated catalog units. Figure 3-21a displays the catalog units that had improvement in worst-case DO after the CWA. Figure 3-21 b displays the catalog units that had degradation in worst-case DO after the CWA. Figure 3-22 presents a frequency distribution of the before- and after-CWA DO data.

Key observations from Figures 3 - 21 and 3 - 22 include the following:

• 167 (68 percent) of the 246 evaluated catalog units had increases in worst-case DO after the CWA; 79 (32 percent) of the catalog units had decreases in worst-case DO after the CWA.

• The percentage of evaluated catalog units characterized by "very low" worst-case DO (< 2 mg/L) was reduced from 11 to 2 percent. Before the CWA, 26 catalog units had very low worst-case DO; after the CWA, only 6 catalog units had very low worst-case DO.

• The percentage of evaluated catalog units characterized by undesirable worst-case DO (below the 5 mg/L threshold) was reduced from 47 to 26 percent. Before the CWA, 115 catalog units had undesirable levels of worst-case DO; after the CWA, 65 catalog units had undesirable levels of worst-case DO.

• The percentage of evaluated catalog units characterized by desirable worst-case DO (above the 5 mg/L threshold) increased from 53 to 74 percent. Before the CWA, 131 catalog units had desirable levels of worst-case DO; after the CWA, 181 catalog units had desirable levels of worst-case DO.

Figure 3-23 is a column graph that describes the changes in worst-case DO that occurred after the CWA for the 246 evaluated catalog units in relation to the 5 mg/L threshold. Key observations from this figure include the following:

• 67 percent of the evaluated catalog units (166 out of 246 units) remained either above (47 percent) or below (20 percent) the 5 mg/L worst-case DO threshold.

• Of the 115 catalog units that had worst-case DO concentrations below the threshold of 5 mg/L before the CWA, 57 percent (65 catalog units) increased to above the threshold after the CWA.

• Of the 131 catalog units that had worst-case DO concentrations above the benchmark threshold of 5 mg/L before the CWA, only 11 percent (15 catalog units) fell below the threshold after the CWA.

Of the 246 evaluated catalog units with paired before- and after-CWA data, 167 catalog units (68 percent) had increased worst-case DO and 79 (32 percent) had decreased worst-case DO after the CWA. Figure 3-24a and b display the magnitude of degradation and improvement, respectively. Key observations from Figure 3-24 include the following:

• Approximately 32 percent of the evaluated catalog units that had increases in worst-case DO (53 of the 167 improving catalog units) increased by 2 mg/L or more.

• Approximately 13 percent of the evaluated catalog units that had decreases in worst-case DO (10 of 76 degrading catalog units) decreased by 2 mg/L or more.

• Approximately 44 percent of all evaluated catalog units either stayed the same or improved or degraded by 1 mg/L or less (108 of the 246 catalog units).

Catalog Units with Greatest Improvements Table 3-2 lists the 25 catalog units with the greatest before- and after-CWA improvements in worst-case DO. Appendix D presents a complete listing of all 246 catalog units with before- and after-CWA changes in DO. Figure 3-25 presents a location map of the top 10 of these units along with a stacked column graph that shows their before- and after-CWA worst-case DO concentration. Key observations from Table 3-2 and Figure 3-25 include the following:

• All of the top 25 catalog units with the greatest before- and after-CWA improvements had before-CWA worst-case DO levels below the benchmark threshold of 5 mg/L. Four catalog units had a before-CWA worst-case DO concentration of 0.0 mg/L.

"Befae" &"After Improvement

"Befae" &"After Improvement

Figure 3-21 Locations and change in worst-case DO concentrations of evaluated catalog units where (a) shows improving units (N = 167) and (b) shows degrading units (N= 79) before (1961-1965) versus after (1986-1990) the CWA. Source: USEPA STORET.

Worst-Case DO (mg/L)

Figure 3-22 Frequency distribution comparing worst-case DO concentration of evaluated catalog units before and after the CWA. N = 246 catalog units. Source: USEPA STORET.

Figure 3-22 Frequency distribution comparing worst-case DO concentration of evaluated catalog units before and after the CWA. N = 246 catalog units. Source: USEPA STORET.

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Magnitude of Decrease in I Magnitude of Increase in

Worst-Case DO After the CWA I Worst-Case DO After the CWA

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Magnitude of Decrease in I Magnitude of Increase in

Worst-Case DO After the CWA I Worst-Case DO After the CWA

Figure 3-24 Frequency distribution of change in worst-case DO for evaluated catalog units that (a) decreased in concentration (n = 79) and (b) increased in concentration (n = 167) before and after the CWA. Source: USEPA STORET.

• For 20 of the 25 catalog units, after-CWA worst-case DO improved to levels greater than 5 mg/L.

• The five catalog units that did not break the threshold value of 5 mg/L after the CWA all had concentrations of 0.6 mg/L or less in the before-CWA time-block.

Long-Term Trends of DO and BOD5 at Catalog Unit Scale Time series for dissolved oxygen and BOD5, averaged over 5-year periods from 1961-1995 and filtered for "dry" hydrologic conditions, are presented in Figures 3-26 through 3 -33 for some of the catalog units identified with the greatest improvements in dissolved oxygen concentrations (Table 3-2). Note that time series data for BOD5 were not available for the Lower Spokane River, Washington, and the Salinas River, California. As can be seen for the Lower Fox River, Wisconsin (Figure 3-26), Cattaraugus Creek, New York (Figure 3-27), Lake Dubay (Wisconsin River), Wisconsin (Figure 3-29), Oconto River, Wisconsin (Figure 3-31), Lower Great Miami River, Ohio (Figure 332), and the Lower Big Sioux River, Iowa (Figure 3-33), the decreasing "before and after" trend in mean ninetieth percentile BOD5 levels is consistent with the progressive improvements in dissolved oxygen over the 35-year period. As can be seen in these time series data sets, many of the catalog units are characterized by mean ninetieth percentile BOD5 concentrations during the 1960s ranging from ~ 3 to 15 mg/L. Very high BOD5 concentrations, however, were recorded for the Lower Big Sioux River, Iowa (62 mg/L in 1961-1965) and Cattaraugus Creek, New York (54 mg/L and 108 mg/L in 1961-1965 and 1966-1970). Appendix D presents a complete listing of before- and after-CWA BOD5 data for the 97 catalog units with paired data sets.

TABLE 3-2 Twenty-five Catalog Units with Greatest Improvements in Worst-Case (Mean Tenth Percentile) DO after the CWA
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