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Note: Units are kg/hectare-year.

a Export coefficients for BOD5 and TSS for agriculture and forest categories from Thomann and Mueller (1987).

b Range of export coefficients for urban land use categories I, II, and III from PLUARG studies (Marsalek, 1978), presented by Novotny and Olem (1994, Table 8.2, p. 449).

c Mean export coefficients for total N and total P for mixed agricultural and forest land uses from Reck-how et al. (1980).

Note: Units are kg/hectare-year.

a Export coefficients for BOD5 and TSS for agriculture and forest categories from Thomann and Mueller (1987).

b Range of export coefficients for urban land use categories I, II, and III from PLUARG studies (Marsalek, 1978), presented by Novotny and Olem (1994, Table 8.2, p. 449).

c Mean export coefficients for total N and total P for mixed agricultural and forest land uses from Reck-how et al. (1980).

and woodland), the transformation of a watershed's land uses progresses over many years through several intermediate stages of development to a fully developed urban-industrial watershed (Novotny and Olem, 1994). With the irreversible transformation to the endpoint of urban-industrial land uses of a watershed, the emphasis in water quality management needs to incorporate strategies for control of both nonpoint sources of runoff and the point source discharges within the "urban-industrial" water cycle. In contrast to the control strategy for point sources (build a wastewater treatment facility) as the most effective technology for removal of pollutants from a point source waste discharge, the reduction of nonpoint source loading of pollutants is focused on the design and implementation of "best management practices" to control, and manage, land use activities and surface runoff. As with urban runoff control measures, the technical aspects of the numerous practices available for controlling nonpoint source runoff from forest, agricultural, and other rural land uses are presented in detail by Novotny and Olem (1994).

As part of its public works infrastructure, practically every town and city in the nation has an urban stormwater sewer system designed to collect and convey water runoff from rainstorms and snowmelt. Depending on the development characteristics of an urban area, stormwater runoff can result in significant intermittent loading of pollutants to surface waterbodies. Based on findings from the National Urban Runoff Project (NURP), conducted by USEPA from 1978 to 1983, USEPA (1983) concluded that urban runoff accounted for significant wet weather loading to the nation's surface waters of pathogens, heavy metals, toxic chemicals, and sediments. The origins of the diffuse discharges of these pollutants include contaminants contained in wet and dry atmospheric deposition, erosion of pervious lands, accumulation of debris on streets, traffic emissions, and washoff of contaminants from impervious land surfaces. Table 2-15 presents typical discharges of conventional and nonconventional pollutants in urban runoff.

BOD5 Loads from the National Water Pollution Control Assessment Model (NWPCAM)

The National Water Pollution Control Assessment Model (NWPCAM) is a national-scale water quality model designed to link point and nonpoint source loadings and resultant calculated in-stream concentrations of CBOD5 , CBODu, DO, TKN, total suspended solids, and fecal coliform bacteria with a "water quality ladder" of beneficial uses (Carson and Mitchell, 1983). The framework for the model is USEPA's Reach File Version 1 (RF1) and Version 3 (RF3) national databases of streams, rivers, lakes, and estuaries. The national model uses mean summer streamflow data to characterize steady-state loads, transport, and fate of water quality constituents. Presented for comparison purposes is current (ca. 1995) BOD5 loading information derived using available NWPCAM national data for municipal and industrial discharges, CSOs, and urban and rural nonpoint sources (Bondelid et al., 2000).

BOD5 Loading from Municipal and Industrial Sources The input data used to estimate municipal and industrial effluent loading of BOD5 within the NWPCAM come from USEPA's Permit Compliance System (PCS), the Clean Water Needs Survey (CWNS) databases, and default assumptions derived from the literature. The PCS database contains discharge monitoring data for major POTWs and industrial dischargers (facilities with a discharge greater than 1 mgd). The CWNS database provides a more comprehensive database of all POTWs and generally reliable population, flow, and treatment level information. Less confidence is placed on the effluent concentration data reported in the CWNS database. Therefore, when actual discharge data were available from PCS, those data were used. PCS data were also used to develop default effluent concentrations to apply when a facility's actual concentration was not available or was outside normal ranges expected for a given level of treatment.

Municipal Table 2-17 presents a compilation of characteristic effluent concentrations of conventional and nonconventional pollutants used in NWPCAM for different types of municipal POTWs. The data sets extracted from USEPA's PCS and CWNS databases are supplemented by influent and effluent data taken from the literature (e.g., AMSA, 1997; Metcalf and Eddy, 1991; Clark et al., 1977; Leo et al.; 1984; Thomann and Mueller, 1987).

A total of 1,632 of the 2,111 hydrologic catalog units in the contiguous United States are subject to municipal effluent loading. Figure 2-18 presents distributions of municipal BOD5 loading by percentile of catalog units with nonzero municipal loads according to (1) loading rate and (2) fraction of total municipal loading. Figure 2-19 presents a map showing the magnitude of municipal effluent loading of BOD5 aggregated for the 1,632 catalog units with nonzero municipal loads. Figure 2-20 displays the proportion of the total nonpoint and point sources load contributed by municipal waste loads.

Key observations from Figures 2-18 through 2-20 include the following:

• Less than 1 percent of the 1,632 catalog units subject to municipal loading receive effluent BOD5 loading at a rate greater than 25 mt/day (Figure 2-18a). About 20 percent of the catalog units account for about 90 percent of the total municipal BOD5 loading to the nation's waterways (Figure 2-18b).

• Relatively low municipal BOD5 loading rates (less than 0.5 mt/day) characterize many of the catalog units within the western and central portions of the contiguous 48 states.

• Higher rates of municipal loading (0.5 to 5 mt/day) are characteristic of the Mississippi River valley and the Northeast, Midwest, and Southeast. The highest loading rates (> 25 mt/day) are for major urban centers, including New York, Boston, Los Angeles, San Diego, Dallas-Ft. Worth, Detroit, and San Francisco.

• The municipal wastewater component of total point and nonpoint source load of BOD5 tracks closely with the results of the loading magnitude calculation. The municipal wastewater component is highest around major urban centers and lowest in the western and central portions of the contiguous 48 states.

TABLE 2-17 Effluent Characteristics for POTWs

Advanced

Advanced Advanced Wastewater

Parameter (mg/L) Raw Primary Primary Secondary Secondary Treatment bod5

Advanced

Advanced Advanced Wastewater

Parameter (mg/L) Raw Primary Primary Secondary Secondary Treatment bod5

Mean

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