decrease (EPA 1983) (Table 9-8). Extended detention facilities dry out between storms and are not effective for removal of dissolved nutrients by biological mechanisms. However, such facilities can sometimes serve multi-purposes as recreational areas when dry. Design guidelines for storage and other passive treatment control devices are provided in references such as Schueler (1987), Camp et al. (1993), Urbonas and Stahre (1993), and American Society of Civil Engineers and Water Environment Federation (1998). Additional information may be found in Novotny and Olem (1994), EPA (1993b), Debo and Reese (1995), and Novotny (1995).

Caution should be exercised when comparing urban BMPs on the basis of removal efficiencies. It is often found that while influent quality varies considerably, effluent quality exhibits a much smaller range (Strecker et al. 1999). Thus, high removal efficiencies may result purely from the fact that influent concentrations are high. Hence, BMPs might be better characterized simply by effluent quality. Alternatives for determining urban BMP performance effectiveness, based on a review of the most complete data set currently available, are provided by Strecker et al. (1999).

Natural wetlands are protected waters by law in the United States and generally cannot be used for waste treatment, except sometimes for a “polishing” purpose. Nonetheless, they provide many functions that enhance water quality (NRC 1991b), including acting as a sink for phosphorus, and facilitating denitrification by converting nitrate to nitrogen gas. Their capacity for nutrient removal can be considerable (Mitsch and Gosselink 1986). Hence, protection of coastal wetlands and tidal exchange is an important water quality consideration for coastal waters. Many factors can act to impair the natural functions of wetlands, including the drainage of wetlands for additional cropland, overgrazing, construction of highways, channelization of an adjoining waterway, deposition of dredged material, and excavation for ports and marinas (EPA 1993a).

TABLE 9-8

Type of Pond

Total Suspended Sediments

Nitrogen

Phosphorus

Extended detention basins

70-80

0 (dissolved)

20-30 (total)

0 (dissolved)

20-50 (total)

Retention ponds

70-80

50-70 (dissolved)

30-40 (total)

50-70 (dissolved)

50-60 (total)

TABLE 9-8 Comparison of nutrient removal percentages from well-designed extended detention basins and retention ponds (EPA 1983).



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