Professionals in the water resource field use many different parameters to characterize water quality. If one's interest centers on protecting fish and other aquatic organisms, however, DO concentration is a key parameter on which to focus. This interest is articulated in section 101 of Title I of the Clean Water Act in the form of a national goal for fishable waters. Fish kills are the most visible symptom of critically low levels of DO. Some species of fish can handle low levels of oxygen better than others. Cold-water fish (salmon, trout) require higher DO concentrations than warm-water fish (bass, catfish). Early life stages usually require higher DO concentrations than adult stages. Table 1-1 presents USEPA's water quality criteria for DO for cold-water and warm-water biota for four temporal categories. The reader should note that a DO concentration of 5 mg/L has been adopted in this study as a general benchmark threshold for defining desirable versus undesirable levels of DO (i.e., the minimum concentration to be achieved at all times for early life stages of warm-water biota).
The concentration of DO in a stream fluctuates according to many natural factors, including water temperature, respiration by algae and other plants, nitrification by au-totrophic nitrifying bacteria, and atmospheric reaeration. By far the biggest factor in determining DO levels in most waterbodies receiving wastewater discharges, however, is the amount of organic matter being decomposed by bacteria and fungi. Twenty-five years after the passage of the CWA, the nation's investment in upgrading POTWs to secondary or greater levels of treatment resulted in significant reductions in BOD loadings. Has the CWA's push to reduce BOD loading resulted in improved water quality in the nation's waterways?
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