Figure 23.5. Dissolved oxygen trends in the Hudson River off of 42nd St., Manhattan, NY, ca. 19102002. Data represent surface and bottom summer average concentrations of 8-14 samples per summer.

rate and river flow (Clark et al., 1995). The bacterial decomposition of high organic carbon loads from untreated sewage can deplete DO, especially in the warm summer months, rendering the water unfit for most aquatic life. DO is therefore used as one of the most universal indicators of overall water quality and a means of determining sewage impacts on habitat and ecosystem conditions.

DO has been depressed in the Hudson River for most of the twentieth century. For example, summer average DO was typically between 2 and 4 mg l-1 in surface and bottom waters off of Manhattan from circa 1910-70 (Fig. 23.5). Minimum values were often less than 1 mg l-1 and average summer percent saturation varied between 25 and 50 percent (Brosnan andO'Shea, 1996b). Despite the construction of several WTPs throughout the estuary since the 1930s, no clear impact on DO in the lower Hudson was observed during this period. This is in contrast to other regional waterways (for example, the East River and the Arthur Kill) where DO increases have been observed since the 1940s (O'Shea and Brosnan, 2000). The most significant abatement of sewage loadings into the lower Hudson River did not occur until after the late 1970s when most of the existing WTPs were upgraded to secondary treatment and additional plants were constructed (Figs. 23.2 and 23.3). Up until the mid-1980s, over 5 m3 s-1 of raw sewage was still being discharged into the lower Hudson from the western shore of Manhattan and the northwestern shore of Brooklyn. Completion of the 3.6 m3 s-1 North River WTP at 125th Street in Manhattan in 1986 and the 1.7 m3 s-1 Red Hook plant in the lower East River in 1987, coupled with upgrades of the Yonkers, New York and Passaic Valley, New Jersey and other regional WTPs, resulted in significant water quality improvements (Brosnan and O'Shea, 1996b). By the late 1990s, summer average DO off of Manhattan was typically between 5 and 7 mg l-1 (Fig. 23.5) and bottom minima typically exceeded 3.5 mg l-1. Average percent saturation values in the late 1990s approached 70-90 percent. Surveys conducted by Clark et al. (1995) from Haverstraw Bay to New York Harbor also document DO improvements from 1978-93.

Limited data from the Albany Poolfrom the 1940s through the mid-1980s indicate average summer concentrations of less than 1 mg l-1 to less than 5 mg l-1 were common prior to 1970 and the minimum recorded value often approached 1 mg l-1 or less (Fig. 23.6). Average saturation values ranged from 10-40 percent. However, with the additional abatement of sewage loadings that accompanied the passage of the Clean Water Act in 1972, DO improved significantly with summer average concentrations typically between 6 and 8 mg l-1 in the 1980s (Fig. 23.6). Summer minima during the 1980s typically exceeded 6-7 mg l-1 and average percent saturation was typically 70-85 percent.

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