Comparison of Trends in Organic Loadings and Dissolved Oxygen Concentrations in Delaware and Narragansett Bays

摘要

Despite great interest in estuarine protection and large expenditures on wastewater treatment there has been little systematic comparative evaluation of estuarine management decisions and impacts on the conditions of estuaries. In both Delaware and Narragansett Bays sewage treatment plants serve major urban systems at the head of the bays and discharge approximately 75 percent of the total sewage derived organic loading to each Bay. Long term records of BOD loadings from these plants indicate that government decisions to build secondary treatment facilities resulted in significantly decreased BOD loadings to these estuaries. Long term surveys of summer (worst case) dissolved oxygen conditions in the upper estuarine portions of both bays indicate a resulting improvement of dissolved oxygen concentrations. Moreover, they also indicate a major improvement starting several decades earlier, coincidental with the construction of primary sewage treatment facilities in the 1950s. In both bays pollution levels had reduced oxygen concentrations to hypoxic (3mg/l) levels even in surface waters during the summer for several miles along upper Narragansett Bay and several tens of miles along upper Delaware Bay. It is difficult, however, to attribute the marked improvement in dissolved oxygen in recent years to government decisions to expand municipal treatment. Industrial activities, with large volumes of potentially high BOD wastewater, have for a long time discharged to these urbanized upper estuaries. Unlike the municipal treatment plants, there is little quantitative information available on these industrial discharges. Changes in industrial processes or relocation of the plants to other states may have significantly affected water quality.