The potential trophic benefits to lakes and reservoirs of a 1-mg/l and zero-discharge; total phosphorus effluent standard for municipal sewage treatment plants were examined by using two phosphorus mass balance models. The analysis included 255 lakes and reservoirs receiving municipal sewage treatment plant effluents and located in the eastern half of the United States. These water bodies, their significant tributaries, and contributing effluents were sampled during the U.S. Environmental Protection Agency's National Eutrophication Survey. Improvement in trophic condition is defined as a predicted decrease of at least 25% in external phosphorus supply, which would cause the predicted in-lake total phosphorus concentration to (1) decrease from greater to less than 40 μg/1 but remain above 20 μg/1, (2) decrease from above 20 μg/1 to less than 20 μg/1, or (3) decrease from less than 20 μg/1 to a lower concentration. The definition stated above being used, the two models indicate that 18–22% of the water bodies would benefit from a 1-mg/l effluent standard. If the requirement were zero phosphorus, 28% of the water bodies would benefit.
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