Hot water (aka “thermal pollution”) is regulated as a pollutant because it is a wide-spread and persistent problem that can have disastrous effects on local ecosystems and communities. Section 316 of the Clean Water Act addresses “thermal discharges.”
According to the permit Entergy has to operate Pilgrim’s once-through cooling system, heated waste water is routinely discharged up to 32⁰F hotter than the ambient temperature of Cape Cod Bay. During periodic thermal backwashes (approximately 4x per year; to clean pipes and screens that get clogged with marine life), Pilgrim discharges up to 255 million gallons of water that is 120⁰F hotter than the ambient temperature of Cape Cod Bay.
Baseline studies in the 70’s and 80’s showed that Pilgrim’s thermal plume would reach nearly five miles (3,000 acres; up to 1⁰C Δ) into the Bay, however findings are based on limited field data and current data are non-existent. The size and effects of the thermal plume could be larger or differ from what has been reported by the facility.
Marine ecosystems may be more sensitive to slight changes in temperature than terrestrial based ecosystems. This is because, for all marine species, only an optimal temperature range allows for successful reproduction and growth. Thermal pollution can negatively affect metabolic rates, feeding behavior, reproduction, and distribution of marine organisms. It also encourages parasites and invasive species.