As a regulatory requirement of Entergy’s Clean Water Act permit (NPDES permit), Entergy must publish monthly reports for pollutants discharged into Cape Cod Bay by Pilgrim Nuclear Power Station. Entergy must collect wastewater samples, conduct tests of the samples, and report the results to the U.S. Environmental Protection Agency (EPA). The monthly reports are called Discharge Monitoring Reports (DMRs).

Pilgrim’s August 2015 DMR states that chlorination of Pilgrim’s condenser cooling water occurred almost the entire month of August (27 of 31 days). This chlorinated water is discharged into Cape Cod Bay. Chlorination of cooling water is used as a form of bio-fouling to control bacterial and algal slime that builds up on the internal pipes, especially in the hot summer months.


Pilgrim also performed a thermal backwash in mid-August. Thermal backwashing is done about 4-5 times each year, and is when super-heated water is pushed back through the intake structure as a form of macro-invertebrate control (controlling seaweed and mussels growing on screens and pipes). Pilgrim’s thermal backwash water cannot exceed 120⁰F. As you can see from the table below (from the August 2015 DMR), the maximum water temperature reached 107.1⁰F.

thermal discharge

Pilgrim’s August 2015 DMR also notes two fish impingement events that occurred on 8/22 and 8/29. Entergy is required to report these “minor” impingement events (when rates are higher than “normal”) to U.S. EPA and MassDEP, as well as list them in the DMRs. See below for more details.


As always, Entergy attributes these large impingement events to “natural circumstances,” not to the once-through cooling system that pulled 446.4 million gallons of water from Cape Cod Bay each day it operated in the month of August (circled in red below). Furthermore, there are no studies that have investigated the lasting, long-term impacts of these impingement events on the species or the ecosystem, but yet these large fish kills are quickly labeled as having “no effect.”

Also noted in the table below, Pilgrim’s heated water (aka, thermal pollution) that was discharged into Cape Cod Bay in August was about 101°F (Pilgrim’s permitted limit is 102°F) – making it about 31°F hotter than the ambient temperature of the Bay (Pilgrim’s permitted limit is 32°F).