FAQ: Transport of Radioactive Contaminants in and around Cape Cod Bay

The study’s authors, led by WHOI physical oceanographer Irina Rypina, used a NOAA state-of-the-art, high-resolution ocean circulation model to study transport by oceanic currents near Cape Cod Bay. The research focused on the near-surface (<2m) transport of wastewater released from Pilgrim, how it varies seasonally, and the role of wind as the driving force behind the spread of the plume. The model output was validated using observational data from 20 drifters passing through the study area between 2019 and 2021 as part of a NOAA Fisheries study.

This study, alone, is not able to draw conclusions about the health and safety impacts of a release of radioactive wastewater from Pilgrim. The study is a critical first step to evaluating the spread of wastewater releases based on known ocean currents and transport pathways, but these pathways do not tell us the fate of the many different radionuclides that we know are present or their ultimate fate in the marine ecosystem, which depends on their individual chemistries in the ocean.

The model results suggest it is unlikely that the bulk of plume waters will leave the Bay in less than a month. More likely, most of the released material will stay in the Bay for longer than a month, coming close to the coastlines, including coastal areas offshore of Dennis, Wellfleet, and Provincetown. If the release were to happen in the spring and summer, a small portion of a plume might leave the Bay in less than a month’s time, passing north of Provincetown and then flowing southward along the outer Cape.

Maps showing most likely pathways for waters released at PNPS during different seasons. Color shows probability in percent (on logarithmic scale) that a virtual water parcel released off PNPS (red star) will pass through a given location. Yellow indicates prominent spreading pathways. Dark blue denotes areas that are not typically reached by a plume. White denotes areas that are never reached by a plume.

The model cannot tell us how long it will take for water that reached the shoreline to eventually leave the Bay. In the model, the Bay is divided into a grid with 700-meter squares, and every 6 hours a value for ocean currents is estimated for each grid cell. Any variations in the ocean currents that happen closer than 700 m from the coast are not resolved by the model. As a result, when the model tracks virtual water parcels across the Bay, parcels can sometimes enter grid cells that lie entirely on land. When this happens, the model will stop tracking the parcels. A different model would be needed to answer questions about when the plume would eventually leave the Bay.

Maps showing the average time it takes in days for a virtual water parcel released off PNPS to reach a given location.

These seasonal changes can be linked to different wind conditions that affect the oceanic circulation in and around the Bay.

This project focused on near-surface transport only; questions related to 3-D circulation and transport will be addressed in future research.