A new study has found that “sunny day flooding,” occurring during high tide, can increase the levels of fecal bacteria in coastal waters. The study, led by researchers at North Carolina State University, aimed to determine whether tidal flooding without rainfall could lead to higher levels of fecal contamination. Historically, the highest levels of fecal bacteria in coastal waters were seen after rainfall, as contaminants were washed into waterways. However, due to sea level rise, there has been an increase in flooding in coastal areas during high tide, even without rainfall.
For the study, researchers collected water samples daily for two summer months at three sites along a waterway in coastal North Carolina. They specifically focused on two perigean spring tides, known for their pronounced high and low tides due to the moon’s gravitational pull. The researchers increased their sampling at each site during the perigean spring tides to capture changes in water quality during the tidal cycle. Floodwaters during the high-water levels of the tides were also sampled, as they pushed up through local storm drains and caused minor flooding. These floodwaters were found to have relatively high levels of fecal bacteria.
The researchers observed that the floodwaters had elevated levels of fecal bacteria above the safe levels recommended for recreational waters. However, the findings regarding coastal waters were more nuanced. While higher concentrations of fecal bacteria were detected as floodwaters and perigean spring tides receded, these levels were not consistent and usually lasted for only a few hours. Overall, the study indicates that perigean spring tides can raise concerns about fecal bacteria and water quality in coastal waters, but the concentrations are not as high as those caused by stormwater runoff from rainfall.
The researchers emphasize that the results are specific to the samples taken in one area along a large waterway and may vary depending on the size of the waterway. They suggest that contamination after flooding may last longer in smaller waterways and call for additional research on this topic. With sea levels expected to continue rising, leading to more sunny day flooding, understanding the impact of tidal floods on water quality is crucial for making informed decisions about public health and safety. Further research is needed to gain a deeper understanding of the implications of tidal flooding on coastal water contamination.
The study, titled “Fecal Bacteria Contamination of Floodwaters and a Coastal Waterway from Tidally-Driven Stormwater Network Inundation,” was published in the open-access journal GeoHealth. The research team, led by Megan Carr, a Ph.D. student at NC State, included other researchers from NC State, the Environmental Defense Fund, and the University of North Carolina at Chapel Hill. The study received support from the National Science Foundation and the U.S. Geological Survey Southeast Climate Adaptation Science Center. As policymakers and public health officials navigate the challenges posed by increasing sunny day flooding, continued research in this area will be essential in protecting coastal water quality and public health.
