Rising sea levels are driving wastewater into the ocean

A new study by Earth scientists at UH Mānoa has revealed that sea level rise is driving wastewater into the ocean. The experts report that sewer lines and other coastal wastewater infrastructure are likely to flood with groundwater as the sea level swells.

The research is the first of its kind to provide direct evidence that wastewater infrastructure is being inundated by tidally-driven groundwater in urban Honolulu.

The study has found that higher sea levels are pushing wastewater into storm drains and into coastal waters, with negative impacts to water quality and ecological health.

To investigate the impacts of future sea levels,  the team analyzed coastal ocean water and storm drain water in low-lying areas during spring tides. The researchers used chemical tracers to detect groundwater discharge and wastewater present at each site.

“Our results confirm that indeed, both groundwater inundation and wastewater discharge to the coast and storm drains are occurring today and that it is tidally-influenced,” said study lead author Trista McKenzie. “While the results were predicted, I was surprised how prevalent the evidence for these processes and the scale of it.”

During spring tides, storm drains can overflow. At the same time, wastewater from compromised infrastructure also discharges into storm drains in low-lying areas, according to the study. This contaminated water poses a risk to human health.

In addition, many of the human-derived contaminants were found at levels that pose a high risk to marine organisms. 

“Many people may think of sea-level rise as a future problem, but in fact, we are already seeing the effects today,” said McKenzie. “Further, these threats to human health, ocean ecosystems and the wastewater infrastructure are expected to occur with even greater frequency and magnitude in the future.”

“Coastal municipalities should pursue mitigation strategies that account for increased connectivity between wastewater infrastructure and recreational and drinking water resources.”

“We need to consider infrastructure that minimizes flooding opportunities and contact with contaminated water; and decreases the number of contaminant sources, such as installation of one-way valves for storm drains, decommissioning cesspools, monitoring defective sewer lines, and construction of raised walkways and streets.”

The study is published in the journal Limnology and Oceanography Letters.

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By Chrissy Sexton, Earth.com Staff Writer