Wastewater discharge harms reefs, study finds

A team of University of Hawaii scientists has completed an intensive study of nearshore ecosystems around Maui that paints a clear picture of the impact of discharged treated wastewater.

And the picture isn’t pretty.

Results from the study were published in the Nov. 3 edition of PLOS ONE, a scientific journal.

Led by botanist Daniel Amato, scientists representing the UH Botany Department and the Department of Geology and Geophysics based their investigations on field experiments and chemical analysis of water and algae at six locations around Maui, including Kahului Bay, which is adjacent to Maui’s highest-volume sewage treatment plant.

The facility has used wastewater injection wells to dispose of treated wastewater, releasing the discharge deep into the ground.

A study of nearshore areas in nearby Kahului Bay found high nutrient levels in marine surface waters and a “thick, fleshy mat” of colonial zoanthids, a coral-like organism, according to a UH news release.

In the published study, the authors noted that the presence of such concentrations of zoanthids is associated with areas close to wastewater injection wells; such concentrations were not found anywhere else during the team’s study.

The authors said the study supports the rarely investigated hypothesis that submarine groundwater discharge is a significant source of harmful nutrients in coastal ecosystems.

The study also found that agriculture has a significant impact on coastal ecosystems, particularly with regard to nitrogen levels.

“By comparing the spatial distribution of (nitrogen) in both water and algal samples among locations with various potential nitrogen sources, the role agriculture and wastewater have in coastal regions has become more clear,” the study reported.

“These land use practices are the most likely sources of excess nitrogen in Maui’s coastal waters.”

Overall, reefs near plantations and wastewater injection wells had the most seaweed, the lowest diversity and the highest nitrogen concentrations in algal tissues, coastal groundwater and marine surface waters, according to the study.