New research from the USGS Pacific Coastal and Marine Science Center and the National Park Service examines how the discharge of submarine groundwater mixes with seawater and potentially impacts the health of coral reefs at Moloka‘i, Hawai‘i.
Groundwater is a precious resource on islands, providing potable water to island communities. However, when groundwater becomes contaminated by ground-based pollution—excess nutrients leached from cesspools or agricultural areas, for example—it can threaten another precious resource found near tropical islands: coral reefs. Healthy reefs protect coastlines from storms and flooding; coastal communities depend on reefs for their sustenance and livelihoods.
It is well known that coral reefs around the world face growing threats from stressors such as global warming, ocean acidification, and habitat destruction. One stressor that remains poorly understood is land-based groundwater pollution, which can introduce nutrients and pathogens to reefs from coastal and submarine springs.
The remote Kalaupapa peninsula on Moloka’i features towering coastal cliffs, vibrant coral reefs, and the volcanic Kauhakō Crater lake. The peninsula is also home to Kalaupapa National Historical Park, a National Park Service site that was once a Hansen’s disease (leprosy) quarantine colony, established in 1866. Today, the site is a refuge for the few remaining residents who are cured but were forced to live their lives in isolation.
Kalaupapa National Historical Park is open to visitors via a permitting system. Prior to the Covid-19 pandemic, the site received about 50,000 visitors per year. Wastewater from the site is mostly untreated, gathering in cesspools which are in direct contact to surficial runoff pathways.
In order to better characterize the impact of groundwater discharge to coral reef health at Kalaupapa peninsula, researchers used geochemical and geophysical methods to study the movement of water and nutrients from land to sea, in what is known as a "ridge-to-reef" approach.
The researchers found that a shallow freshwater lens underlies Kalaupapa peninsula, connecting it to Kauhakō Crater lake. The freshwater lens is tidally influenced, with discharge rates falling during high tide and peaking during low tide. This underground connection between Kauhakō Crater lake and the ocean gives credence to conflicting historical accounts of the lake’s salinity: some had reported the lake to be freshwater, while others said it was brackish.
A digital elevation model derived from Lidar data revealed that groundwater flows past multiple cesspools in the Kalaupapa settlement, likely contributing to higher nutrient loading in the coastal zone. This was further supported by observed nutrient concentrations in the nearshore zone that were strongly associated with submarine groundwater discharge (SGD), with the highest nutrient levels coincident with coastal springs. Previous research has shown that locations with higher SGD also exhibited lower coral cover.
“This research highlights the crucial importance of managing anthropogenic nutrients, such as those that enter the shallow freshwater lens through sources like cesspools, that can be discharged onto coral reef habitats via groundwater,” said Ferdinand Oberle, Research Geologist and lead author of the study. “This emphasis on understanding ‘ridge-to-reef' water movement is a step towards a more integrated approach to preserving coral reefs in the Anthropocene, connecting land use and groundwater to the health of coral reef habitat.”