Hydrogeology and Reef Health

Science Center Objects

We are conducting geophysical and geochemical research to address questions about coastal groundwater-to-reef flow and coral reef health, with the goal of informing management decisions related to planning and implementing activities in priority watershed-coral reef systems.

Cartoon showing the cross-section of a hill near an ocean and the movement of groundwater underneath.

Schematic image showing the main pathways for groundwater discharge directly to the coastal ocean, including fresh groundwater discharge and recirculation of seawater through the coastal aquifer (saline groundwater)
Photo credit: Ferdinand Oberle, USGS

The Problem

Loss and degradation of coral-reef ecosystems are occurring at increasing rates worldwide and submarine groundwater discharge (SGD) likely plays an important role in ways we still do not fully understand. Submarine groundwater discharge appears to be an important factor for determining the chemistry of the coastal ocean. As fresh groundwater flows toward the sea and coral reefs, it rises over salty and denser water. The salty and fresh water mix along the interface, and the resulting fluid often discharges near the shoreline and into coral reef environments. This interface between underground water types has recently been called a “subterranean estuary,” a zone where fresh and salty water mixes - analogous to regions where a creeks or rivers meet the ocean.

A man wearing sun protection gear stands in water near a tropical beach holding onto equipment attached to a flotation device.

USGS physical scientist Cordell Johnson installs a special buoy in the waters of the National Park of American Samoa on Ofu in the Manuʻa Islands Group, American Samoa. The special buoy, developed by USGS scientists, measures radon on the reef. Radon is a marker of submarine groundwater discharge that brings low-salinity waters, and sometimes nutrient- and contaminant-laden waters, to the coral reefs. Photo credit: Curt Storlazzi, USGS

In nearshore locations with high groundwater flows to the ocean, these persistent, cooler plumes can potentially counteract the exposure of corals to elevated and rising sea-surface temperatures that cause coral bleaching. However, coastal groundwater can also alter the nutrient loading of seawater and introduce substances from land, including toxins, pathogens, and other pollutants. The flux of groundwater into coral-reef ecosystems is highly variable, being controlled by geology, climate, land use, and ocean dynamics. Groundwater movement to the ocean also respond to changes in sea level, precipitation, and coastal groundwater withdrawals. In addition, assessing the impact of coastal groundwater flows on coral reefs fundamentally requires an integrated understanding of nearshore currents and wave movements and, often, species-specific information on coral tolerance to groundwater quality and quantity.

Our Approach

Ecosystem functions of coastal groundwater flow to coral-reef ecosystems are not well quantified but can be both advantageous (low temperature buffers thermal stress from rising sea-surface temperatures, dependable supply of new nutrients, and freshened water masses) and harmful (excess nutrient loadings, pollution and pH-driven bioerosion) to coral-reef communities. Although an excess of groundwater borne nutrients has been invoked to explain complete phase shifts from coral- to macroalgae-dominated ecosystems, exact linkages are not fully understood. All components of coastal groundwater flow-affected ecosystem functions will be significantly impacted by future climate change and sea-level rise scenarios, as well as by expected land-use changes. We are addressing critical gaps in our understanding of the role of coastal aquifers in shaping, sustaining, or adversely impacting coral reef ecosystems.

A thermal image of people standing near the water's edge on a beach.

Thermal infrared image of two USGS researchers standing on the coastline and looking out (up on the image) over a coastal groundwater plume that is non-visible to the naked eye but can be observed in this thermal image due to the temperature differences between the cooler (blue) groundwater and warmer (pink) ocean water over the coral reefs.
Photo credit: Ferdinand Oberle, USGS

We are conducting applied geophysical and geochemical research to address scientific questions about coastal groundwater-to-reef flow and coral reef health, with the goal of informing management decisions related to planning and implementing activities in priority coral-reef ecosystems and associated watersheds. For example, our research is identifying pathways and magnitudes of coastal groundwater flow and related coral reef pollution, and estimating load reductions expected from implementation of specific management measures. We are working on constructing predictive coastal groundwater-related coral reef ecosystem models to guide management decisions.

Caption for large image at top of page: Example of one of one of the geophysical methods (Electrical resistivity tomography) used to detect the freshwater-saltwater interface and the location where freshwater flows onto a reef. Superimposed are the results from chemical analysis of groundwater samples that show that salinity measurements correspond with freshwater, mixing an saline zones. Credit: Oberle et al., 2017  

Please also see the associated efforts on the Coral Reef Ecosystem Studies (CREST) Project website: