Interbedded basalt from 3 different volcanoes makes a complex, Heterogenous aquifer. Precipitation enters into the young basalt and flows into the sub layers before discharging to the coastal ocean. At the same time tides and waves push seawater into the coastal aquifer creating a complex subterranean estuary. Credit: McKenzie, et al. The figure is designed by SOEST Publication Services.
The discharge of water from the coastal aquifers into the ocean is called a submarine groundwater discharge. This can be freshwater or salty water that intruded into the ocean's edge. The University of Hawai'i at Mnoa has published a new study that shows that sea level plays a role in determining the amount of water that goes into the coastal zone.
Humans and the environment rely on the Groundwater for its vital resources. Adding water to the nearshore supports many traditional Native Hawaiian aquacultural practices, such as loko ia. Climate variations and sea-level rise are affecting the discharge of submarine and coastal water.
It is difficult to understand and predict the volume and timing of submarine discharge due to the complex nature of seasonal seawater intrusion, tidal pumping, and wave set-up occurring on daily time scales. Long-term observations are the only way to understand the process in places like West Hawai'i.
The professor of Earth Sciences in SOEST and his colleagues constructed an autonomously-operated monitor to calculate the magnitude of submarine discharge. The instrument was deployed in Kholo Bay for several years with the help of The Nature Conservancy and was able to collect data at an unprecedented length and resolution.
The instrument captured a revealing set of data, as it was deployed through significant weather events and ocean conditions. Traditional and novel time-series analysis techniques were used by the authors to understand how the patterns of submarine discharge are affected by precipitation, tides, and waves.
As cool, the water in the ocean floats on the surface, providing a source of water for the coastal environment. There is a clear layer of about 1 m underlain by the murky ocean water. Credit: Joseph Kennedy.
Data shows the drivers of discharge.
The researchers found that when the rain is low, the discharge of water and the amount of salt in the water are affected by tides. Lower salinities at the coastline can be caused by a major driver pushing water out of the aquifer when precipitation is above a threshold. This pattern is more pronounced during El Nio periods.
freshwater flowing off the land surface causes coastal salinity to decrease immediately during heavy rains. The team's analysis showed that high precipitation resulted in higher levels of water in the ground and a gradual increase in submarine discharge with a time lag of several weeks.
Supporting sustainable management.
"Understanding patterns like these helps improve the management of the main water resource for the islands, as well as the management of coastal ecosystems and aquaculture," said Dulai. Native Hawaiians were well aware of submarine discharge and were successful in managing coastal resources for centuries, but ongoing changes in precipitation patterns and sea-level rise due to climate change and humans lead to changes in submarine discharge patterns.
The team is going to apply the knowledge gained from the analyses to their management decisions, as the first step in understanding what to expect under normal and extreme climate events.
Trista McKenzie and her team show submarine discharge dynamics under baseline and extreme event conditions. The DOI is 10.1038/s41598-021-01920
Scientific reports are in the journal.
The ocean tides are a key part of the discharge of water into the coastal zone.
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