By Burt Lum
For many of us the coastline is where we connect with the ocean. It is the place where we swim, fish, surf, relax and explore. It is at this interface, where the land meets the ocean, that so much marine life finds its origins and is the source of what draws so many millions of tourists back to the islands. In the 2002 Final Report: Economic valuation of the coral reefs of Hawaii, funded by the National Oceanic and Atmospheric Administration, which considered data on fisheries, recreation, property value, and biodiversity found that the value of Hawaiʻi’s coral reefs were estimated to be $9.7B.
The obvious impacts to the coastline are pollution from land based sources like stormwater runoff, sediment from agricultural lands and sewage spills into the ocean. This is commonly reported in the news cycle as storms stress drainage systems and sewers, resulting in brown water alerts.
A recent study at the University of Hawaii, funded by the Sea Grant College Program helped shed some new light into submarine groundwater discharge (SGD) as another source of land based contaminants into the coastal ecosystem.
This report gave me pause to think for a moment about submarine groundwater discharge in the context of the work we are doing with the ʻIke Wai project. As rainfall in the watershed replenish or recharge the freshwater aquifers, water will naturally find its way out. This could be in the form of springs or pūnāwai where they come up on land or wai puaʻi i ke kai, fresh water springs in the sea.
Anyone who has spent time in the ocean knows of these fresh water ocean springs as the cold water you feel as you swim along the shoreline. They could also appear as water flowing out from the sand forming small streams to the ocean or as a spring of clear water bubbling up from the coral rubble.
Unlike the springs and artesian wells that provide us with our drinking water, these coastal springs are less familiar to us in modern day. In the not too distant past, ancient Hawaiian practitioners used these coastal springs to feed their fishponds creating a brackish mix of ocean and freshwater to raise shrimp and fish. One example that still exists and continues in operation is the Heʻeia fishpond on the windward side of Oʻahu.
The relationship of the SGD to the coastal ecosystem was appreciated by the native practitioners and is now being studied more closely as seen by the Maui groundwater report. I was able to catch up with Henrietta Dulai one of the co-authors of the paper who explained her research on SGD and how it provides new insights not only on the freshwater aquifer but the impact of groundwater contamination and nutrients on the coastal marine ecosystems.
One of the tools to gain a baseline understanding of SGD flux into the ocean is through radon detection. Radon is a colorless, odorless, tasteless gas occurring naturally as a radioactive decay product of radium found in igneous rock. Although radon is a gas, as it decays, it produces other radioactive elements called radon progeny or radon daughters. These decay products can be detected in water.
This natural production of radon signatures in the freshwater starts as the rain water gets absorbed in the watersheds and percolates through the geologic structure of the island aquifers, much of which is volcanic and igneous in composition. Likewise, the oceanʻs water has lower levels of radon since it is not commonly in contact with volcanic rock.
I got a chance to visit Dulaiʻs lab where one of the radon detection sniffer buoys was being retrofitted with a new detector. These buoys are positioned in the ocean to detect levels of radon over periods of months and years. The goal is to measure the amount of radon and salinity which would directly correlate with the amount of SGD entering into the ocean. Once data are collected over extended periods of time, patterns of tidal changes and seasonality can be derived from the data. This helps researchers better understand the freshwater discharge during El Nino and La Nina years, and to also observe the impact of climate change and sea-level rise on aquifer capacity and levels.
From the lab we take a short field trip out to two locations on east Oʻahu. Our first stop is on the shoreline at Maunalua Bay. Here you can see freshwater bubbling up from the sand and forming small streamlets into the ocean. The ocean is murky but there are areas where you can see clear water mixing in with the sea water.
With her water quality sonde and multi-parameter display, Dulai shows me how to gather readings of oxygen levels and temperature. Unlike the radon sniffer buoy, this method is primarily for spot checking for specific sensor readings.
We then head over to the Black Point area along Kahala Ave. Here Dulai tells me that many of the homes have cesspools instead of sewer lines. We can also see where the groundwater is discharging into the ocean. In these areas, besides the increased bacterial count, researchers have detected anthropogenic pollutants like pesticides as well as nitrogen originating from cesspools.
With advances in sensor technology, data collection, and the ability to trace signatures from groundwater that originated on land and enter into the ocean, researchers can see the direct impact to the marine ecology and general health of the coastline. This in turn helps coastal resource managers and lawmakers craft appropriate policies and procedures to protect this valuable natural resource. As shown in the Maui groundwater study, impact of these contaminents do have a direct effect on algae and limu of the coastline. Future work is currently underway to determine the impact on reef ecosystems and marine life.
Dulai said, “This work will provide much needed data on water, nutrient, and pollutant fluxes as well as inform the hydrological models on the amount of freshwater discharge. By no means is this leakage a waste, in fact it is a resource of freshwater and nutrients for all coastal ecosystems. I hope to help the hydrology team determine sustainable water use levels that take into account SGD and the needs of coastal communities.”