By Burt Lum.
To understand groundwater in Hawaiʻi scientists first start with a conceptual model. Then through systematic exploration and testing determine whether that model can sufficiently predict how much water is stored, where it’s stored, how it’s replenished and its long term sustainability. These are critical questions especially in light of demands place on Hawaii’s freshwater resources by agriculture and municipal/domestic uses. At a recent Honolulu Science Cafe talk, I was able to catch up with Don Thomas, Geochemist and Director of Center for the Study of Active Volcanoes at the University of Hawaiʻi. During his talk he shared how these models, developed in the 1930 – 1940ʻs have remained relatively unchanged since then but are now being revisited based on new field research and discovery.
Back in 1935, Harold T. Stearns and Gordon MacDonald, both geologists with the U.S. Geological Survey, published an extensive body of work on the geology and water resources in the Territory of Hawaiʻi. As Thomas explains, “The conceptual model consisted of freshwater saturated rock that forms a freshwater lens near sea level and a transition zone where the freshwater is mixing with saltwater. Below that we have saltwater as deep as we care to go into the islands underlying geology.”
Based on this model, the major storage of groundwater is the basal lens and the systems of dike complexes found within the islands’ volcanoes. These dike complexes occur in the vicinity of the core of the volcano or in features called rift zones. The rift zone is an area of structural weakness in the volcano where underground intrusions of molten magma occur during growth of the volcano. The magma cools in place to form dense impermeable rocks that can compartmentalize the groundwater and hold it in the system of dikes.
For the last 75 years this was the prevailing model for groundwater in mantle plume volcanic islands like Hawai’i. And this was the model that framed the geologic discussions about the underlying structure of what a volcanic island would look like as it grew from the ocean floor to the size of mountain like Maunakea. There were, however, many unknowns about the details of the growth of an ocean island volcano like Maunakea and Thomas recalls, “There were many discussions and lots of arguments over a period of more than 10 years and to our surprise, once an investigation was undertaken of the deep structure of Maunakea, the only thing in common amongst all the theories of what we would find was that we were all in some fashion wrong.”
Thomas admits to backing into the study of groundwater. It in fact started with the Hawaii Scientific Drilling Project (HSDP) which in 1996 was charged (and funded) by the National Science Foundation to recover and characterize a continuous set of lava samples from a single Hawaiian volcano. The spot chosen was near Hilo on the flank of Mauna Loa and Maunakea.
To shed new light on the old model, researchers used the type of drill bit called a coring bit, referred to by Thomas as a cookie cutter, to recover the lava samples to a depth of as much as 4000 meters. Thomas noted in his talk that, in the previous 4500 wells drilled for groundwater, all of them used a rotary drill bit, whose function is to grind through rock and discard it, but the intent of the HSDP was to preserve samples of each lava flow drilled in the form of a solid, continuous core. With the cookie cutter drilling system they could retrieve individual sections of core, bring it to the surface and go back to drilling. The result was the rock samples were representative of a continuous record of the geology as they drilled through the multiple lava flows. But in the process of studying the rock samples, researchers were able to also learn about the type of water encountered underground.
Initial understanding of the groundwater on Hawaiʻi Island was that the young rocks were very permeable. Any rainfall recharge would sink rapidly into the island, would go down to the basal lens and escape out into the ocean. At the HSDP drill site, in the first hundred meters they found the typical freshwater which transitioned to saltwater in the layers of pahoehoe and ʻaʻa flows of Mauna Loa. At about 250 meters, in a geologic transition zone from Mauna Loa to Maunakea, drilling produced multiple soil and ash layers. Then at about 300 meters below sea level the well started to produce freshwater. This aquifer was under substantial pressure and produced water at a rate of 2100 gallons per minute.
As they drilled deeper, to depths of 2000 meters, more freshwater was discovered in areas called pillow lavas or structures formed by the extrusion of the lava under water. These deep freshwater layers would produce water at pressures of 160 pounds per square inch (psi). This raised more questions as to how did these aquifers form so deep underground and so deep below sea level, and led to the Humuʻula Groundwater Research Project. The drilling site in the Pōhakuloa Training Area (PTA) is at an elevation of about 2000 meters above sea level in the saddle region of Hawaiʻi Island. Drilling less than 200 meters down, Thomas and his team found what is called a perched aquifer that extended down to about 350 m depth; although the rocks were dry below the perched aquifer, at 550 meters below ground, and about 1400 meters above sea level, Thomas encountered another saturated zone that extended to the total depth of the hole at 1760 meters. Thomas estimates this regional, dike confined perched aquifer to be about 200 square kilometers and represents a substantial freshwater resource in this area.
Thereʻs good news and bad news associated with these findings. On one hand the good news is that thereʻs a lot more groundwater located in both perched and deep underground aquifers. But studies show the age of the water in these aquifers is anywhere from 2000 years old for the deep underground aquifers and 9600 years for the dike confined perched aquifers. Which brings recharging these aquifers into question.
The bad news is this distribution of water potentially impacts the regulatory framework for freshwater which is based on a sustainable yield. Current groundwater regulations are set by the Commission on Water Resource Management, and establish 40 percent of the total freshwater resource as the target that can be used, saving 60 percent of the recharge in the aquifer. The question then becomes, what is the total freshwater resource and do these perched and deep underground aquifers measure into that total?
Through the ʻIke Wai project some of these questions will get addressed, like better understanding the direction of groundwater flow from these perched aquifers, how these system of dikes might contain the groundwater, what is the storage and reserve of the aquifers?
Thomas said, “The hope is the ʻIke Wai project will help build an interactive database where we will aggregate much of the water chemistry and relevant geologic data into a user friendly application. We could visualize changes in water levels, distribution of chemistry throughout an aquifer and educate the regulators on some of these critical questions on contaminant transport, storage capacity and whether we using the water at a sustainable rate.”