Climate Change, and Response To It, Could Affect Islands’ Water Supplies
For islands, a reliable source of drinking water defines what is habitable and what is not. In Hawai`i, most drinking water sources are underground lenses of buoyant freshwater that float atop the denser sea water.
Keeping withdrawals in balance with recharges to these aquifers is vitally important. Global warming might easily change the equation. If droughts occur with increasing frequency and severity — events that are a part of many scientists’ global warming scenarios — the sustainable yields of Hawai`i’s aquifers would likely be reduced.
Since pumpage figures for O`ahu’s most important water source, the Pearl Harbor aquifer, already approach the present estimate of sustainable yield, drought-induced changes could result in water shortages for O`ahu. A similar situation exists on Maui, where the `Iao aquifer is being pumped at limits close to the sustainable yield.
But reductions in the quantity of island freshwater sources are not the only possible outcome of global warming. In the Climate Change Action Plan published by the Department of Business, Economic Development, and Tourism, the chapter devoted to agricultural activities suggests there may be changes in the quality of freshwater as well.
The plan recommends that improved soil management measures could lead the soil to hold greater quantities of carbon dioxide and nitrogen (another greenhouse gas) than it now does. One of the traditional ways of putting nitrogen into the soil naturally is through the planting of legumes.
But this may not be as straightforward as it seems. “Soybeans and most other legumes are not very acid-tolerant,” the Climate Change Action Plan notes, “and farmers in this area are currently increasing their applications of lime and other alkalai sources to raise the pH of Hawai`i’s naturally acidic soils. Unfortunately, it was recently discovered that a large lens of nitrate currently resides in the upper 35-40 feet below the surface in this area, a lens which would be liberated to enter the island’s main aquifer if the soil’s pH rises above a certain level.”
Nitrate is present in most of Hawai`i’s freshwater sources, but in amounts below the action levels of 10 parts-per-million nitrogen set by the Environmental Protection Agency. When present at high levels in water, nitrates can lead to what’s called the blue-baby syndrome, in which infants suffer from impaired oxygen levels. In extreme cases, death can result.
If the level of nitrate in gruondwater exceeds the 10 ppm concentration of nitrogen allowed by the EPA, the water would have to be treated before it could enter the Board of Water Supply transmission lines. Already, groundwater in Central O`ahu is already displaying concentrations of nitrates more than half the maximum contaminant levels allowed by the EPA. Nor does this reflect the full extent of the problem. According to a knowledgeable source who did not want his name used in this article, the Board of Water Supply pumps take water from the lower part of the aquifer, where the water is relatively uninfluenced by influxes of the agricultural chemicals carried through the soil with the irrigation water that recharges the aquifer. At the higher levels of the aquifer, he said, water is not drawn, since the Board of Water Supply knows at the top of the lens it will find “bad water” heavily influenced by agricultural chemicals.
A Nitrate Surge?
Jonathan Deenik, a graduate student at the University of Hawai`i, has studied the presence of nitrate in Central O`ahu soils. In an interview with Environment Hawai`i, he explained that the ability of the soil to hold nitrate depends on its keeping a positive electrical charge. Most Hawai`i soils have a peculiar property of being able to change their electrical charge. As the acidity of the soils increase, the positive charge increases. As it diminishes — as occurs with liming — the negative charge increases.
Nitrate exists in the soil as a negatively charged ion. As such, it binds with the positively charged minerals in the soil. But change the pH in the soil and the nitrate is freed.
The degree of soil weathering also has an effect on its ability to hold nitrate. The more weathered the soil, the greater its ability to latch onto nitrate. Thus, one would expect — and Deenik found — that the soils closer to the surface would hold more nitrate, as long as the pH remained weakly acidic.
But Deenik also found that unweathered soil at the lowest depths in the core samples also held nitrate. “At the lowest elevation, adsorption decreased, but the soil was still adsorbing nitrate. So the nitrate is being held back by electrochemical process,” he said.
Does the liming pose a problem for groundwater supplies? Deenik acknowledges much work remains to be done. Some studies suggest applications of lime do not change the pH of soil below the plow layer. Yet even without changes in acidity of the subsoil, somehow nitrate is getting through the 300 meters to the aquifer of Central O`ahu.
“My study just begins the story,” Deenik says. “It confirms that these soils have peculiar charge properties and takes it one step further: you can change charge properties, and potentially increase the movement of nitrate through the soils.” But the next chapter is not being written. A proposal that Deenik and others made for further work on the long-term effects of changing the surface layers of soils did not get financial support.
What To Do?
Should the ongoing practice of liming result in a sudden release of nitrates to the aquifer, the BWS would be hard pressed to deal with the consequences, the source close to the BWS told Environment Hawai`i. While many of the wells in Central O`ahu have associated treatment plants that use granular activated carbon for removal of organic chemicals, GAC is not appropriate treatment for removing nitrates. The Pearl Harbor aquifer underlying the Central O`ahu plain is the source of drinking water for most of leeward O`ahu. Should nitrates rise above the maximum level allowed for drinking water supplies, there is no alternative source that could supply the quantity of water needed for such time until the BWS could install the type of treatment — ion-exchange resin — needed to remove the nitrates.
According to a staffer with the BWS, ion-exchange resin treatment is expensive, even when compared to the GAC filters — “and those are pretty expensive already,” he said.
Surface Flows
Streams in Hawai`i are fed by both runoff from land and springs from underground water sources. On all major islands, stream diversions provide an important, cheap source of irrigation water for agriculture. On Maui, stream water makes up a large component of drinking water supplies.
Increased periods of drought would affect streams as well as aquifers. The consequences of dramatically reduced stream flows would reach not only into the commercial agricultural sector and, on Maui, the supplies of municipal water, but would also extend to taro lo`i and native freshwater fish, already at risk because of diversions and introductions of non-native aquarium fish.
— Patricia Tummons
Volume 10, Number 1 July 1999
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