O‘ahu is in a Geological Sweet Spot for Fresh Water. Here’s Why

We looked into the story of the island’s freshwater, including where it comes from, how good it is and whether we’re taking care of it.
We all know it: Water is life.

With California’s drought in the headlines, climate change looming and some troubling local numbers – base stream flow, rainfall and aquifer levels are all down on Oʻahu – now is a good time to ask the same question of our water that we ask of our farm-to-table food: What’s its story?

Where does our water come from? How good is it and is it better, or worse, than bottled water? Are we taking care of it? Who’s using it?

Most important, will there be enough for the future?

Here is the Hawaii Business guide to the fresh water that we share. Included is information on each island, but each island is different, so we have focused on O’ahu, both the most populous and the “Goldilocks Island.”


376It’s All Connected

Thought ocean was ocean and rock was rock? Think again. UH hydrologist Aly El-Kadi sums it up when he says: “It’s all connected.”

  1. On average, an estimated 1.8 billion gallons of rain per day fall on Oʻahu.
  2. Some rain runs off: historically, about a third.
  3. Historically, about a third of that rain soaks in and recharges the groundwater. It takes between one year and 25 to reach the aquifer.
  4. Some rain is absorbed by plants and then evaporates: about a third.
  5. An aquifer is not a big hole in the island, says Board of Water Supply employee Arthur Aiu, “It’s saturated rock.” Our aquifers consist of porous lava rock that has been slowly infiltrated by rain and sea over millions of years. Depending on how porous it is, lava rock can act like a sponge – or the wall of a container.
  6. Oʻahu is surrounded by a skirt of nonporous caprock on its low-lying plains. It’s like the sides of a pool that keep water in (although water leaks out above and below the caprock line).
  7. Below the fresh groundwater comes a brackish layer, whose thickness varies according to many factors, including tides and how much humans disturb the system.
  8. It seems bizarre, but the island’s groundwater sits atop underground seawater. That’s because freshwater is less dense than saltwater; rain soaks into the island and forms a floating freshwater lens atop bottom layers of ocean water. In proportion, the freshwater lens is something like an iceberg: for every foot of water above sea level, there are 40 feet below it.
  9. In the heart of the mountains, at the source of old volcanic activity, volcanic dikes catch and store water at much higher elevations. The Honolulu Board of Water Supply taps some of this water with horizontal dike tunnels.
  10. The island’s geological water system isn’t a one-way arrangement. Groundwater is always flowing somewhere, and eventually it will all exit, either through human-created means or through the island’s system of freshwater submarine springs.


Water Vocabulary: What Is an Artesian Well?

If you grew up in Hawaiʻi, your mother probably told you that our water comes from artesian wells. Those don’t refer to the vast stores of freshwater beneath the island (those are called aquifers), but to the wells originally used to tap the aquifers.

What makes a well artesian? It’s located at an elevation between the groundwater level and sea level, and, because the water is under pressure, it flows all the time, whether needed or not. That became a problem in the early part of the last century, when many of the island’s artesian wells had to be capped.

It’s also one reason you’ll be hearing less and less about artesian wells as time goes by. The water table is much lower than it was in the late 1800s. And most of our groundwater these days comes from non-artesian sources: tunnels, shafts and deep wells, which need to be pumped.


Oʻahu, the Goldilocks Island

Everybody who works closely with our water system knows it: Oʻahu is in a geological sweet spot for fresh water.

“Talk about a Goldilocks situation, where some islands are too old and some islands are too young,” enthuses Scot Izuka, a Hawaiʻi-based hydrologist with the U.S. Geological Survey.

“Oʻahu has got the geology that makes for big, robust, clean groundwater,” says Lenore Ohye, hydrologic planning program manager for the state water commission: “No other [Hawaiian] island has a huge aquifer like Pearl Harbor’s under there.”

A lot of it has to do with timing, so enjoy these geologic conditions while they last – for the next few hundred thousand years. Here’s why we are in the sweet spot.

  • Hawaiian Islands are shield volcanoes, for the most part built up gradually in thin, evenly distributed layers of lava flow, which allow for lots of water storage in the pukas. And we’re at the right point in our island’s life, too, says Ohye: “Kauaʻi is a really old island, so the rock is really compacted; the little pukas have collapsed. It’s hard to find a really good well on Kauaʻi.”
  • Oʻahu is old enough, though, to have built up a large coastal plain made up of unporous, water-containing caprock, which requires a few thousand millennia of erosion, coral-reef formation and sea-level change, says Izuka. The caprock, he continues, “is fabulous for Oʻahu. It’s like no other island in the chain.” Caprock acts like the sides of a bowl, and a lot of it at higher elevations means that our island can hold in more fresh water. “If you didn’t have the caprock, [the freshwater lens] would only be 20, 40 feet thick,” says Barry Usagawa, program administrator of the Board of Water Supply’s Water Resources Division. “You see it on the Big Island. Their [freshwater lens is] only tens of feet thick, because they don’t have this caprock, so the water is just flowing out.”
  • Our mountains have eroded to just the right height. Any higher, and clouds would get trapped on the Windward side and deny the Leeward side its share of rain. “On the Big Island, where I’m from,” says Usagawa, “the Hilo side is always raining, but the mountain is so high that the water, when it falls as rain, cannot get to the Kona side. Here, it’s just the right height that, when it starts to fall, the trade winds push it over to the Leeward side, and that’s where all the big aquifers are.”But lower mountains than ours don’t attract rain effectively, says Usagawa. “If we didn’t have the mountains, it would be dry, like Kahoolawe.”
  • Our water tastes good in part because it has not been sitting around, in geological terms, for very long; our island is no more than 3 million years old and most of our fresh water is much younger than that. Groundwater in continental formations has often been in contact with rock for millions, or tens of millions, of years. The longer water is in contact with rock, the more minerals it picks up and the stronger the mineral taste will be.


There’s a Hole in the Bucket

Scot Izuka, U.S. Geological Survey hydrologist and project chief for the current Hawaiʻi Volcanic-Rock Aquifer study, likens Oʻahu to “a bucket with holes.” Don’t take it personally – it’s the best way for him to explain that our aquifer water doesn’t just sit there after it soaks in. “Groundwater is dynamic,” says Izuka. “It’s moving. It’s not just a bucket of water, sitting there. It flows through the aquifer and discharges into streams and the ocean.”

“Submarine groundwater discharge” – the huge volume of fresh water that exits the island beneath the ocean’s surface – is one of the hot research topics at UH Manoa’s Department of Geology and Geophysics. Hawaiʻi’s submarine springs flow colder than the surrounding ocean, so scientists can identify the huge freshwater plumes created by these springs with thermal infrared remote images taken from the air.

If you’re swimming near shore and suddenly encounter a very cold flush of water, you may be swimming through a freshwater plume. Pre-contact Hawaiians knew about these submarine springs, sometimes diving deep and collecting fresh water in gourds when other clean fresh water was hard to access.


The Bullet We Dodged

In 1879, James Campbell commissioned Oʻahu’s first well to be drilled at Honouliuli, and he hit what would become liquid gold for the irrigation-greedy sugar-cane industry: a seemingly endless fount of fresh water.

Campbell’s well touched off a “water rush” on Oʻahu that went hand in hand with the boom in plantation agriculture. Soon, there were 400 artesian wells on the island. But some of the wells began “salting up” and, by the 1920s,  Oʻahu’s water crisis was in full swing, with some tunnels running dry and water restricted to certain hours of the day in some neighborhoods.

In 1929, Honolulu’s Board of Water Supply was established, with the goals of metering all water and sealing or repairing all leaking artesian wells. Other regulatory bodies were founded and, though diminished, the island’s water supply began to stabilize. 

The Honolulu Board of Water Supply offers occasional tours of the Halawa Shaft, this inclined “skimming” tunnel, and its working pump room, which first connected the vast Pearl Harbor aquifer to the city of Honolulu when it became operational in 1944. Photo: Courtesy of the Honolulu Board of Water supply

The Honolulu Board of Water Supply offers occasional tours of the Halawa Shaft, this inclined “skimming” tunnel, and its working pump room, which first connected the vast Pearl Harbor aquifer to the city of Honolulu when it became operational in 1944. Photo: Courtesy of the Honolulu Board of Water supply


Living Above the Store

Are we really sleeping above our water supply?

Yes–especially if you live on the south-central Oʻahu plain, above the massive Pearl Harbor aquifer, by far the state’s largest. Pumping about 105 million gallons per day (mgd), the Pearl Harbor aquifer supplies about two-thirds of Oʻahu’s municipal water.

There are dozens of smaller wells across the island and it’s likely that at least some of your water comes from a local well, too. Different aquifers’ waters can blend in the pipes before they reach your tap, although, for the most part, potable water generated on the Windward side stays on the Windward side.

If you live on Oʻahu and want to know exactly where on the island your water comes from, check the annual water-quality report the Board of Water Supply mails to your address. (Find the report for your address using this shortcut: tinyurl.com/Oahuwater.) The wells and pumping stations of origin are printed on it.


The Waterworks: From Well to Cup

Oʻahu’s municipal water delivery, run by the Honolulu Board of Water Supply, works much like a one-way circulatory system. First, enormous pumps move water from wells through underground pipes, some of them 3.5 feet in diameter. Unlike many continental water supplies, Oʻahu’s water all has a local destination, traveling less than 30 miles from its source.

Much of the water goes straight from the aquifer to your tap, but because it’s better for the pumps to operate more or less continuously despite fluctuations in demand, any excess goes to one of 171 enclosed aboveground reservoirs – the green tanks you see near the tops of residential hills and mountains. At peak times – in the morning and evening when many people are showering and making meals – water from the reservoirs supplements the pumps.

The board likes to keep the tanks between 75 and 100 percent full, not only for normal high-demand times, but also for emergencies. Pumps need electricity to function; if, in a worst-case scenario, the electricity went off islandwide, Oʻahu’s water supply would be limited to whatever was stored in the tanks. The board’s Marc Chun says the tanks are designed to hold about 24 hours’ worth of a “max day” of water demand for the population they serve.

All in all, the board maintains 2,100 miles of pipeline (nearly the distance from here to San Francisco) laid “at least four feet down” beneath Honolulu’s roads, says Arthur Aiu. The water pipe system so closely mirrors our road system that a place on the Board of Water Supply’s board of directors is reserved for a member of the city’s Department of Transportation.



It Tastes Good, But Is It Really Pure?

Yes, Hawaiʻi’s water is pure, and there are independent numbers to prove it. Using an algorithm developed in collaboration with the Department of Biostatistics at Harvard University’s School for Public Health, the independent annual Purdex water-quality survey takes state and federal water contaminant data and produces a score from 1 to 1,000 of drinking water’s purity by geographic area and system.

A higher Purdex score is better. Purdex describes any score above 900 as “Exceptional” – among the best anywhere. In 2014, Kaunakakai and Hilo tied for first place in the United States, with scores of 933. Hawaiʻi’s state average for this year: 905.

Here’s how some Oʻahu locations scored for 2015:

  • Kahuku: 936
  • Queens Medical Center: 921
  • Honolulu-Pearl Harbor-Windward: 920
  • Wahiawa: 913

Other Islands:

  • Hilo: 943
  • Lihue-Kapaa: 904
  • Lahaina: 874

For comparison, here are 2015 scores for some mainland municipalities:

  • San Francisco Regional Water System: 844
  • Chicago: 828
  • Seattle Public Utilities: 657
  • Portland Water Bureau: 776
  • North Las Vegas Utilities: 639
  • City of Phoenix: 638
  • Cleveland Public Water System: 626


Should You Buy Bottled Water?

We’ll let these Purdex water quality scores for popular bottled water brands speak for themselves:

  • Aquafina: 786
  • Crystal Geyser: 743
  • Fiji: 733
  • Pure Life: 730
  • Dasani: 721
  • Volvic: 663

No bottled water brand on Purdex’s list topped 800. Only non-mineral, non-sparkling water from spring or municipal sources were used.

Source: www.purdex.com/Bottled_Water.aspx



Tinkering With Our Water

What happens to your drinking water before it hits your glass? That depends on where it came from.

Chlorine: Until about 10 years ago, about 60 percent of the Island’s water did not need to be treated at all; enclosed volcanic rock does a fantastic job of filtering water and then keeping it pure. These days, because of revised EPA regulations, it all gets minimally chlorinated – but much less than in many cities on the North American mainland, which depend on easily contaminated surface water.

Activated Carbon: At the other end of the spectrum, the now-mostly defunct pineapple industry really did a number on some of the Kunia, Wahiawa and Mililani area aquifers. Former generations of pesticides were so effective in part because they lasted for ages, often contaminating the aquifer below. If some of your water comes from Kunia or Mililani, be thankful it’s passed through an activated carbon filtration system first. It’s like a big version of the carbon filters you can get for your faucet or water bottle (or fish tank). No carbon actually ends up in the water – but the pesticides get filtered out.

Fluoride: Not. One additive you won’t find in Oʻahu’s municipal water is fluoride. Did you know that we actually banned the compound in 2004? Most U.S. cities fluoridate, but most Western European countries don’t. Our ban has a cultural component, too, with Honolulu’s ordinance invoking water as the “spiritual fount of all life” and therefore not to be used “as a means for delivery of chemicals for medical or dental purposes when other alternatives are available.” If you want fluoride, you’ll have to join the military, whose independent water sources are fluoridated, or brush with fluoride toothpaste.


Shrunken Aquifer

Here’s bad news and good news. Oʻahu’s aquifer is a fraction of the size it was before the island’s first well was drilled at

Honouliuli, in 1879. But it has grown slightly in the past few years.

Freshwater lens thickness at Pearl Harbor

  • 1878: 1,599 ft (estimate)
  • 1951: 1,025 ft
  • 2003: 615 ft
  • 2015: 677 ft

That the aquifers are lower than before they were tapped is to be expected, says Lenore Ohye, hydrologic planning program manager for the state water commission. “You have to accept that the aquifer will shrink if you allow withdrawals.” The trick, Ohye says, is to find a “sustainable yield:” a withdrawal level where groundwater equilibrium is preserved and what’s going out won’t decrease the aquifer levels further.

Source: Water levels derived from Hawaii Business research based on above-ground freshwater head levels across time.


whosusingitSomething We Get Cheap

Many of Hawaiʻi’s liquids cost the Earth: Our milk and gasoline prices are some of the most expensive in the nation. Good news on water, though: Hawaiʻi gets some of America’s best water for prices that are low compared to many of the municipalities surveyed.

Estimated monthly charge for 10,000 gallons

  • Atlanta: $89.06
  • Baltimore: $85.93
  • San Francisco: $78.02
  • Portland: $77.21
  • Los Angeles: $63.17
  • Kauaʻi County: $59.50
  • Austin: $58.09
  • Maui County: $48.50
  • Boston: $45.41
  • Honolulu County: $44.50
  • Dallas: $34.47
  • Hawaiʻi County: $33.70
  • Denver: $33.38
  • Seattle: $27.95

Source: Honolulu Board of Water Supply, 2014 data.


It Was Too Cheap

Honolulu’s water prices are reasonable, but they were too low before a 70-percent rise in potable water rates began in 2011. Why the increase? The Honolulu Board of Water Supply says that at pre-2011 prices, it was cheaper for customers to let leaks run than to go through the hassle of fixing them – and that was bad for the water supply.


The Big Disrupter: Climate Change

We’ve learned how to limit our water use and regulate our aquifers. If our current conditions stayed constant, we’d have a pretty sweet deal: enough water, effectively forever.

But Hawaiʻi’s population is growing, slowly but surely – and climate change is coming. Weather patterns are already transforming, as we wrote about in our December 2014 issue: For Oʻahu, total rainfall is going down, base stream flow is down and temperatures are up, evapotranspiration is increasing (water that evaporates or is released by plants), and absorption is very likely falling.

For the state water commission, which controls water permitting, “the huge issue is climate change,” says Lenore Ohye. No one knows for sure how much water will be flowing into Oʻahu’s aquifers in the future, she continues, but “it sounds like there’s going to be less.”

Climate change creates a lot to worry about, but not regarding rising sea levels affecting Oʻahu’s water, says Usagawa of the Board of Water Supply. Even if Waikīkī is underwater, the Island’s shield of caprock would likely protect the freshwater lens from inundation.


Other Risks to the Water System: Contamination

Having our freshwater supply stored beneath our feet comes with risks.

A main one is the risk of contamination; even small amounts of toxic chemicals we use every day can make vast stores of water unfit to drink. A single gallon of fuel or oil can contaminate a million gallons of water beyond safe EPA levels. A small amount of many pesticides can similarly poison whole reservoirs: acceptable background levels of pesticide are often measured in parts per billion.

So don’t pour that oil change onto the ground, says Ellen Kitamura, the Board of Water Supply’s deputy manager and chief engineer. You might not see its effects in the next year or two, but “maybe your grandkids end up drinking it.”

Development: Impermeable urban living surfaces like streets, driveways and roofs, along with concretized drainage systems and streambeds, mean that more rainfall is shunted straight to the ocean and never gets a chance to soak into the island and replenish the aquifers. To get a sense of what is lost, consider that an inch of rain on 1,000 square feet of urban surface (about seven parking stalls) is 420 gallons of usable water.

Even unexpected man-made changes can affect the freshwater lens. Aly El-Kadi, a UH hydrology professor and assistant director of the university’s Water Resources Research Center, says the blasting of a boat marina can “break the caprock, which is what stops the fresh water from escaping” – creating an unintended and continuous outlet for fresh water. 

Invasive Species: It’s received wisdom that one raindrop out of every three that falls in Hawaiʻi is destined to be absorbed by plants’ root systems. But is that still true? El-Kadi says invasive plant species can change the equation for the worse, encouraging erosion with shallow root systems or retarding the growth of other plants in a radius around them, which can also lead to soil erosion. Erosion washes away the upper layers of soil, which retain moisture, and leads to increased runoff from the relatively impermeable clays that remain. “Native species are better,” says El-Kadi.


Hooray for Low-Flow Toilets

Did you know: Oʻahu is actually using less water as a municipality than we did a generation ago? For many decades, until 2014, the Board of Water Supply’s official estimate of daily use was 155 to 165 million gallons per day. In 2014, the Board revised that number down to 145-150 mgd, despite a population that has been increasing on average about 1 percent per year.

Yes, low-flow toilets make a difference. Toilets are the single largest in-the-house residential user of water. When low-flow toilets were first required in the early 1990s, says Usagawa, “You could just see the (total water) use going down.”

That’s just one of a number of measures that have reduced municipal water use over the last 25 years.


An Enigma Solved

An island works in mysterious ways.

When I visited Arthur Aiu at the Water Museum in Kalihi, he told me a funny thing when I asked him about stream flow. In addition to confirming that “everything’s lower,” he mentioned that turning on aquifer pumps can affect the flow of streams in the area. That surprised me – and it still surprises him, too.

Around a decade ago, Aiu says, “We did an experiment in Makaha. When we turned on the pumps, the stream was lower. We turned them off again, and the stream came back. We thought we were pumping it from an underground aquifer and it wouldn’t affect the stream – but it did.”

How could pumping water from deep underground affect the flow of an overland stream? We got our answer from Scot Izuka at the USGS. You know all those streams that flow whether or not it’s rained recently? They’re fed by water that’s leaking out of its underground aquifer, either near the coast, from the freshwater lens, or inland, from high-elevation dikes of groundwater deep in the Koolaus. It’s called “base flow,” and that’s what goes down when the pumps go on.


What Are the Reservoirs For?

If we lived in California – or even on Kauaʻi – we might be drinking from the open-air reservoirs. These days, Oʻahu’s open-air reservoirs are strictly for flood control and, occasionally, recreational fishing.

What’s that medieval-looking tower in the middle of the Nuuanu Reservoir? (Shown above.)It’s not for a damsel in distress – it’s part of the flood control system. The reservoirs are kept deliberately low. During a very heavy rain, water flows into and fills up the reservoir. Openings in the tower let the water join Nuuanu Stream in a slower, controlled way, preventing flash floods all the way through downtown Honolulu.

The Honolulu Board of Water Supply maintains 2,100 miles of pipe laid at least four feet down beneath roads, says the board’s Arthur Aiu. Photo: Aaron Yoshino

The Honolulu Board of Water Supply maintains 2,100 miles of pipe laid at least four feet down beneath roads, says the board’s Arthur Aiu. Photo: Aaron Yoshino



The Future of Water

With education and incentives, municipal demand for water is down, but not expected to stay that way forever. To keep up with a growing population and a changing climate, the Board of Water Supply is trying new tools. Here are three processes that may ensure our water supply for the next generation, no matter what the future holds.

Recycling: Oʻahu already recycles about 12 million gallons a day – about 8 percent of daily potable water use – through the Honouliuli Water Recycling Facility, treating wastewater until it meets state Department of Health standards for industry, landscaping, agriculture and golf course irrigation. None of it is used for drinking, and all recycled water is currently used in West Oʻahu, where the separate infrastructure for recycled water exists. It’s not available to residential users. Yet.

7-10k-gallonBy the way, if it’s a water pipe and it’s purple, recycled water runs through it. Pipes are color-coded for instant identification.

Desalination: It sounds like a futuristic option for desert places, but Barry Usagawa of the Board of Water Supply hopes desalination will play a role in Oʻahu’s near future. The U.S. government has made land available for a desalination plant near Ewa, but the offer sunsets in 2020 (already extended from 2003). The desalination process costs more than the current water system, but Usagawa describes it as a financially viable and culturally sensitive alternative to taking more water from the Windward side to fund the ongoing population boom in West Oʻahu.

Desalination is also immune to drought, says Usagawa, who describes the plant as part of an islandwide long-term sustainability plan. “The whole strategy on resource management is to diversify your water supply just like you diversify your investments: so much in stocks, so much in bonds and so much in fixed accounts. Our fixed account is groundwater and our stocks are desal[ination]. Bonds are the recycled water. So you diversify your portfolio, and you get a more adequate and steady supply.”

Water Harvesting: With climate change projected to lead to more intense storms (which, in turn, lead to a higher percentage of rainwater running off, rather than being absorbed), redirecting some of that storm runoff into the aquifer has become a possibility worth researching. Aly El-Kadi, a core researcher with UH’s Water Resources Research Center, is partnering with the Board of Water Supply to explore “water harvesting” at Nuuanu Reservoir No. 4. Surface water will be captured at the reservoir and then injected into the ground. Right now, says El-Kadi, it’s just a feasibility study. But, if it works, it could help offset climate change’s effect on Hawaiʻi’s water supply.

El-Kadi says he got the idea from South Korea, where the country’s monsoon-or-drought weather patterns created a need for water harvesting. 

Disaster Prep

You already know to fill up all your big pots and your bathtub in preparation. But when a natural disaster actually strikes and you lose access to water from the mains, here are further steps to take.

1. Add bleach. Yes, the thing you use to clean toilets. If water might be standing for several days, chlorine should be added, even to normal tapwater. The state Department of Health recommends adding “one to eight drops of new, unscented liquid bleach with a strength of 5 to 6 percent (like Clorox)” for every gallon of standard tapwater. If you suspect it’s contaminated or compromised, keep adding Clorox drops until the water maintains a very slight chlorine odor 30 minutes after mixing.

2. Boil it. Boiling water for one to three minutes is a good alternative to Clorox, says the DOH. Don’t boil it too long – which can concentrate salts and minerals – but spending a little bit of time at 212 degrees F will kill bacteria and make tapwater safe for short-term use.

3. Store drinking water in a covered, clean, food-grade container. The DOH recommends 2-liter plastic drink bottles as good emergency water containers. Who knows, having a bunch on hand might save you from the annual Hurricane Prep Run, and the accompanying dozens of little water servings that will start to taste like plastic within a few months’ time.

4. Don’t drink from the bathtub. On its website, the DOH provides a rather graphic description of the “aerosols from every flush” that uncovered bathtub water will receive from a neighboring toilet. Drink treated water from covered containers where possible, and save bathtub water for other uses.



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