Can You Drink Tap Water in Hawaii?

Many health officials advise not to drink tap water in different countries while traveling abroad. This is because in many underdeveloped countries, the water is not filtered as well or treated, and as a result, could potentially contain hazardous bacteria that could harm your health. But, what about other areas that are found across the ocean, like Hawaii?

Can you drink tap water in Hawaii? Although there are a few remote areas of Hawaii in which the water is not safe to drink, most tap water in Hawaii is safe to drink due to its natural origins and filtration process.

Many people take advantage of the fact that we have access to safe, clean water every day. But, when it comes to places we may not be as familiar with, like Hawaii, we must ask ourselves these questions: where does the water come from? What treatment process does it have to go through in order for it to be safe for us to drink?

Understanding Tap Water & Its Origins

Tap water in the United States usually comes from two sources: surface water and groundwater. Surface water consists of natural bodies of water, such as lakes, streams, rivers, or oceans, while groundwater is made up of water found beneath the surface (more often seen with well systems).

However, even though both forms of water come from natural sources, it does not mean that they are immune to contamination. For that reason, public drinking water systems exist to remove water-borne germs and other pathogens from the water before it enters homes.[1]

Water Regulation & Safety Standards

The Safe Drinking Water Act (SDWA) was originally passed by Congress  in 1974 to “ensure and protect the quality of Americans’ drinking water.”[2] This act–and its later amendments in 1986 and 1996–gives the United States Environmental Protection Agency (EPA) authority to oversee the regulation of community water systems.

The EPA has set a large number of standards for all public water systems in the United States to ensure we have access to safe drinking water; they have “set maximum contaminant levels and/or treatment technique requirements for over 90 different contaminants in public drinking water.”[3]

Water Treatment Process

The water treatment process can be summarized in four main steps: Coagulation and flocculation, sedimentation, filtration, and disinfection.

Coagulation & Flocculation

First, from the source, water is taken through an intake point to a treatment plant. In the plant, positively-charged chemicals are added to the water. The chemicals’ positive charge allows them to bind to the dirt and dissolved particles in the water, which have a negative charge. This results in the formation of larger particles in the water called floc.


The floc is heavier than its surroundings, so it ends up settling at the bottom of the water supply. This process is known as sedimentation.


The clear water above the floc will pass through various natural filters with different compositions and pore sizes–including sand, gravel, and charcoal–in order to remove any dissolved particles such as dust, bacteria, viruses, parasites, and chemicals.


Once the water has been filtered, a type of disinfectant (usually chlorine or chloramine) may be added to in order to eliminate lingering bacteria, viruses, and parasites that were not as easily removed during the filtration process. The disinfectant may also be dispensed through the water to protect it from new germs as it is directed to homes and businesses through the piping systems.

Differences in Water Treatment

Water can be treated differently in different areas depending on the source and quality of the water that is brought into the treatment plant. For example, surface water usually requires more treatment and filtration than groundwater because it contains more sediment and is more likely to be polluted or contaminated compared to groundwater.

A variety of other chemicals may also be added to the water in order to adjust for hardness or pH levels or even prevent corrosion, depending on the water source. Many public drinking water systems also add fluoride to the water in order to aid in tooth decay prevention.

After treatment, the water is stored in a storage tank and then brought to thousands to millions of homes through a series of pipe systems.[4]

Why Question Hawaii’s Tap Water?

If Hawaii’s tap water is considered safe to drink because of federal regulations, why are there often questions related to just how safe it is? There are a few reasons why first-time visitors to the faraway state may be skeptical about the tap water.


Many officials warn travelers not to drink the tap water when traveling abroad because their water treatment standards may differ from our own. In addition, water treatment may not be available in all regions of the country you may be visiting.

Because Hawaii is technically across the pond (albeit a short skip across in comparison to visiting Europe or Asia), many people automatically apply the same logic. It can be easy for some to forget that Hawaii is a part of the United States and therefore follows the same strict water treatment regulations as any other state.


Another reason for people to believe that the tap water in Hawaii is not safe to drink: Cesspools. A cesspool is a hole in the ground in which untreated human waste is deposited.

Hawaii has had a history of cesspool problems within the past few years. In fact, they have had a record number of 88,000 cesspools in existence across its major islands, which is higher than any other state in America. Over 53 million gallons of raw sewage were deposited into the ground each day in previous years. Because over 90% of Hawaii’s drinking water comes from groundwater wells, the cesspool issue could lead to serious consequences for residents.

It is for this reason that state lawmakers outlawed the creation of new cesspools back in 2016, and have been working since then to replace the state’s cesspools with alternative forms of sewage systems.[5]

Where Does the Drinking Water in Hawaii Come From?

Although Hawaii has its own share of water treatment facilities to follow United States EPA regulations, some of the filtration processes may differ, depending on what region you are in.

Each of Hawaii’s six (of eight) inhabited islands (Hawaii, Maui, Oahu, Kauai, Molokai, and Lanai) have access to multiple natural sources of water, including surface water (rainfall and watersheds) and groundwater (aquifers and springs). Therefore, the water treatment process for each island depends on its main source of water.[6]

Surface Water

Hawaii’s freshwater supply simply would not exist without the water cycle continuously circling water to and from the ocean.

Rainfall Catchment

First, Northeast Trade Winds travel across the ocean, picking up the moisture that has evaporated from the nearby ocean water. On islands like Oahu, the moisture-filled winds arrive and are deflected upward once they hit the steep cliffs of the mountainside.

As the rising moisture rises closer to the mountain tops, it begins to cool and form clouds. Over time, these clouds condense and eventually produces rain that falls back down to the ground. In Oahu, a third of the two billion gallons of rain the island receives in one day replenishes aquifers, the second third helps nourish vegetation before evaporating, and the final third is runoff that flows back into the ocean.


A watershed is a large area of land (usually encompassing a mountain or valley) that catches, collects, and stores rainwater as it falls. The geologic nature of the watershed can either direct water as surface water to nearby rivers and streams or underground.

Areas of Hawaii that use surface water as their main source of tap water run it through their various treatment facilities. This is more common in major cities, such as Honolulu.


Nearby rainforest plants and trees can also catch rain as it falls. Because their roots help stabilize the top few layers of soil in the ground, the rainwater is able to filter through to the deeper layers of earth gradually.

The water seeps into a variety of geological structures, including cracks, crevices, and various-sized pockets that help channel, store, and purify water before it is fed back through the soil during the evaporation phase of the water cycle.


In a process called percolation, the water is also filtered through porous volcanic rock within the earth to the water table within the lava. From there, the water ends up in aquifers, which are “deep reservoirs within porous rock.”[7]


In some instances, percolating water will become trapped between layers of volcanic ash or clay-like soil. As a result, rather than continue moving downward, the perched water collects and begins flowing sideways, sometimes appearing as a natural spring.

Dike-Impounded Water

Some groundwater can become trapped in large vertical structures formed by volcanic dikes. Dikes are the result of volcanic magma no longer flowing to the surface; the magma cools over time, producing vertical sheets of nonporous rock. With enough collected water and internal pressure, dikes can create a freshwater spring on their own.

Areas of Hawaii that rely mainly on groundwater as their source of water may sometimes run it through treatment facilities, although it is more common to send water to homes and businesses directly from wells.

Water Systems in Hawaii

Because Hawaii is also a part of the United States, its water treatment and disinfecting standards follow the same water regulations set by the Environmental Protection Agency and state-wide policies.

Hawaii’s drinking water systems “are comprised of groundwater, surface water, water catchment, wastewater injection, and desalination.” [8]

Water Sources in Hawaii

As mentioned previously, different areas of Hawaii use multiple main sources of water, although 90% of the water in Hawaii comes from groundwater sources.

Hawaii (The Big Island)

The Big Island’s source of water comes from a combination of surface and groundwater, in addition to rainwater catchments. Surface water is treated and disinfected within various water treatment facilities. Groundwater is disinfected before being distributed to nearby homes. The County of Hawaii Department of Water Supply provides the majority of water distribution.


The water in upper Maui comes from surface water, primarily from the streams in East Maui. The water is pulled and treated at one of Maui’s three water treatment facilities before it is finally delivered to homes.

In central Maui (Kahului, Wailuku, Waihee, Maalaea, Kihei, and Paia), most water comes from groundwater sources, specifically the Iao Aquifer located under the West Maui Mountains. That water is naturally filtered through lava rocks and then sent to a treatment facility to disinfect with chlorine before delivering it to homes through the pipe system.

Other areas of Maui rely on a mix of surface and groundwater sources for their water supply, but the treatment for the water remains the same.

Overall, Maui’s tap water is safe to drink; it meets or exceeds all standards set by Hawaii’s state government in addition to federal requirements. The United States EPA and the Hawaii State Department of Health routinely test the water supply to make sure it is safe.[9]


Oahu’s freshwater mainly comes directly from deep groundwater aquifers. The water is already purified through over 25 years of percolating downward through both soil and volcanic rock, so it is extracted from the ground via a large network of shafts, water tunnels, and wells, and immediately pumped to mains and reservoirs to be accessed by homes and businesses nearby. The Honolulu Board of Water Supply distributes the majority of the drinking water in Oahu.

However, there do exist about 13 treatment plants in Oahu that are responsible for adding small amounts of chlorine and granular activated carbon (GAC) in roughly 60% of the total water supply to guarantee the safety of the water.[10]


Kauai’s tap water source comes from mostly groundwater, although a small percentage of water is supplemented by surface water sources. Both sources of water are mostly treated and distributed by the County of Kauai’s Department of Water. The water is also very much safe to drink due to being compliant with water treatment standards; like other Hawaiian islands, its drinking water meets or exceeds state and federal regulations. The Department of Water regularly tests the water in compliance with EPA standards.[11]


Because the majority of the land on Molokai is for agricultural use, most of the water distributed on Molokai is for farms. It comes from the Molokai Irrigation System (MIS), which distributes water to over 230 users in the central region.

The main water source is in Waikolu Valley, where rain can fall for an average of 120 inches each year. The water is pumped from the valley’s main stream, in addition to seven wells drilled in the region. It is fed through a series of underground pipes to transition boxes, which direct the water to the nearby Kualapu’u reservoir for storage and later use in nearby farms.[12]


Lanai uses groundwater as its main source of tap water, which is safe to drink. Although most of the land on Lanai is undeveloped, they have multiple wells in which water systems pump water into takes for storage and daily use.

The Lanai Water Company is the main provider of water on Lanai. To ensure that the water remains safe to use and rink, the company conducts regular testing in accordance with the Hawaii State Department of Health and Clean Drinking Water Branch in Lanai. Their testing and treatment meet the standards of state and federal public health requirements.[13]

Where in Hawaii is the Drinking Water Not Safe?

You are most likely to find unsafe drinking water in the other two large islands of Hawaii that are not as populated or developed as the other six: Niihau and Kahoolawe.

  • Niihau is only 69 square miles big and is mostly used for raising livestock, so the main water source there is natural groundwater from a well. However, because the island is privately owned, there is not as much monitoring of the main water supply.
  • Kahoolawe is the smallest of Hawaii’s eight major islands, measuring at 45 square miles. The island is completely uninhabited, and therefore access to the island is prohibited by the state and federal government. In the past, the island was actually used as a target by the United States military, and still has remains of artillery shells. For obvious reasons, the water on Kahoolawe is not safe to drink at this time.


In conclusion, as of right now, most of Hawaii’s tap water is safe to drink; however, it is still important that you do your research on the area you plan on visiting before traveling to be sure. There are still a few regions where health officials would not recommend drinking the tap water. In addition, recent outbreaks or storms can also affect the safety of drinking water.

Like with any other new place you visit, it is better to be safe than sorry, so you are still encouraged to drink bottled water over tap whenever you can.

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