When many people think of Honolulu, Hawai’i, the first things to come to mind are world-class beaches, sunshine, and surfing, but the city of Honolulu is much more than that. Besides being the capital of a state with 1.4 million residents, Honolulu is an economic and cultural center in the maritime Pacific region. At present and into the future, the well-being of this vibrant city will depend on careful water resources management on the island of Oahu.

Honolulu needs to diversify its water supply portfolio to supplement recharge that will be lost by climate change. Additionally, Honolulu will experience marine inundation and groundwater flooding as sea levels rise. There is a range of options that the city could pursue to augment its water supply, such as desalination, expanded water recycling, and rainwater harvesting. This article will examine the possibility of brackish water pumping and flushing, which seeks to build resilience by addressing both the challenge of sea level rise as well as the risk of reduced water supply due to climate change and population growth.  

The city of Honolulu is growing.

Honolulu is located on the leeward side of the island of Oahu. The island is 597 square miles, and it is covered in its entirety by Honolulu County. Honolulu city proper has approximately 350,000 residents, which is about a third of the island’s total population. Tourism and agriculture are both leading sectors of the island’s economy. Honolulu itself is both a vacation destination and the capital of the state of Hawai’i.

Each of the Hawaiian Islands has unique, sensitive ecosystems and Oahu is no exception. Approximately 1/3 of the island’s land is off limits for development, thanks to Hawaii’s strong statewide land use laws. The island also has a historically strong agriculture sector, but the agriculture sector on the island has been shrinking because of global competition and increasing land values, which will likely change the balance of water users on the island in the coming years.

Oahu’s water supply is primarily influenced by climate and geology.

The unique climate and geology on Oahu are determinants of supply-side water resource dynamics. The Northeast trade winds and the rain shadow effect on the Ko’olau Range are the lifeline for the island. Water evaporates off the North Pacific and the trade winds push the resulting clouds towards the mountains. When the clouds reach the higher elevation of the mountain range, precipitation falls on the island. This precipitation goes back into the atmosphere via evapotranspiration, recharges groundwater aquifers by seeping through the soil, or becomes runoff. The runoff reaches the ocean through streams and rivers, seeps down into aquifers, or evaporates. The spatial distribution of the island’s annual rainfall reflects the impact of the rain shadow effect. The uplands receive close to 280 inches of rain per year, while lowlands and coastal areas receive about 20 inches of rainfall on average (see Figure 1).

There are three types of aquifers on Oahu, each with a distinct set of geological conditions. Dike-confined water is groundwater in higher elevation areas that is constrained by rock structures that were formed by prehistoric lava flows. At higher elevations, tunnels can be drilled vertically to extract the dike-confined water. Fresh basal water is found below the middle elevations and in the island’s valley. This freshwater floats above a vast saltwater aquifer deep below the island. Lastly, brackish caprock water is found in the island’s coastal aquifers that occur at the saltwater/freshwater mixing zone. Because the caprock aquifers are brackish water, wells that drill into them are only used for irrigation. For example, in Figure 2, wells (A) and (B) both produce brackish water that cannot be used for drinking.

Because most of Oahu’s population is in lowland and coastal areas and the highest quality drinking water is found in the uplands, an extensive pumping system is used to transfer water from the source to the consumers. Overall, nearly 200 well pumps and approximately 200 other “booster” pumps are maintained to transport potable water to Oahu’s residents. There are also 14 dams on the island that form reservoirs to collect drinking water and wastewater.

Oahu currently uses recycled water as one alternative water source for the island. The waste water recycling facility was constructed in 2000 to reach a capacity of 12 million gallons/day (mgd). Currently, the plant only produces 8 mgd to irrigate golf courses and cool a nearby power plant. Water from the plant is not safe for human consumption so it is only permitted to travel through “purple pipes” that separate the recycled water from the potable water supply.

Honolulu’s per capita water consumption is consistent with the yield of its aquifers.

Municipal water uses account for approximately 64%, or 146 mgd, of water used on Oahu. Other leading water consumers include the military, agriculture, irrigation, and industry (see Figure Z). The average per capita water use on the island is 157 gallons/day and per capita water use in Honolulu has decreased since 1990 even though the population has increased over the same time period. The water management agency believes that efficient appliances have played a large role in keeping water use stable despite population growth.  The downward trend of per capita water use is not projected to continue past 2020 based on the demand projection published by the Board of Water Supply in 2016.                

Overall, water consumption on Oahu is well below the existing sustainable yield of its aquifers (407 mgd), but the portfolio of sources on the island is not diverse. 100% of the island’s drinking water is extracted from groundwater aquifers, and brackish water and recycled water only account for a small portion of the water used for industry and irrigation (see Figure 3).

Climate change is a leading threat to Honolulu’s water system.

In the future, climate change and population growth on Oahu will put more stress on Honolulu’s water system. In terms of climate change, reduced rainfall and sea level rise have the most potential to impact the island. The projected population increase in Honolulu will multiply these threats by increasing demand on the existing potable water supply.

Over the past 30 years, rainfall levels on Oahu have been in decline. This pattern is projected to continue as the global average temperature trends upward, which means that the aquifers on the island will be recharged more slowly. Climate change models also predict that there will be a larger imbalance between wet and dry areas on the island, and more rain in the mountains will arrive as drenching downpours. This type of rainfall will cause more erosion and more flash flooding. It will also prevent water in the island’s streams from percolating through the soil into the aquifers. Instead, the water will rush towards the sea, furthering reducing the potential for rainfall to recharge aquifers.                

Sea level rise is another threat associated with climate change. Sea level rise will impact Oahu in at least two ways. First, marine inundation will flood shoreline areas and submerge some of the existing coastline. Then, the water table in coastal areas will also rise (see Figure 4). As the water table rises, it will cause flooding in coastal areas where the water table is shallow. According to Kolja Rotzoll and Charles Fletcher, this type of flooding will at first occur during high tides that coincide with heavy rainfall, but over time the impact will become more frequent: “It is likely that future urban settings will be characterized by standing pools of brackish water, maximized at high tide. This may affect traffic, walkways, and any movement in urbanized coastal areas.” The brackish water that seeps into urban areas will also corrode infrastructure more quickly than freshwater would, resulting in higher maintenance costs and a system that is generally more vulnerable.

Brackish water flushing is a versatile solution for future water system challenges.

To deal with looming climate change threats, Honolulu needs a solution for its water system that can be adapted to changing conditions and consider risks that stem from both rainfall variation and sea level rise. Inspired by Hong Kong’s seawater flushing system and emerging studies of groundwater inundation, brackish water flushing is one such solution.

At first glance, Hong Kong does not look like a city that has many similarities with Honolulu. Hong Kong has nearly 25x the population as Honolulu and Honolulu is located on a small island. However, a closer look reveals that the two cities have similar threats and opportunities within their water systems. Hong Kong also deals with a limited water supply portfolio, and both cities have a supply of salt water and brackish water at their disposal. Hong Kong has utilized its seawater resource by building a system that uses seawater for flushing. Toilet flushing is one of the most taxing domestic water uses, so the city has been able to take a massive burden off its potable water supply by introducing saltwater flushing for 85% of the city’s population.

Salt water flushing is an option for Honolulu, but it is unclear how a salt water intake pipe would impact the island’s sensitive coastal waterways. The prospect of marine inundation would also complicate any proposal to build new coastal infrastructure. Fortunately, there is another nascent water supply that Honolulu could tap into: Brackish water aquifers. Rotzoll & Fletcher believe that “groundwater withdraws could be used to mitigate the effect of a rising water table, even if it means pumping brackish water to avoid inundation.” Instead of simply pumping draining brackish water aquifers into the ocean, Honolulu could use its brackish water for flushing. Brackish water flushing from the coastal aquifers would reduce reliance on fresh aquifers. Additionally, using the brackish water will also lower the water table and prevent groundwater flooding during high tides and reduce localized coastal-plain flooding that is predicting to result from rising sea levels.

This solution will take the water that Honolulu does not want and turn it into water that the city needs. Investing in a purple-pipe brackish water flushing system will also create the opportunity to expand recycled water use if necessary. This solution also calls attention to groundwater inundation, which is one of the more poorly understood sea level rise risks that all coastal cities should be planning for as they think about climate change.

Unfortunately, creating a system of “purple pipes” and drilling into the brackish water aquifer is not a low-cost solution for Honolulu. It would be best to begin this practice with new development areas first and then progress toward retrofitting existing areas. This would also be a large, distributed infrastructure system, so the cost of maintenance may be high. Distributed infrastructure does have its benefits though: issues in one part of the system will be isolated from the other components. It is also important to acknowledge that this system will not augment the potable water supply, it will only reduce stress on the demand side. If rainfall on Oahu was to catastrophically decline, Honolulu would need to seek other options for recharging aquifers.

Honolulu isn’t in crisis mode – yet.

At present, Honolulu has a good balance of aquifer recharge and consumer demand. However, the Board of Water Supply should still think about the future and the challenges that the island may face in the future. It’s difficult to predict exactly how climate change is going to impact Oahu. As such, the Board of Water Supply should consider different impact scenarios and invest in solutions that can easily be adapted. Measures like desalination require a large upfront investment and would only address one of Honolulu’s water system issues. Brackish water flushing could address supply stress while mitigating the impact of groundwater inundation at the same time. In sum, Honolulu should think ahead and look for water system solutions that consider the interconnected nature of Oahu’s water.

Sources

Hill, K. (2018, December). Maps of a rising water table: The hidden component of sea level rise. Presented at the Berkeley Distinguished Lectures in Data Science, University of California, Berkeley. Retrieved from https://bids.berkeley.edu/events/maps-rising-water-table-hidden-component-sea-level-rise

Honolulu Board of Water Supply. (2016). 2016 Water Master Plan. City and County of Honolulu, HI.

Rainfall Atlas of Hawai’i. (n.d.). Retrieved from http://rainfall.geography.hawaii.edu/downloads.html

Rotzoll, K., & Fletcher, C. H. (2013). Assessment of groundwater inundation as a consequence of sea-level rise. Nature Climate Change, 3(5), 477–481. https://doi.org/10.1038/nclimate1725

Water Supplies Department. (n.d.). Seawater for Flushing. Retrieved from https://www.wsd.gov.hk/en/core-businesses/total-water-management-strategy/seawater-for-flushing/index.html