The supply of available groundwater for the island of O‘ahu is estimated to be 415 million gallons per day (mgd). The current groundwater allocation to agricultural, military, urban, tourism, and domestic use is approximately 333 mgd, leaving 82 mgd currently unallocated. New demands for domestic water through 2020 could exceed the current capacity of the island. For example, development in the ‘Ewa, Central, Wai‘anae, and Honolulu districts alone is estimated to increase water consumption by 90 mgd by the year 2020. This estimate does not take into consideration any new military or agricultural needs, or potential increased water needs in other areas of O‘ahu (Hardy undated). Conserving the island’s water resources and maximizing their effective use will play a critical role in sustaining O‘ahu’s growing population.
O‘ahu relies completely on groundwater for its drinking water. Draining aquifers too deeply can disrupt the balance of the aquifer’s freshwater lens, causing saltwater contamination of the freshwater supply. The Wai‘anae moku is already experiencing a shortfall in meeting its own needs, with supplemental water supply brought in from the Pearl Harbor aquifer.
Saltwater intrusion is a potential problem near the coast. Diagram ‘A’ is a well completed in a volcanic-rock aquifer in which withdrawal is much less than recharge. Diagram ‘B’ shows the same well under conditions of large groundwater withdrawals. Saltwater has intruded the aquifer and brackish water has reached the well.
Source: U.S. Geological SurveyMany of Wai‘anae’s streams have been diverted as irrigation for commercial agriculture. The Wai‘anae coast has already seen the reduction of kalo lo‘i and other crop lands that results from water diversion and misuse. With increases in population expected for the area and the island, Wai‘anae needs to prepare for the increased demands on its water supply.
Maintaining the quality of water resources on O‘ahu demands creative strategies for water use and reuse. Water reclamation efforts that are being used at the nearby ‘Ewa Water Recycling Facility, adjacent to the Hono‘ulī‘ulī Wastewater Treatment Plant, provide as much as 12 mgd for irrigation of landscaping, parks, school grounds, and crops. Though desalination is expensive at $3.20 per thousand gallons, innovative technology is driving the cost down. Construction of a proposed desalination plant at Kalaeloa could eventually provide up to 35 mgd to the dry ‘Ewa Plain (Lanzilotti 2003).
Golf courses can be nonpoint-source contributors of pollutants, such as nutrients and pesticides, to surface water and groundwater.
Source: CZM Hawai‘iSources of contamination to the water resources of the Wai‘anae moku fall into two categories: point sources and nonpoint sources. Point sources generally are discharged from a pipe into receiving waters such as a stream or bay, and include waste streams from industrial facilities. These pollution sources are federally regulated and generally affect only surface water, although wastewater discharged to injection wells may affect groundwater if not properly treated prior to injection. The larger threat to both surface and groundwater reserves are nonpoint pollutant sources. These sources of pollutants are associated with particular land uses (such as agriculture and urban use) and occur over wide areas with no real defined individual source.
Agricultural irrigation is responsible for some of the recharge of groundwater resources, known as "return flow." This return flow can contain chemicals from fertilizers and pesticides, including chlorides and nitrates, which can contaminate groundwater and make it nonpotable (Shirazi 2004). The Wai‘anae district is currently the largest and most commercially important center for livestock production in the state of Hawai‘i (City and County of Honolulu, Department of Planning and Permitting 2000). As of January 2004, production in the region included three of the eight major commercial dairies in the State of Hawai‘i, which accounted for roughly half of the State’s milk production; 40 of the State’s 230 piggery operations, which produced 45 percent of the State’s total hog inventory; and poultry operations, which produced the vast majority of the State’s eggs, broiler/fryer chickens, and pullets (Nils Morita, Hawaii Department of Agriculture, personal communication, 2004). There are also many small noncommercial truck, family, and subsistence farms that sell mostly to the Honolulu markets.
Changes in land use also threaten the quality and quantity of water resources in the moku. Population growth creates a greater demand for housing that could, in turn, lead to development of current open space. In 1998, the Department of Planning and Permitting estimated that by 2020, an additional 4,500 units of housing would be needed in the moku, mostly in the Nānākuli area (City and County of Honolulu, Department of Planning and Permitting 2000). Urban development includes not only residences, but also commercial and recreation areas.
Developing open space could affect the return flow of water from streams back into the aquifer either by altering natural stream channels or by disrupting the interaction between soil and ground (through paving, construction, and so forth). This disruption can increase the speed at which surface water flows to the coastal areas; increase polluted runoff from roads, parking lots, residential landscaping, and golf courses; and adversely affect the quality of water that empties into the coastal zone.
"Before, when the water from up mauka ran through the old ‘auwai and streams, it was drinkable. Take the old Wai’anae Tunnel [constructed to divert water to the old sugarcane fields]—the overflow came down to the taro [fields]. We could fish the opae, hīhīwai, and o’opa in the streams. But that all stopped in ’55 or ’56. Now we worry about what’s above in the watershed, what’s polluting our water?"
– Josiah "Black" Ho‘ohuli, Nānākuli Homesteads resident
El Niño and other similar weather events, which are caused by changes in the interaction between ocean and atmosphere, are associated in Hawai‘i with relatively dry winter weather. There is an increased potential for drought in the years that El Niño occurs. As El Niño and similar events become more frequent and the occurrence of drought increases, even greater demands are placed on the finite resources of groundwater and surface water. To learn more about the effects of weather and drought conditions on Wai‘anae's water resources, please see the Ke a‘o Ulu: Drought Conditions section.
Because of the importance of a sustained water supply to both traditional and commercial agriculture in the Wai‘anae moku, protecting the quality and quantity of local waters is essential to maintaining the rural quality of life that is so important to its residents (City and County of Honolulu, Department of Planning and Permitting 2000).
City and County of Honolulu, Department of Planning and Permitting. 2000. Waianae Sustainable Communities Plan. http://www.honoluludpp.org/Planning/DevSust_Waianae.asp
Hardy, R. Undated. Sustaining Hawaii's Natural Drinking Water Supplies. State of Hawaii Commission on Water Resource Management. http://www.hawaii.gov/dlnr/DRINKING.HTML
Lanzilotti, S.S. 2003. WHO-KOBE Report: Water Quality. City and County of Honolulu. Department of Emergency Services. http://www.co.honolulu.hi.us/esd/reportc.htm
Shirazi, T. 2004. Urban Groundwater Database: Honolulu. International Association of Hydrogeologists. http://www.scar.utoronto.ca/~gwater/IAHCGUA/UGD/honolulu2.html
Honolulu Board of Water Supply. 2004b. Water Conservation Website. http://bws.starrtech.com/cssweb/display.cfm?sid=1061
