The Pearl Harbor Aquifer of Oahu, Hawaii

Megan M. Flory

Spring 2007

GO571 Geohydrology

Dr. Marcia K. Schulmeister

Pearl Harbor, Hawaii
(Butler, 1994)

Introduction

Oahu, often called "the gathering place," is an island of extremes. At one extreme, there is metropolitan sprawl of Hawaii’s capital city, Honolulu, and the playground of Waikiki with its sophisticated restaurants and excellent shopping boutiques. At the other extreme, there is the paradise of the island’s miles of white sandy beaches, the crystal clear waters of the Pacific and the majestic Koolau and Waianae mountain ranges. (The Hawaiian Islands) With an area of 602 square miles, the island is home to the largest population of all the Hawaiian Islands, and is the most developed of the islands as well. (Oki, et. al. 2005) Figure 1 shows the main islands of Hawaii and the relative location of Oahu.

Figure 1. The Hawaiian Islands
(WorldAtlas.Com, 2007)

Because of the high population of Oahu and the isolation of the Hawaiian Islands, freshwater resources are very important to residents on this island. This webpage focuses on the Pearl Harbor Aquifer, (shown in Figure 2), which is one of the most important aquifers in the state.

Figure 2. Extent of the Pearl Harbor Aquifer
(Loague, 1996)

Regional Climate

Oahu hosts an array of climatological conditions, each depending on geographic position and elevation. However the prevailing northeasterly trade winds are the dominant factor that governs Oahu’s pleasant, tropical climate. The island has two mountain ranges separating the moist, cooler windward side from the drier, hotter leeward coast. The Koolau Mountains extend along the northeastern side and to the west are the Waianae Mountains. (Shema) The high temperatures on the windward side rarely ever exceed 90oF, while on the leeward side temperatures can exceed 90oF several times a year (Hawaii Weather as Locals Know It). Average year-round temperatures for the island range from 88oF to 74oF with moderate humidity of 53% during the day. Both mountain ranges block trade wind moisture. As a result, showers occur almost daily on the windward side while on the leeward side showers are light. (Shema) These daily showers add up to an average annual rainfall range from about 20 inches in the Waianae area, which is along the west coast of the island, to 158 inches in the Upper Manoa Valley, which is on the east side. (Oahu Profile)

As in most tropical climates, there are two seasons in Hawaii. The cooler wetter season called Ho'oilo, which lasts from October to April, and the hotter, drier season called Kau, which lasts from May to October. At sea level, the all day average air temperature varies from about 78oF in August to about 72oF in February. (Seasons and Months, Hawaii Weather as Locals Know It)

Figure 3 shows the mean annual precipitation based on data from 1961 to 1990 for the island of Oahu. The windward, or north through northeast facing sections of the island, and the leeward, or southwest through southeast sides of the island, are both evident here.

Figure 3. PRISM 1961-1990 Mean Annual Precipitation Oahu, Hawaii
(PRISM, 2002)

Another attribute this map demonstrates is the steep rainfall gradients. In fact, there are few areas elsewhere in the World where rainfall gradients are as steep as they are in Hawaii (Climate of Hawaii).

Recharge Mechanisms and Relationship of the Aquifer to the Unsaturated Zone

Rainfall is Oahu's primary water resource for streams and groundwater supply (Watershed Partnerships, 2007). Natural recharge rates within the Pearl Harbor basin vary from 4,000 mm yr along the upper slopes of the Koolau mountain range to less than 100 mm yr along the leeward coast (Giambelluca, 1996). Basal groundwater in the area originates as rainwater falling in higher drainage basins to the north and northeast and percolating vertically downward to the basal aquifer within the basalt bedrock. Fresh water of the basal aquifer floats on and displaces the salt water, which saturates the highly permeable basalts at the base of the island Oahu. (EPA Superfund Record of Decision) The impermeable caprock at the coast blocks the flow of freshwater and causes the water to bulge into the shape of a lens. Beneath this freshwater lens is the transition zone where the freshwater mixes with the salty ocean water. These features are shown in Figure 4 below.

Figure 4. Hydrology of an Island Aquifer
(Liu, 2005)

Discharge is primarily to wells and shafts and to springs in the Pearl Harbor area. Some ground water also flows out of the southern Oahu ground-water area to the adjacent southeastern Oahu ground-water area to the east (Oki, et. al. 2005).

Unconsolidated and consolidated sedimentary deposits form a thick confining unit near the coast. Coralline limestone within the sedimentary deposits is at shallow depths and is extremely permeable but commonly contains brackish water that is withdrawn for cooling and industrial purposes. The confining unit locally is more than 1,000 feet thick near the coast. (Oki, et. al. 2005)

In some coastal areas, caprock overlies and confines the aquifers, impeding freshwater discharge and impounding basal water to a thickness of as much as 550 m. (Taskin, 1999)

Distribution of Surface Waters

Oahu’s tropical rainstorms and steep topography result in many rivers and streams flowing off the mountains. Many of these are shown in Figure 5.

Figure 5. Rivers of Oahu
(Hawaii Water Quality Extension Program)

In addition to this rivers and streams, Oahu has several reservoirs on the island. The locations of these reservoirs are shown in Figure 6.

Figure 6. Pearl Harbor, Hawaii
(Hawaii Water Quality Extension Program)

Hydrologic Properties of the Aquifer

In general, the Pearl Harbor Aquifer can be described as a regional, unconfined, basal groundwater aquifer composed of Koolau basalt, except near the coast where there are thick confining units of unconsolidated and consolidated sedimentary deposits. (EPA Superfund Record of Decision) Basal groundwater refers to water beneath the water table, and in a basal water lens, the freshwater head is found near sea level. (Taskin 1999) The sedimentary deposits near the coast can be more than 1,000 feet thick in some places. (Oki 2005)

The actual thickness of the aquifers of Oahu is not known, but probably is at least a 1000 meters. The lack of a definable aquifer thickness and the partial penetration of wells introduce ambiguity into estimates derived from aquifer tests. Estimates of specific yield range from about 1-20% percent; most values lie within a narrow range of about 5-10%. Estimates of confined-storage coefficient typically are higher than the range of 5 x 10-3 to 5 x 10-5 cited for confined aquifers. (Taskin 1999) The documented range of vertical hydraulic conductivities for the basal Pearl Harbor Aquifer is extremely high, ranging from approximately a few hundred to a few thousand feet per day. In contrast, the horizontal hydraulic conductivities of the Pearl Harbor Aquifer range from approximately 0.09 centimeters per second to 0.7 centimeters per second. The potentiometric surface of the basal aquifer gently slopes toward the shore of Pearl Harbor, and the regional hydraulic gradient averages approximately 0.3 meter per kilometer. (EPA Superfund Record of Decision)

Geologic Description of Aquifer Medium

The following figure shows a basic representation of the main geologic features of Oahu. Through numerous eruptions, the Koolau and Wainanae lava flows eventually joined to create the Schofield Plateau.

Figure 7. Geology of Oahu
(Dave Black Online, )

The Pearl Harbor Aquifer lies just south of the plateau and is mostly composed of basaltic rocks ranging in age from Pliocene to Holocene. Quaternary-age consolidated sedimentary deposits form productive aquifers in lowlands and near shore areas but usually contain brackish water, which is fresh water mixed with saltwater. These deposits are underlain by and interbedded with Quaternary-age low-permeability sedimentary deposits that form confining units that impede the discharge of freshwater into the ocean and, thus, allow the freshwater lens to build up to a greater thickness than would be possible in an unconfined setting. Sedimentary deposits include coralline limestone, dunes, sand, lagoonal deposits, and alluvium. (Oki, et. al. 2005)

Natural Water chemistry of the Aquifer

Volcanic rocks and soils are a source of numerous inorganic constituents, including calcium, magnesium, sodium, potassium and some heavy metals, in the water of the island. For example, most silica found in stream water is derived from volcanic rocks and soil. (Oki 2003)

The following information was collected from a well in Honolulu County. The land-surface elevation is 620 feet above sea level and the depth of the well is 750 feet below land surface. The well is completed in the Koolau volcanic series, lava flows local aquifer and also in the Hawaii volcanic-rock national aquifer.

Water Temp: 20.6 oC Turbidity NTU: 0.09Specific Conductance: 163 µS/cmDissolved Oxygen: 8.9 mg/L
pH (field measurement): 7.4Bicarbonate: 64 mg/LAmmonia: <0.02 mg/L/td>Nitrite: <0.01 mg/L
Phosphorous: 0.07 mg/LOrthophosphate: 0.06 mg/LCalcium: 8.1 mg/LMagnesium: 6.89 mg/L
Sodium: 13.2 mg/LPotassium: 0.88 mg/LChloride: 15.3 mg/LSulfate: 2.7 mg/L
Flouride: <0.1 mg/LSilica: 42.7 mg/LArsenic: <0.9 µg/LBarium: 3 µg/L
Boron: 21 µg/LCadium: <1.00 µg/LChromium: 2.7 µg/LCopper: 1.9 µg/L
Iron: <10 µg/LLead: <1.00 µg/LZinc: 3.5 µg/L

(USGS Water Data for the Nation)

The Pearl Harbor Aquifer as a Water Resource

In 2004, Oahu had a resident population of 899,593 (Oahu Profile), and most of these people live in the Honolulu and Pearl Harbor areas. (Water Resources Research Grant Proposal) In addition to this large resident population, the island also sees many tourists in a year. Because so many people live near the Pearl Harbor Aquifer, it is safe to say that a large percentage of the Oahu population depends on this aquifer for their fresh water resources, which is one reason why it is the most productive aquifer in the state producing about 87.5 million gallons of potable water per day. (Water Resources Research Grant Proposal) This number does not include the non-potable, brackish water being pumped from the aquifer and used for cooling and industrial puposes. (Oki, et. al. 2005) In addition to water usage in the Pearl Harbor area, 4.5 million gallons of water per day are pumped from the Pearl Harbor Aquifer to Waianae. (Waianae Watershed Management Plan)

As can be seen on the land use map below, most of the surface above the Pearl Harbor Aquifer is developed as either residential or urban areas. Therefore, most of the water pumped from the aquifer is for domestic and commercial use with agricultural representing only about 2% of the total water use. (Pitafi, et. al. 2006)

Figure 8. Land Use Map of the Pearl Harbor Area
(“Oahu Land Use Sample Map”, DATE???)

Most sources agree that the Pearl Harbor aquifer is being pumped at a rate close to that of its sustainable yield, which is 165 million gallons per day, (Groundwater Hydrology Modeling) and the population of the island is increasing exponentially. As the population increases, there will be a higher demand for fresh water; a demand that in the near future might not be met. In addition to the increase in demand, another implication of increasing population is the decrease of agricultural lands. When these agricultural areas are irrigated, more than half of this water returns to the basalt aquifer by infiltrating through the highly permeable soils (Mink, 1962). Developed landscapes tend to increase runoff, which mostly goes directly into the ocean.

Aquifer Contamination

One of the biggest contamination threats to the freshwater aquifers of Oahu is saltwater intrusion. This problem is mainly a function of the geology of the island and the manner in which island aquifers operate. As previously discussed, freshwater floats on top of the saltwater in the aquifer, creating a delicate equilibrium. Groundwater withdrawals induce upward and landward movement of saltwater. Wells completed in the freshwater lens near the coast, such as the Pearl Harbor area, are particularly likely to induce brackish water or saltwater to move into the well as pumping continues. (Oki, et. al. 2005) This problem can be seen by the increased salinity occurring in the Pearl Harbor Aquifer. The tropical climate and the thick, rich soils of Oahu make it an ideal place for raising pineapple and other vegetables. Unfortunately with agriculture also come pesticides and fertilizers, which can percolate downward and pollute the underlying aquifer. Wastes from septic-tank systems, sewers, industry, and storm runoff also can introduce undesirable constituents into the aquifers, (Oki, et. al. 2005), and because this aquifer is so deep and large, once it has been contaminated it is extremely difficult to clean up. (“Pesticides and Groundwater in Hawaii”)

At one time it was thought that contamination of the aquifer was highly unlikely because of the great depth to the water table and the filtering action of the soil. However, recent studies of several Oahu wells have indicated pollutants in the water supply. (“Pesticides and Groundwater in Hawaii”) “Due to past agricultural application of pesticides and perhaps past fuel spills or improper disposal of waste chemicals, the Pearl Harbor Aquifer is contaminated with trace concentrations of ethylene dibromide (EDB), l,2-dibromo-3-chloropropane (DBCP) and 1,2,3-trichloropropane (TCP). Since the discovery and characterization of the extent of the contamination in the late 1970's and early 1980's, the concentrations detected have not decreased. In addition, there is evidence that the contamination is spreading rather than subsiding” (Water Resources Research Grant Proposal).

Agencies Responsible for the Protection and Cleanup of Oahu’s Groundwater

There are several state agencies involved in the protection of Hawaii’s water. These agencies include: The Hawaii State Department of Health, Clean Water Branch, the Department of Environmental Services, Land and Natural Resources Department, Board of Water Supply (BWS), Hawaii Association of Conservation Districts (HACD), University of Hawaii College of Tropical Agriculture and Human Resources (UHCTAHR), State Agriculture Department, Coastal Management Program, and Resource Conservation and Development (RC&D), State of Hawaii Commission on Water Resource Management.

At the federal level, The National Resources Conservation Service (NRCS), the US Environmental Protection Agency (USEPA), and the Agency for Toxic Substances and Disease Registry (ATSDR) have all helped in the protection and/or cleanup of ground water on Oahu.

According to National Water Rights Digest, when dealing with water rights, Hawaii is described as a regulated riparian state. (National Water Rights Digest Reference)

Conclusions

The Pearl Harbor aquifer is a very important, but diminishing and threatened resource. Its volcanic origin leads to high productivity, while its island location makes it vulnerable to saltwater intrusion. Agricultural contaminants and the increasing population of Oahu are also sources of pollution threatening this resource. Proper monitoring and conservation techniques are needed to ensure that this aquifer’s high productivity and quality are not compromised for future use.

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