Alessandra Pistoia
Pope
Applied Hydrogeology
13 April 2012
Hawaiian Islands Aquifers Case Study
The islands of Hawaii are a long chain of volcanoes known as the Hawaiian Ridge, which stretches northwestward across the central Pacific Ocean (Figure 1). Each shield volcano was formed as the plate moved northwestward across a hot spot. Some of the volcanoes grew above sea level, qualifying the mass as an island. As the islands age and move away from the hot spot, they erode and subside; this is why the newer volcanoes are the highest above sea level and, for the most part, larger than the older, eroded volcanoes.
Given the volcanic origin of the islands, basaltic lava is the foundation of every island. Three main groups define volcanic rocks: lava flows, dikes and pyroclastic deposits. Pahoehoe and aa are the two main types of lava flows. A layered sequence of lava flows creates void spaces: vesicular, fracture, interflow, intergranular, and conduit porosity. Since pahoehoe flows tend to spread out, the void spaces found in a sequence of these flows will have high intrinsic permeability. Aa flows form productive aquifers with a permeability common to coarse-grained gravel; however, it should be noted that the lava in the core of an aa flow has low permeability. The hydraulic conductivity in flat-lying lava flows are greatest parallel to the direction of the flows, and least perpendicular to the layered sequence of flows. Lava tubes may also occur, which cause extremely permeable features with a high hydraulic conductivity. Dikes lower overall rock porosity and permeability, and also have very low hydraulic conductivity. In some places, dikes compartmentalize more permeable rock, in which case ground water can be impounded. Dikes channelize ground water flow parallel to the general trend of the dikes. Pyroclastic deposits include ash, cinder, and larger blocks. Hydraulic conductivity of pyroclastic deposits range from 1 to 1,000 feet per day. Pyroclastic deposits have fairly good permeability, unless they are compacted.
The most productive aquifer in Hawaii is the volcanic-rock aquifer. These aquifers are heterogeneous; water flows more efficiently horizontally than vertically due to the way the lava settles. The thickness of the aquifer depends on the thickness of lava flow; generally, thicker flows are less permeable. Volcanic-rock aquifers made up of high-permeability, dike-free, and shield-building-stage lava flows have the highest well specific capacity (Figure 2). Volcanic-rock aquifers that are intruded with dikes are less permeable, thus has lower well specific capacity; these wells are commonly found in rift zones and caldera complexes. The Hawaiian Islands’ climate causes a great deal of weathering, which reduces the permeability in the rocks. From this weathering,clasticsedimentary deposits accumulate in some areas (Figure 3)on the volcanic rock. Sedimentary deposits are mostly found near the coast. Deposits of alluvium, coralline limestone, and cemented beach or dune sand are considered to be productive aquifers in much of the United States, however this is not the case in Hawaii. These sedimentary deposits form a caprock, or a low-permeable material that lays over highly permeable volcanic rock (due to the vescularity in basalt). Caprocks impede the discharge of freshwater to the ocean, making the freshwater lens thicker than what would otherwise occur (Figure 4).
Fresh ground water can be found in three settings: a freshwater-lens system, a dike-impounded system, or a perched system (Figure 5). The freshwater-lens and dike-impounded systems exist below the lowest water table;the perched system exists above the lowest water table. In some settings the freshwater-lens and dike-impounded systems are adjacent, in which case they form a hydrologically connected ground water flow system. The most dependable groundwater sources in Hawaii are from the freshwater-lens system. Water levels are directly affected by ocean tides and withdrawals from wells (Figure 6). Water levels also change in response to precipitation; the more precipitation, the higher the water level (Figure 7). In Oahu, the water levels have begun to decline due to an increase in ground water withdrawals (Figure 8).
On an annual basis, the amount of recharge available to enter the aquifers is about equal to the difference between average annual precipitation and water losses (average annual runoff and evapotranspiration). Average annual rainfall differs from island to island(Figure 9). Kauai receives on average more than 435 inches of rainfall annually; whereas the island of Lanai receives on average no more than 10 inches of rainfall annually because it is in the rain shadow of Molokai and Maui. Not only does rainfall differ between islands, but also within the island itself. Generally speaking, the northeastern sides of the islands, between 2,000 and 6,000 feet, are the wettest (Figure 10). Average annual runoff of the islands range from less than 5 to 200 inches per year. Evapotranspiration is a major factor of the hydrologic budget of the islands. Annual pan evaporation over the open ocean is estimated to be 65 inches. Leeward coastal area increases annual pan-evaporation rates to around 100 inches. On the other hand, altitude between 2,000 and 4,000 feet reduces annual pan-evaporation about 17 inches.
Discharge in a freshwater-lens system in highly permeable rocks is due toleakagenear the coast and to springs; spring discharge is an indicator of aquifer water levels. In the most permeable volcanic rocks, the water table is only a few feet above sea level, and the slope of the water table is nearly flat. In a low-permeable aquifer, the water table is several hundreds or thousands of feet above sea level. In vertically extensive freshwater-lens systems, discharge occurs directly to stream valley. Caprock confining units impede discharge of groundwater to the ocean.
In 1995, total freshwater withdrawals from aquifers were 516 million gallons per day. Each island withdrew different amounts of ground water; Oahu accounted for 47% of these withdrawals, which is not surprising because Honolulu, the most populated city in Hawaii, is found here. The use of ground water withdrawals varied between the islands (Figure 11). Oahu used most of its water towards public supply, where as Maui County and Kaui used more than half of its total withdrawals for agriculture (Figure 11). Over withdrawing can cause saltwater intrusion.
Ground water contamination is anthropogenically induced. Agricultural activities have the largest impact on contamination; pineapple and sugarcane cultivation have contaminated the greatest number of well sites. These contaminates are in low concentrations, thus they are not a huge concern for Hawaiians.
All figures are from the USGS.
References:
Fetter, C. W.Applied Hydrogeology. Upper Saddle River, NJ: Prentice Hall, 2001.
Print.
"Ground Water in Hawaii."USGS. Web. 12 Apr. 2012.
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"HA 730-N Hawaii Ground-water Problems Text."USGS. Web. 12 Apr. 2012.
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"HA 730-N Hawaii Regional Summary Text."USGS. Web. 12 Apr. 2012.
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"HA 730-N Hawaii Volcanic-rock Aquifers Text."USGS. Web. 12 Apr. 2012.
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"Hawaiian Volcanic-rock Aquifers Extent."USGS. Web. 12 Apr. 2012.
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