By William Meyer1
The U.S. Geological Survey has recently published the results of a study that describes a new form of ground water occurrence in Hawai`i. As described in this study, ground water in the area between Ke`anae Valley and Kuhiwa Stream in northeast Maui occurs as a vertically extensive fresh water body extending from below sea level to altitudes in the hundreds and thousands of feet. This type of ground water body has also been recently discovered by the USGS in the Lihu`e Basin in Kaua`i. Prior to the USGS studies this type of ground water occurrence had not been reported for the islands and their discovery in two areas of the state suggests the possibility that they may occur in other areas. For instance, available data suggest that vertically extensive ground water bodies may underlie areas of Kaua`i outside of the Lihu`e Basin.2
One of the results of these studies is that the relationship between ground water and surface water in an area underlain by a vertically extensive groundwater body is significantly different from that previously assumed. Earlier descriptions of ground water occurrence for the two areas held that pumping from a well constructed to a depth near or below sea level would not affect overlying streams. Because a vertically extensive ground water body underlies the areas, however, pumping from a well constructed to depths near or below sea level will affect or reduce the flow in overlying streams.
Basal Ground Water
Prior to the USGS studies, the general modes of occurrence of ground water in Hawai`i have been divided into basal and high-level ground water bodies. Basal ground water is characterized as a lens-shaped water body floating on saltwater. The altitude of the uppermost surface of a basal lens (its water table) is generally considered to be below 50 feet. In general, pumping from a basal water body is not expected to affect an overlying stream or spring if the streambed or spring is higher than the water table. This is generally true except at or near the shoreline.
Perched Water
High-level groundwater bodies are found at altitudes higher than basal water bodies. The water table of high-level water bodies can be in the hundreds to thousands of feet. High-level water bodies are generally described as either perched or dike impounded. Perched water bodies occur where a rock layer of low permeability impedes the vertical movement of ground water. Water bodies above these layers are considered to be perched and the rock below them is considered to be unsaturated. The major feature that has been used to identify perched water bodies has been high level springs. An abrupt decline of the water level in a well that occurs over a few feet as the well is being drilled is also assumed to indicate the presence of a perched water body. It is generally assumed that a basal water body underlies perched water bodies and that pumping from the basal aquifer will not affect overlying streams or springs.
Dike-Impounded Water
Dikes are thin, near vertical sheets of low permeability rock that intrude existing rocks. In some areas of Hawai`i, there can be as many as 2,000 dikes in a linear mile. Dikes can intersect and also extend over long distances. Their presence impedes the horizontal movement of ground water, thereby creating dike-impounded water bodies. Water from dike-impounded water bodies often discharges into overlying streams or springs and therefore wells located in these bodies can reduce the flow of both streams and springs. Dike-impounded water bodies can also be in contact with saltwater.
Ground Water Occurrence in Northeast Maui
USGS study results for northeast Maui conclude that ground water occurs under two general conditions: 1) From Ke`anae Valley eastward to and beyond Kuhiwa Stream the area is underlain by a vertically extensive ground water body that extends from below sea level to at least 2,100 feet above sea level and 2) westward from Ke`anae Valley, the area is immediately underlain by a zone of high elevation perched water separated from a basal water body by a zone of unsaturated rock. The USGS also concludes, however, that no reliable water level measurements exist from wells drilled to sea level between Kakipi Gulch and Ke`anae Valley to confirm the study conclusions for this area. Previous reports had concluded that ground water in all of Northeast Maui consisted of perched water bodies underlain by basal water. It was assumed that the springs and streams existed because of ground water discharge from the perched water bodies to them. It was assumed that the springs and streams existed because of ground water discharge from the perched water bodies to them. It was also assumed that withdrawal of water from wells completed to depths near sea level would not affect the flow of streams and springs in the area.
From Ke`anae Valley eastward, the average depth below land surface to the vertically extensive ground water body is about 50 feet. The general movement of ground water is toward the ocean and the major streams and springs in the area. Discharge from the water body occurs to streams, springs, and the ocean. Streams that are sufficiently incised below the land surface intersect the vertically extensive water body at altitudes higher than 2,000 feet and continue to gain flow because of ground water discharge into them at lower altitudes. Because ground water discharges into them, these streams flow year-round except at locations where their flow is diverted into Ko`olau ditch. Because the rocks are fully saturated, withdrawal of ground water from wells open to any part of the water body will reduce streamflow and ground water discharge at the coast. The discharge of nearby springs will also be reduced. Streams east of Makapipi Stream are not sufficiently incised to intersect the vertically extensive water body and therefore do not have persistent flow of perched water bodies underlain by basal water.
The northeast Maui area was thes ubject of the first contested case before the Commission on Water Resources Management. The case involved the proposal of Maui Pineapple Inc. [Maui Land and Pineapple?] to pump Kuhiwa well at a rate of one million gallons per day for about three months of each year. The commission ultimately granted this request. Kuhiwa well is located about two miles inland from the ocean and the town of Nahiku. The well is adjacent to the East Branch of Makapipi Stream at an elevation of 1,396 feet above sea level. It is completed at a depth of nine feet below sea level. At the time of the contested case, most hydrologists believed that the area the well is located in was underlain by perched water bodies and a basal aquifer.
The concern of those contesting the use of the well was that withdrawal of water from it would reduce the flow in nearby streams and springs, including Big Springs. This spring is the biggest spring on Maui. Testimony based on the original interpretation of ground water occurrence in the area stated that withdrawal of ground water from Kuhiwa well would not affect streamflow or the flow of Big Springs. Other testimony held that withdrawal of water from Kuhiwa well would affect nearby streamflow and potentially reduce the rate of discharge from Big Springs.
The results of the recently published USGS study indicate that Kuhiwa well is located in the area underlain by the vertically extensive ground water body and that withdrawal of water from the well will reduce flow to nearby streams, springs, and the ocean. The maximum combined reduction would be equal to the pumpage of the well. These conclusions, however, have yet to be considered by the Water Commission and ground water withdrawal from Kuhiwa well is still permitted.
The USGS concludes that the vertically extensive water body does not exist west of Ke`anae Valley. Here a high elevation perched zone contained within low permeability rocks immediately underlies the area. This water body is encountered a few tens of feet below land surface. A zone of unsaturated rock separates the perched zone from an underlying basal water body. As stated by the USGS, however, no reliable water level data exist for wells drilled in the area between Ke`anae Valley and Kakipi Gulch to confirm the conclusions for the area.
The general movement of water in the perched zone is to streams, springs, and downward to the basal water body. Incised streams gain water from the perched zone but lose water upon flowing onto the unsaturated rock. Streams that remain within the perched zone all the way to the ocean continue to gain perched ground water all the way to the ocean.
Removal of ground water at wells completed in the basal ground water body will reduce the discharge of ground water to the ocean by an amount equal to the withdrawal rate, but would not be expected to affect overlying streams or springs that derive their water from the perched water body. Removal of ground water at wells completed in the perched water body, however, would be expected to reduce the discharge of ground water into nearby streams and/or springs and also the amount of perched water infiltrating downward to the basal water body. The cumulative reduction would be equal to the rate of withdrawal.
- 1.William Meyer, now retired, used to be head of the U.S. Geological Survey in Hawai`i.
2. The Kaua`i study was conducted by the USGS in cooperation with the county of Kaua`i Department of Water and the study on Maui was conducted by the USGS in cooperation with the state of Hawai`i Commission on Water Resources Management and the County of Maui Department of Water Supply. USGS study results for Northeast Maui are documented in Water Resources Investigation Reports 98-4142, 99-4090, and USGS Professional Paper 1618. USGS study results fro the Lihu`e Basin are documented in Water Resources Investigation Report 98-4031. Information on these reports may be obtained from the District Chief, U.S. Geological Survey, 677 Ala Moana Blvd., Suite 415 Honolulu, HI 96813.
— William Meyer
Volume 11, Number 9 March 2001
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