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Pearl Harbor, Oahu
PEARL HARBOR AREA GROUND-WATER-LEVEL
SURVEYS, On October 31, 2002 and May 15, 2003, multi-agency ground-water-level surveys were made in the Pearl Harbor area of Oahu, Hawaii. On October 31, 2002, water levels at 24 wells were measured during a two and a half hour time period (9:00 a.m. to 11:30 a.m.). (Water levels measured at two additional wells are not reported because the reference-mark elevations for these wells are not currently known.) On May 15, 2003, water levels at 23 wells were measured between 9:00 a.m. and 11:30 a.m. (The water level measured at one additional well is not reported because the reference-mark elevation for this well is not currently known.) Agencies participating included the U.S. Geological Survey, the Honolulu Board of Water Supply, the Hawaii Commission on Water Resource Management, the U.S. Air Force (through their consultant, URS Corporation), and the U.S. Navy. To avoid errors associated with different measuring tapes, all participating agencies (with the exception of the U.S. Navy, which measured one water level on October 31, 2002) used water-level tapes calibrated with a reference steel tape maintained by the U.S. Geological Survey. However, errors associated with (1) elevations of reference marks for the wells and (2) wells that are out of plumb may be included in the reported water levels. These sources of error are briefly described below. The tapes that were used in the water-level survey measure the distance (d) between a reference mark at the top of the well and the water level in the well. If the elevation (z) (relative to mean sea level) of the reference mark is known, then the water level (relative to mean sea level) can be determined by subtracting d from z. Although reference-mark elevations are measured using surveying equipment, any error in the reference-mark elevation for a well will affect the reported water level (referenced to mean sea level) for that well. Reference-mark elevations were not resurveyed to a common bench mark for the water-level measurements of the Pearl Harbor area. If the tape used to measure a water level does not hang vertically because the well is out of plumb, then the measured depth to water in the well will be too great. This will result in an estimated water level that is too low relative to mean sea level. Two wells measured by URS Corporation have measured vertical-alignment errors that were accounted for in the reported water levels. However, the error associated with vertical alignment of the remaining wells is not known. Water levels measured on October 31, 2002 in the Pearl Harbor area ranged from 13.6 to 20.4 feet above mean sea level. Water levels measured on May 15, 2003 in the Pearl Harbor area ranged from 13.1 to 19.7 feet above mean sea level. In general, measured water levels were lowest near the southeastern and southwestern parts of the Pearl Harbor area and were highest in the inland, northern part of the area. The water-level surveys of October 31, 2002 and May 15, 2003 shared a total of 21 well sites in common. At each of these common sites, the change in water level can be determined. For three wells in the western part of the Pearl Harbor area, water levels changed by 0.1 foot or less. For the remaining 18 common sites, water levels measured on May 15, 2003 were about 0.4 to 1.6 feet lower than water levels measured on October 31, 2002.
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