THE ARTESIAN CONDITIONS.
Oahu presents two series of diversified sheets of rock dipping gently toward the sea from high central points ; but the material is volcanic. In the early days successful artesian wells had been sunk through sedimentary strata, whence it was inferred that it would be useless to attempt borings in the so-called unstratified rocks. Many were dissuaded from such attempts by that con sideration, yet any geologist would quickly observe the re semblance between these volcanic sheets and a nearly horizontal stratification. There is an alternation of hard basaltic sheets, volcanic clays, ashes, and sometimes limestones which offer the necessary condition for subterranean currents—as they dip gently outwardly on all sides.
The meterological conditions explain the source and spread of the waters. Rain is profusely abundant on the highlands. The trade winds, laden with moisture, drop their burdens on coming in contact with the land surfaces. The maximum rainfall is at the altitude of about 1,200 feet. The preponderance of the dis charge, being upon the windward side, determines the place of the most copious streams and the more effective erosion. Hence the domes have been worn away unequally. One side may be entirely removed, and the other be scarcely affected at the sur face. If the ridge is narrow at the altitude of greatest precipi tation both sides will be extensively worn down. This is well shown on the Koolau upland, where the southeast end has been greatly denuded upon both sides from Mokapu point to the Pali, while to the north, at a greater height, the canyons are less con spicuous on the west side.
The laying bare of the interior of the dome allows the water to sink into the pervious layers, and to flow beneath the surface towards Kaala and the southwest. Only the needful alternation of pervious and impervious strata is necessary to give rise to the subterranean streams which will send water to the surface when pierced by the artesian wells.
The borings upon Oahu prove the alternation of basalt, clay, earth and limestone to the depth of several hundred feet. The principal water-bearing stratum is a very porous basalt, from three hundred to four hundred feet below the sea level by the shore. It has a hard, impervious cover, sufficiently tight to pre vent the passage of water through it. The following general statements concerning the artesian conditions seem to be well es tablished: 1. The presence of a porous water-bearing stratum beneath an
impervious cover.
2. Water is reached usually at the depth of from three hun dred to five hundred feet.
3. The water flows freely without pumping only in a narrow belt of territory adjacent to the coast line, where the surface is but slightly elevated ; which is forty-two feet at Honolulu, thirty two feet at Ewa and twenty-six feet at Kahuku, at the northeast angle of the island. Wells sunk in higher ground shows the water rising to the level of forty-two feet at Honolulu, above which it will discharge only by the application of a pump.
4. For convenience in obtaining a proper supply several wells are sunk adjacent to each other. Naturally, as development takes place, the number of the wells increases. Thus the Ewa planta tion had at first six ten-inch wells some thirty feet apart con nected by a single pump, which lifted the water about sixty feet. Later the wells are a foot in diameter in groups of ten for each pump. The water is forced through steel pipes twenty-four and thirty inches in diameter to a maximum elevation of five hundred feet. From various points ditches are dug which carry the water to every field of the plantation. Though the pumps act without cessation, the water never fails ; 5,00o acres of land are irrigated from these wells.
5. These wells at Ewa are found to be slightly affected by the brine of the sea. The natural waters of the island contain .0073 per cent. of salt according to Dr. Walter Maxwell ; 6 Pa cific water holds 2.921 per cent. of the same. One hundred grains to the gallon of water represents 0.14 per cent. The analyst of the Ewa company found that the chlorine present (sodium chloride) was more abundant in the wells nearest the ocean. At station No. r the chlorine amounted to 17.61 grains in a gallon. At stations Nos. 2 and 3, farther inland, the chlorine had dimin ished to 8.18 and 11.97 grains to the gallon. By experiment at several localities it has been found that the salinity increases when the pumping becomes excessive. At Ewa it is stated that vegeta tion is not at all affected when the number of grains per gallon is less than sixty. At Molokai, where the salinity is greater, it is stated that the cane is not affected unless the number of grains per gallon exceeds one hundred.