Popular Science Monthly/Volume 6/February 1875/Water-Supply of Islands

From Wikisource
Jump to navigation Jump to search



ON islands of considerable size and height, composed of rocks and various earthy beds, springs of fresh water in the valleys are not uncommon, and their presence excites no remark. The rainfall of the island itself is laid up in its strata exactly as in the hills of the main-land, and the small size of the reservoir is made up for by the frequent rains and fogs to which islands are subject.

There are cases of islands near the main-land where springs are fed by streams from the continent following the rock-strata below the dividing straits. But on islands composed as many of those on our Southern coasts are, of pure sand and of very small elevation, and hence with no raised reservoir to supply springs, the fact that pure fresh water may be obtained in large quantities by digging, is a mystery even to many well-informed people, although the explanation is very simple. To say that the "sea-water filters through the sand into these wells and becomes as sweet and pure as spring-water," is simply to display profound ignorance of chemistry and facts.

From time immemorial the ash-leach has been in use in many civilized, that is, soap-making, countries. Essentially an ash-leach is a vessel tight enough to hold wood-ashes, but not tight enough to hold water. Being first filled with ashes, water is then poured in gradually, and, after a time, runs out below, highly charged with the soluble salt of the ashes.

But, although this machine has been so long in use, the principle on which it acts does not seem to have been fully understood until quite lately. About the year 1833 Messrs. Boullay, of Paris, applied the same apparatus to the manufacture of pharmaceutical preparations, tinctures, infusions, extracts, etc., and it at once became popular with pharmaceutists, under the name of Boullays' Filter, or the Displacement Apparatus. Its mode of operation is simple: the first portion of liquid poured in sinks into the powder that is to be exhausted, and saturates itself with the soluble parts of it. The later additions of liquid, instead of mixing with the first, drive it down before them and take its place, to yield it in turn to the next portions poured in. Thus the first portions of liquid that run from the bottom of the filter will, if it has been properly managed, contain nearly all the soluble matter, and the last will be almost unchanged. For example, if an ounce of powdered ginger be put into a glass tube, as a small lamp-chimney, over the lower end of which a piece of cotton cloth has been tied, and alcohol be slowly poured through it, the first fluid-ounce (f ℥ j) that comes through will contain about all the strength of the ginger. Looking through the glass, we can watch the whole process, see the first alcohol dissolve the resinous matter of the ginger, becoming thick and dark-colored in consequence, and then falling down before the new colorless alcohol added above.

Applying this principle of displacement to a sand-island, we have to start with a great heap of sand with its level or hollow top just above the reach of the waves, while the great mass of sand is below the sea-level. Of course all the submerged part is full of salt-water, while capillary attraction carries some of the water above the sea-level. The first rain that falls sinks at once into the loose sand. It rains very heavily in the gulf islands, but I never saw water run off of the surface; it sinks at once, and, in sinking, drives the salt-water before it, "displaces" it as the apothecaries say, but does not mix with it. Repeated rains continue the operation until there is a lake of fresh water held by the sand in the midst of the sea.

Fine sand will hold, between its grains, over one-third of its bulk of water; thus, within twelve feet of the sea-level, we have a lake of fresh water four feet deep, or about 1,250,000 gallons to each acre of island, and this is the supply reached by digging. But, should a long drought occur, evaporation would rapidly reduce the supply, and the sea-water following in will take its old place. Hence it was that, in 1864, we had plenty of good water at Ship Island and Santa Rosa, while at Brazos Santiago, where no rain had fallen, at the time of my visit, for ten months, the water was brackish and unfit for use, and the government was obliged to distill water from the sea for the use of the army, and the only vegetation on the island appeared to be

cactus and mesquite, showing that drought was not of rare occurrence there.

Our men encamped on Ship Island complained that the water in many of the wells which they dug soon became bad. Perhaps this was owing to the surface-water of the swamps getting into them, or perhaps the drainage of the camps. Those who know the habits of our men in camp can judge for themselves.

A glance at the map and profiles of Ship Island will show how admirably the island is adapted to collecting and holding rain-water. The broader parts of the island are completely surrounded by a raised beach, making large basins. The basin west of the lighthouse is comparatively new. All the islands in that chain grow at their western ends and wash away on the east. The only vegetation in this western basin was, at the time of my sojourn there, a few low, creeping herbs. The beach was so low that a heavy September gale blew the waves over it, and the whole basin became a salt lake, around-which I walked the next day on the beach. The sea having by that time fallen rather below its usual height, there was a difference of several feet between it and the surface of the lake. Observing that at one place the water was just level with the top of the beach, I scratched a shallow channel across it with a stick. In five minutes a strong brook was running out, and, in ten more, a roaring river. This part of the island being subject to such overflows, of course no wells are sunk in it. The fort depends on cisterns of rain-water; but it would be easy to run a pipe underground to a well above the lighthouse and get plenty of water.

The basin east of the lighthouse was a swamp in my time, thickly overgrown with grasses. Afterward, the commandant had it ploughed and made into a garden, said to have been very productive, especially in melons. The neck, of course, was barren, shifting sand, while the large basin beyond was not only swampy, but had a fresh pond in it. The drier parts were covered with a beautiful purple-topped grass, and had some showy flowers; while the wetter parts, and even the pond, were thickly set with woody shrubs of considerable size, and the wide, shallow mouth of the pond was so obstructed by them that the quiet waters of the Mississippi Sound never penetrated it. Beyond the pond were formerly some live-oaks and pines; but the oaks had all been cut down, and the pines were rapidly following them, being carried off for fuel by the prison-camp near the light-house. All around this end of the island, stumps and dead trees standing far out in the water showed that the sand was being gradually swept away by the waves.

Santa Rosa is Ship Island on a larger scale. It has several freshwater ponds, and many bushes and trees. On its barren western end I obtained an abundance of good waiter by sinking a well some four or five feet. The wooden curb was built larger at the bottom, and set in a shallow pit scooped in the sand. A man was then put inside to shovel out the sand, and, as he dug, the curb sank around him. Presently he was waist-deep in water, and the well was finished and yielded freely all summer.