# Memoir on the Island of Navassa (West Indies)/Research for Fresh Water

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IV.—Research for Fresh Water.
J. D. Dana states that in coral islands water is to be found commonly in sufficient quantity for the use of the natives, although the land is so low and flat. They dig wells five to ten feet deep in many points of the dry islets and generally obtain a constant supply. These wells are sometimes fenced around with special care, and the houses of the villagers, as at Fakanto, are often clustered about them. On Aratica (Carlshoff) there is a watering place 50 feet in diameter, from which several hundred gallons may be obtained in a few hours.
The Tarawan Islands are generally provided with a supply sufficient for bathing. On Taritari there is a long trench or canal several miles long and eight feet deep. These islands have been elevated a little above the sea, but are not over 15 feet above the sea.
Kotzebue states that in the interior of Oldia, one of the Marshall Islands, there is a lake of sweet water, and in Tabual a marshy ground exists. There is no want of fresh water in the largest islands; it rises in abundance in pits dug for the purpose.
The only source of this water, says Dana, is the rain which percolating through the loose surface settles upon the harden-ed rock that forms the basis of the island; as the rock is white or nearly so, it receives the heat but slowly, and there is consequently but little vaporization of the water that is once absorbed.
What may be my deference for the opinions of so eminent an authority as Prof. J. D. Dana, I must say that I cannot accept this explanation for the presence of fresh water on the coral islands, considering the nature of such formation so full of cavities of every size through which the fresh water percolates with all facility to the level of the sea. By far more rational seems to be the explanation of this same phenomenon by R. J. Nelson, who by several years of residence in the West Indies had a full opportunity to observe and study this important fact.
R. J. Nelson in his memoir on the geology of the Bermudas, states that fresh water may be had in nearly all parts of the ​islands, provided the bottom of the well is not sunk under the level of low tide.
Generally speaking it is deemed prudent to be contented with twelve or eighteen inches of water, rather than incur the risk of rendering the well brackish, since from the porous nature of the rock the salt water always filtrates through it.
At the Naval Wells where large quantities of water are required for shipping, it is only taken between the half flood and half ebb, to avoid the brackish quality of water drawn at improper seasons, for all the wells on the sea coast are affected by the tide, and more or less so those at a distance from it.
Again, Nelson in his memoir on the Bahamas, has recorded his observations relative to the occurrence of fresh water in those islands, and says: It appears that from the universally porous character of the constituent rock, all the rain that falls passes directly down, percolating through it to nearly the sea level. Here it meets with a body of salt water that permeates the lower portions of the rocky structure, and from its lighter specific gravity the fresh water floats upon the sea water, rising and falling with the tide, not contemporaneously at all points, but at a time peculiar to each spot, according to its distance from the sea, and the more or less porous condition of the intervening stone.
In digging wells, therefore, the shaft is not carried lower down after the first appearance of the "spring" than is necessary to give room for the bucket, so that the salt water may not follow the fresh water into the well. Both salt and fresh water may thus be most certainly met with by sinking a pit at any spot or in any variety of the rock; from the dense kind in the large islands to the mere isolated sand bank, such as at the southwest of Memory rock and Little Bahama Bank, where the wells are mere pits in the sand, a few feet deep.
This explanation of Nelson seems the more conclusive, seconded as it is by the repetition of the same fact in so many distant coral islands. It is well known, and some very simple experiments will convince every one, that with proper attention and patience in the absence of all agitation, a liquid of less specific gravity may be deposited on the top of another one of more gravity, and when the first sheet has been successfully ​deposited, the thickness of the lighter liquid may be increased with less care; the two liquids remaining unmixed for a length of time, a drop of the upper liquid may then easily be sucked through a hollow tube. On a large scale the operation becomes more easy, and we may readily understand how it is successfully enacted in the deep recesses formed by the coral rock, where a lasting tranquillity opposes the mixture of the fresh and salt water, when the rain water percolating in minute drops, gently settles on the top of the sea brine.
Therefore the existence of this porous stratum of lime rock near the level of the sea, that I considered at first as an objection to the existence of a fresh water level, probably has a most important part to play, by being, as it were, an immense filter through which the rain water slowly drips on the top of the sea water, contributing more effectively to this separation of the two liquids, which explains so satisfactorily the presence of fresh water on coral islands.
Consequently the researches for fresh water on the coral islands must be directed by principles entirely different from those in other geological formations, and may be undertaken on the Island of Navassa with justifiable expectations of success.
Besides the general features that I have described at some length on account of their great importance, several local facts point out at Navassa where the first trials ought to be commenced with the most probable success.
The existence of many caves or holes through which the rain water evidently passes to the lower levels, and the existence of the water cave or "spring" to which I have alluded before, are points near which the first diggings ought to be done. This water cave is found on a line of depressions passing through the shore platform and corresponding with other depressions on the summit of the Island. On this course at the foot of the upper cliff, the first well ought to be started.
This experiment, if successful, will be of so much importance to the Island, and the source of so great an economy and comfort; in case of failure would not be without much advantage to the interests involved. Through this well it would be possible to pump the water running and lost to-day through the water cave; besides this boring, if properly conducted, ​would be most important for the confirmation of my views of the structure of the Island, the character and depth of the deposits of the Phosphate, and consequently for the best system of working the diggings. For these reasons I most earnestly recommend this trial so important as a scientific and more so as an economical question.