CAUSE OF IRON IN GROUND-WATERS.
Natural sands, gravels, and rocks almost always contain iron, often in considerable amount. The iron is usually combined with oxygen as ferric oxide, and in this condition it is insoluble in water. Water passing through iron containing materials will not ordinarily take up iron. When, however, the water contains a large amount of organic matter in solution, this organic matter takes part of the oxygen away from the iron, and reduces the ferric oxide to ferrous oxide. The ferrous oxide combines with carbonic acid, always present under these conditions, forming ferrous carbonate, which is soluble and which goes into solution.
Surface-waters nearly always carry free oxygen, and when such waters enter the ground they carry oxygen with them, and the organic matters in the water use up the free oxygen before they commence to take oxygen away from the iron of the ground. It is thus only in the presence of organic matters, and in the absence of free oxygen, that the solution of iron is possible. It sometimes happens that the organic matters which reduce the iron are contained in the soil itself, in which case iron may be taken up even by water originally very pure, as for instance, by rain-water.
Generally speaking, iron is everywhere present in sufficient quantity in the strata from which ground-waters are obtained, and wherever the conditions of the organic matters and oxygen necessary for solution occur, iron-containing waters are secured, and the iron is usually present in the earth in such quantity that the water can dissolve as much as it will take up for a long series of years, or for centuries, without exhausting the supply. There is thus little prospect of improvement of such waters from exhaustion of the supply of iron.
The circumstances which control the solution of iron are very complicated and difficult to determine. Wells near a river, and drawing their water largely from it by seepage, are apt to yield a water containing iron sooner or later, especially where the river-water carries a large amount of organic matter in solution. Waters drawn from extensive gravel deposits, in which the water is renewed principally by the rainfall upon the surface of the deposits themselves, often remain entirely free from iron indefinitely. The rain-water is almost free from organic matter, and the air is able to take care of decomposing organic matters in the surface soil, and below this there are no accumulations of organic matter sufficient to cause the solution of iron. Under other conditions there are subterranean sources of organic matter which result in the solution of iron under conditions which, on the surface, appear most favorable for securing good water. Wells are often used for many years without developing iron, when suddenly iron will appear. This appearance of iron is often connected with increasing consumption of water. In some cases it may result from drawing water from areas not previously drawn upon.
When iron once makes its appearance in a water, it seldom disappears completely afterward, although it often fluctuates widely at different seasons of the year and under different conditions of pumping. In some cases a decrease in the quantity of iron is noted after a number of years, but in other cases this does not happen.
In a few cases manganese has been found in ground-waters. Manganese in water behaves much like iron, but there are some points of difference, so that the possibility of the presence of this substance should be borne in mind.
Iron-containing waters are generally entirely free from oxygen, and when first drawn from the ground they are bright and clear and do not differ in appearance from other ground-waters. On exposure to the air they quickly become turbid from the oxidation of the iron, and its precipitation as ferric hydrate. At West Superior, Wisconsin, a water was found containing both iron and dissolved oxygen. It was turbid as pumped from the well. This condition of affairs seemed abnormal, but was repeatedly checked, and the theory was advanced by Mr. R. S. Weston, who made the observations, that it resulted from a mixture in the wells of two entirely different waters, namely, a water resulting from the rainfall on sand deposits back of the wells, containing dissolved oxygen and no iron, and water from the lake which had seeped through the sand, and which contained a considerable amount of iron in solution but no dissolved oxygen. The wells thus drew water from opposite directions, and the two waters were entirely different in character, and the mixture thus had a composition which would not have been possible in a water all of which came from a single source.