In limestone regions springs are charged with calcium carbonate (the carbonate of lime), and where the limestone is magnesian they contain magnesium carbonate also.  Such waters are “hard”; when used in washing, the minerals which they contain combine with the fatty acids of soap to form insoluble curdy compounds.  When springs rise from rocks containing gypsum they are hard with calcium sulphate.  In granite regions they contain more or less soda and potash from the decay of feldspar.

The flow of springs varies much less during the different seasons of the year than does that of surface streams.  So slow is the movement of ground water through the rocks that even during long droughts large amounts remain stored above the levels of surface drainage.

Movements of ground water.  Ground water is in constant movement toward its outlets.  Its rate varies according to many conditions, but always is extremely slow.  Even through loose sands beneath the beds of rivers it sometimes does not exceed a fifth of a mile a year.

In any region two zones of flow may be distinguished.  The upper zone of flow extends from the ground-water surface downward through the waste mantle and any permeable rocks on which the mantle rests, as far as the first impermeable layer, where the descending movement of the water is stopped.  The deep zones of flow occupy any pervious rocks which may be found below the impervious layer which lies nearest to the surface.  The upper zone is a vast sheet of water saturating the soil and rocks and slowly seeping downward through their pores and interstices along the slopes to the valleys, where in part it discharges in springs and often unites also in a wide underflowing stream which supports and feeds the river (Fig. 24).

A city in a region of copious rains, built on the narrow flood plain of a river, overlooked by hills, depends for its water supply on driven wells, within the city limits, sunk in the sand a few yards from the edge of the stream.  Are these wells fed by water from the river percolating through the sand, or by ground water on its way to the stream and possibly contaminated with the sewage of the town?

At what height does underground water stand in the wells of your region?  Does it vary with the season?  Have you ever known wells to go dry?  It may be possible to get data from different wells and to draw a diagram showing the ground-water surface as compared with the surface of the ground.