wells in .the area of about the same depth, and plate 4 which indicates quality of water at different depths in the Everglades. This difference in quality is due to several factors, mainly to the very low permeability of the rocks that underlie the Everglades (the Fort Thompson formation and the Caloosahatchee marl, principally), to the very flat gradients of the water table that cannot force water to move readily in these rocks of low permeability; and to the fact that not enough time has elapsed since the ocean last covered the Everglades area to allow the rain water falling there to completely flush out the remnants of the Pleistocene (Ice Age) sea water. Actually, most of this water has been greatly modified, partly by dilution with fresh rain water and partly by chemical reactions, mainly of the baseexchange variety, with the surrounding rocks and organic matter. There are extensive areas in the Everglades where it is not possible to develop sizeable supplies of suitable ground water.
Quality of the ground water may change locally along arterial canals when the canal water changes. Along the lower reaches of those canals connecting with Biscayne Bay the adjacent ground water changes with advances and retreats of the salt water from the ocean; and along these canals that drain the upper part of the Everglades, notably the North New River Canal
in which the quality of water changes seasonally with discharge, there is a slight local effect when more highly mineralized canal water spreads into the nearby ground; this is especially noticeable in the area immediately upstream from each dam.
Salt Water Encroachment in the Aquifer.--With respect to the encroachment of salt water from the ocean at depth in the aquifer it is significant
to note that in Miami, before the dredging of the drainage canals, perennially