A. Trevor Hodge
All fresh water begins as rainfall, and (apart from modern industrial pollution creating acid rain), as rainfall, all water is completely pure. When it reaches the earth it is then liable to be affected by the various elements that it encounters there. On falling to the ground, the water is stored in three different ways: these are surface water, soil water, and ground water. Surface water is self-explanatory, including lakes and streams. Soil water is water retained in the soil and enabling plants to grow. Ground water is what the layman would call “underground water,” either in the form of underground rivers, or, much more often, absorbed in geologic strata of some porous material (e.g., limestone) that is saturated with water, and known to geologists as an aquifer. It was this ground water that satisfied most of the ancients' hydraulic needs.
True, some of the aqueducts drew their supplies from surface water. At Rome, the Anio Vetus and the Anio Novus, as their names imply, drew their supplies from the Anio River, and at Segovia (Spain) the aqueduct was fed by the Rio Acebeda, a small stream some 12 km south of the city. Indeed, Frontinus assures as that for the first four centuries of its life Rome drew its water from the Tiber, though this emphasises rather its primitive state than its deliberate choice of hydraulic priorities (Fron. Aq. 4). And one point must be noted. Cities, even when built on major rivers for commerce and communications, seldom used river water. For one thing, it was often polluted (that, after all, was there they usually dumped their sewage), and for another, given gravity flow, a river cannot satisfactorily serve a town built on its banks and hence above the water surface. The water has to come from a higher source.
We thus return to springs and the geological principles underlying them. In the Mediterranean area there is a general shortage of surface water, and the water has to be found deeper down;1 in particular
1 “The geological configuration prevalent on most of the ancient Greek lands on
the rim and the islands of the Aegean is a permeable limestone cap superimposed