OCR Text |
Show be determined in specific areas when all of the facts are known. In California, where the great- est use of ground water is made, many ground- water basins have been surveyed and their safe, dependable yield determined. The Behavior of Ground Water The forces of nature are so infinite and varied that there must be great flexibility in all processes of adjustment. No two streams or rivers or bodies of surface waters are just alike, and the same is true of all underground reservoirs and percola- tion of ground water; they differ infinitely. A ground water reservior may be defined as any local body of earth or rock sotonstituted by nature as to be suitable for receiving, storing, and trans- mitting water. It must have a relatively impervi- ous bottom layer or it could not retain the water. Occasionally, as in the Panhandle of Texas, a ground water basin may be closed, but generally it connects with other reservoirs from which it receives input, and by both underground chan- nels and percolation may receive recharge from streams of higher surface level cutting it. On the other hand, such basins lose water by percolation, small channels, and springs, or in crossing rivers of lower surface level, and to other reservoirs with which they are connected. They may lose water through a gully that has cut down to the water table, or by evaporation or transpiration where the ground water surface is close enough to the land surface. A ground water reservoir is therefore dynamic rather than static in character. Its water is in flow like that of surface runoff-but much more slowly: inches a day, feet a day, feet a year. (Ex- ceptions such as the flow that feeds Mammoth Springs, Ark., are rare.) Some of the water a reservoir holds today may have taken years to come from a distant point; some of it may have percolated through from yesterday's precipitation on the surface. With respect to practically all ground water reservoirs, nature has established a reservoir balance, much like the balance of the hydrologic cycle but on a smaller scale. Where the underground flow is substantial, there is op- portunity for man to pump water from the reser- voir that otherwise would be discharged to the streams where, if not used, it is wasted by flow to the ocean. This is without any serious deple- tion of the reservoir. However, if the rate of re- plenishment is limited, it offers the danger of over- pumping and depletion; and if the reservoir is near the ocean, there is danger of contamination by encroachment of salt waters. In any particular reservoir certain other factors are controlling-the earth structure and composi- tion at the site of the reservoir. Such structure and composition determine the holding capacity, condition of discharge and recharge, and flow. The structure and substances of a reservoir may consist of varying proportions of the following earth materials: clay and shale, which are po- rous but not very yielding of water, the water particles adhering to the fine earth particles and not given up readily; fine sand or silt, which, having more permeability, give up water more easily; gravel, commonly very permeable and forming some of our best reservoirs; sandstones, which have both less holding and less yielding capacity but sometimes may be excellent reser- voirs; limestone, when occasionally pierced by channels of many kinds, forming a myriad of in- terconnecting and holding small passages to a few great caverns of immense storage; basalt, likewise with caverns and crevices in many places; and occasional massive rocks with crevices, fracture zones and partings, that are good bearers and holders of water. No two adjoining reservoirs are exactly alike in the proportions and arrangements of these structural substances, and it is these factors, plus the climate, and the permeability of overlying soils, that determine the quantity of water stored, discharged, and recharged. Moraine fills, of glacial origin, present a further condition and prospect. Therefore, each reservoir presents an individual problem of potential use to man. Quantities of water safely drawn from one may not safely be drawn from another; and measures designed by man to promote storage in one may not be suitable and effective in another. 113 |