1、外文资料 The Ground-water The ources of water which supply water front below the earths surface are called sub-surface sources or ground-water source.Groundwater storage is considerably in excess of all artificial and nature surface storage in the United States.Groundwater distribution may be generally
2、categorize into zones of aeration and asterisked. The saturated zone ia one in which all the voids are filled with water under hydrostatic pressure.The aeration zone in whiche the interstices are filled partly with air and partly with waters, may be subdivided into three subsonic. The soil-water zon
3、e begins at the ground surface and extends downward through the major root zone of fire. Its total depth is variable and dependent on soil type and vegetation.The zone is unsaturated except during period of heavy infiltration.Threecategorise of water calssification may be encountered in this regiona
4、l: hygroscopic water content, which is adsorbed from the air separation； capillary water rat, whiche is held by suifacetension；and gravitational waters, which is excess soil water draining through the soiled. The intermediate zone extends from the bottom of the soil-water zone to the top of the capi
5、llary fringe and may vary from nonexistence to several huntween the near-ground suiface region and the near-water-water table region through which infiltrating waters must passed. The capillary zone extends from the water table to a height determined by zone thickness ia a function of soil texture a
6、nd may vary not ongly from region to region but also within a local area network. The water that can be drained from a soil by gravity is known as the specific yielding. It is expressed as the ratio of the volume of water that can be drained by gravity to the gross volume of the soil.Values of speci
7、fic yield are dependent on soil particle size average, shape and distribution of pore, and degree of completion of the soiled. Average values of specific yield for alluvial aquifers range frome 10% to 20%. An aquifer is a water-bearing stratum or formation capble of transmitting water in quantities
8、sufficient to permit development.Aquifers may be considered as falling into two categorise,confined and unconfined,depending on whether or not a water table or free within an aquifer is change whenever water is recharged to or discharged from an aquifer.Forsaturated,confinedaquifer,pressure changes
9、produce only slight changs in storage volume.In this cases, the weight of the overburden is supposed partly by hydrostatic pressure and pattly by the soild material in the aquifer.When the hydrostatic pressure in a confined aquifer is reduced by pumping or other means,the load on the aquifer increas
10、e, causing its compressional, with the result that some water is forced from its. Decreasing the hydrostatic pressure also causes a small expansione, which in turn produces an additional release of water.For confined aquifer, the water yield is expressed in terms of a storage coefficient Scarcely. T
11、his strong coefficient may be defined as the volume of water that an aquifer takes in or relleases per unit surface area of aquifer per unit change in head normal to the surface.In addition to water-bearing strata exhibiting satisfactory rates of yield,there are also non-water-bearing and impermeabl
12、e strata.Anaquiclude is an impermeable stratum that may contain large quantities of water but whose transmission rates are ot high enough to permit effective development.Anaquifuge is a formation that is impermeable and devoid of waters. Any circumstance that alters the pressure imposed on undergrou
13、nd water will also cause a variation in the groundwater level.Seasonalfactorshare, change in stream and river stages,evapotranspiration,atmospheric pressure change, windsor, ides, external load,various forms of withdrawal and recharge,and earthquakes all may produce fluctuations in the level of the
14、water table or the piezometricsurface,depending on whether the aquifer is free or confined.It is important that the engineer concerned with the development and utilization of groundwater supplies be aware of these factors.He should also be able to evaluate their important relative to the operation o
15、f a specific groundwater basin. The rate of movement of water through the ground is of an entirely different magnitude than that through natural or artificial channels or canddits.Typical value range from 5 fr/day to a few feet per year.The collection of groundwater is accomplished primarily through
16、 the construction of wells or infiltration galleries.Numerous factors are involved in the numerical estimation of the performance of these collection works.Some cases are amenable to solution through the utilization of relatively simpie mathematical expectation.Other cases can be solved only through
17、 graphical analysis or the use of various kinds of models.A well system may be considered to be composed of three elements:the well structure,thepump,and the discharge piping.The well itself contains an open section through which flow enders and a casing through which the flow is transported to the
18、groungsurface.The open section is usually a perforated casing or a slotted metal screen that permit the flow to enter and at the same time pervents collapse of the hole.Occasionally gravel is placed at the bottom the well casing aroung the screen.When a well is pumped,water is removed from the aquif
19、er immediately adjacent to the screen.Flow then becomes established at locations some distance from the well in order to replenish this withdrawal.Owing to the resistance to flow offered by the soil,ahead loss is encountered and the piezometric surface adjacent to the well is depressed.This is known
20、 as the cone of depression.The cone of depression spreads until a condition of equilibrium is reached and steady-state conditions are established. Groundwater quantity is influenced considerably by the quality of the source.Changes in source waters or degraded quality of source source supplies may s
21、eriously impair the quality of the groundwater supply.Municipal and industrial wastes entering an aquifer are major sources of organic and inorganic pollution.Large-scale organic pollution of groundwater is infrequent,however,sincesignficant quantities of organic wastes usually cannot be easily intr
22、oduced underground.The problem is quite different with inordinance are removed only with great difficulty.Inaddition,the effects of such pollution may continue for indefinite periods since dilution is slow and artificial flushing or treatment is generally impractical or too expensive.As the water pa
23、sses through the soiled, a significant increase in the amounts of dissolved salt may occur.Theses salts are added by soluble products of soil weathering and of erosion by rainfall and flowing water.Locations downstream from heavily irrigation areas may find that the water they are receiving is too s
24、aline for satisfactory crop production.These saline contaminates are different to control because removal methods are receive methods are exceedingly expensive.A possible solution is to dilute with water of lower salt concentration(wastewater treatment plant effluent,for example)so that the average
25、water produced by mixing will be suitable for use.Considerable care should be exercised to protect groundwater storage capacity from irreparable harm through the disposal of waste materials. The volumes of groundwater replaced annually through natural mechanisms are relatively small because of the s
26、low rates of movement of groundwaters and the limited opportunity for surface waters to penetrate the earths surface.To supplement this natural recharge process,a recent toward artificial recharge has been developing. In California,forexample,artificial recharge is presently a primary method of wate
27、r conservation.Numerous methods are employed in artificial recharge operation.One of the most common plans is the utilization of holding basis.The usual practice is to impound the water in a series of reservoir arranged so that the overflow of one will enter the next,and so on.These artificial storage works are generally formed by the construction of dikes or levee.A second method is the modified streambed,which makes use of the natural water supply.The stream channel is
