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1、地 下 水从地表下面提供水的水源叫做地下水。在美国地下水的储量相当大，以至于超过了所有人工地上水与自然地上睡的储量，一般来说，地下水的分布可分为通气层和水饱和层两类。在水饱和层，由于吸湿作用，所有的孔隙杯水充满者。而在通气层中，孔隙中充满着部分空气和部分水。通气层可细分为三层。（1）湿土层。他从地表开始往下延伸到主干层。湿土层的总深度是不断变化的并取决于土壤类型及其植被。在湿土层除了重渗透外，是不能达到饱和的。在这一层，水可以分为以下三类：从空气中吸收而来的湿水；由表面张力保持的毛细管水。还有从土壤中吸取的大量的重力水。（2）中间层从湿土层底部一直延伸到毛细管作用层的顶部。并且从不存在到几百英
2、尺的厚度不断变化。中间层是近地面地区和近地下水位区的一个重要连接环。渗透水就是从这一环节通过。（3）毛细管作用层从地下水位延伸到一定的厚度，这一厚度取决于从土壤中吸收毛细管的高低。毛细管作用层的厚度随土质的变化而变化，不仅因地域不同而变化还可能在同一地方其厚度也会不同。由于重力作用，从土壤中吸收的水叫给水度。给水度可以表示为由重力汲取水量与土壤总量之比。给水度的值取决于土壤颗粒的大小，形状孔隙分布的密度及土壤的压实程度。含水层的给水度的平均值在10%到20%之间。含水层是一个能保持水的岩层或者说是一个能够传输足量的水以供水的流动和循环的形成层。含水层可以被看做两种：受限制的和不受限制的。这取决
3、于在大气压下地下水或者自由水面是否存在。在含水层，水可能得到补给或者从含水层排出。每当这时含水层的储水量的细微变化。在这种情况下，覆盖层的重量部分是由流体静压力支撑，部分是由含水层的固体物质支撑。受限的含水层中的流体静压力由于用泵抽水或其他方法而降低，那么含水层的负荷就会增加。造成了含水层的压缩。导致一些水从受限含水层中排出。降低流体静压力还会造成受限含水层微小的膨胀。其结果是导致另一些水分的流失。对于受限含水层来说，给谁的容积度是由储水系数来表示的。这个存储系数可以被定义为含水层所存储的水的容积或含水层每单位表面积、垂直于地面的水头的每单位变化释放的水量。除了能够提供让人满意的给水度率的持水
4、岩层外，还有不持水岩层和不透水岩层。隔水层就是一种不透水岩层，可以容纳大量的水，但是他没有足够的导水速率以使的水得到高效发展和利用。无水层是一种不透水并且不含水的底层。在任何情况下，只要改变压在地下水上的压力，就会导致地下水位的变化。季节因素，江河水位变化，蒸发，大气压变化，风力，潮汐，外部压力，抽水及补水的各种形式以及地震，所有这些都可以引起地下水位的变化或测压水面的变化。这取决于岩层的自由还是受限的。对于一个从事有关地下水供给的发展和利用工程师说，时刻注意这些因素非常重要，工程师还应该能够判断库操作的重要关系。上诉因素与具体地下水。水在地下流动的速率与水在自然或人工制成饿水渠和水管中流动的
5、速率具有完全不同的重要性。具体的速率值从每天5英尺到每年几英寸不等。地下水的收集最初是通过修建水井和渗透坑来完成的。在这些收集工作能力的数字评估涉及到很多因素，有些适用于用相对简单的数学表达式来解决的。系统考虑起来，可能要有三部分组成。井的结构，抽水机和排水管道系统。就井自身而言，它就包含一个敞开的部分进入井内并且通过管被输送到地表，敞开的部分通常是一个表面有许多小孔饿管或者是开有沟槽饿铁板，这块铁板允许水流进入的同时也阻止了小孔的破损。有时候一些沙砾在井管的底部堆积没把铁板围住。当从井中抽水，水就立刻从铁板附近的含水层中流出。然后水流就在离井有一段距离的地方变得固定起来，以确保对这次抽水的补
6、给。由于土壤对水流的阻力，抽水会有很多损失并且附近抽水井的量压面就会凹陷。这被称作降混漏斗。降混漏斗会一直扩大直到均衡的状况达到稳定饿环境确定为止。地下水的质量受源地的影响特别大。水源的变化或是源地供水的降级质量都可能严重地损害地下供水的质量。进入含水层的城市和工业废弃物是有机污染和无机污染饿主要来源。然而，由于大量的有机废物通常不易进入到地下，所以大规模的地下水有机污染不会经常发生。而有关无机污染的问题就不同了，这是因为无机污染会很容易穿梭于土壤并且一旦进入土壤就会费很大劲才能被清除。另外，这种无机污染的不良影响会无限期地进行，因为稀释非常慢，并且人工冲洗或处理往往是行不通或者是造价太高。当
7、水流从土壤中流过，溶解盐的数量会大大增加，降水和水流会造成土壤饿风蚀和侵蚀，然后形成可溶物，这些溶解盐被这些可容物吸收。从大水漫灌的下游地区可以发现，浇灌的水太咸，而达不到令人满意饿庄稼收成。这些含盐的污染物很难控制，因为驱除方法过于昂贵。一个可能解决办法就是用含盐污染物较低的水来稀释（例如，废水处理工厂的污水）。这样通过混合产出的平均盐度的水适合于灌溉。消除废弃物，保护地下水储量免于不可治愈的伤害，还需要大量的努力。由于地下水流动速度相当缓慢以及极少有地表水入渗到地球内部，每年地下水中只有一部分才得以通过自然物理过程得以替换。为了弥补这种自然回灌过程的不足，近年兴起一种人工回灌的新潮流。例如
8、，在加利福尼亚，人工回灌是现在蓄水饿一种主要方式。人们在人工回灌操作时采用很多方法，其中最常见的方法之一就是使用蓄水池蓄水。通常的做法是把水拦截到一连串事先已建好的蓄水池中，因此水从一个蓄水池里流出来将会进入下一个蓄水池，以此类推。这些人工蓄水工程通常得挖建沟壕或者筑堤。另一种方式是改良河床，这种方式运用的是自然水供给。人们将河床拓宽，整平毁掉，或者是采用一系列方法去增加它的蓄水量。沟和槽在这里是被应用：沟梁布设的基本类型有等高线型，即沟梁沿地表面等高线布设；侧向型，即把水从侧向进入,引入许多小沟梁中：树枝状货分支型，即把水从生河道引入连接在一起饿较小渠和沟中。当坡度相对平缓和一致是，洪水就是
9、一种经济的回灌方式。通常的做法是将相对较少的回灌水撒于地表上，这样做就不会破坏土壤货原始植被。另一种做法就是使水灌溉井。在基本条件相同时，灌溉井饿回灌效率通常要低于水泵的灌溉效率，这是因为在临近水井的区域经常会形成堵塞，截留含水层颗粒在回灌井中被滤出的并且滤器附近沉淀下来的悬浮物，气塞，会灌水于天然水体之间的化学反应均可造成阻塞。因此最好的回灌水应该是清洁的，不含或含少量而且已经消过毒的水。The Ground-waterThe ources of water which supply water front below the earths surface are called sub-su
10、rface 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 categorize into zones of aeration and asterisked. The saturated zone ia one in which all the voids are filled
11、 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 zone begins at the ground surface and extends downward through the major root zone of fire. Its total depth is v
12、ariable and dependent on soil type and vegetation.The zone is unsaturated except during period of heavy infiltration.Three categorise of water calssification may be encountered in this regional: hygroscopic water content, which is adsorbed from the air separation; capillary water rat, whiche is held
13、 by suiface tension;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 capillary fringe and may vary from nonexistence to several huntween the near-ground suiface region and the near
14、-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 and may vary not ongly from region to region but also within a local area network. The water that can be dra
15、ined 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 specific yield are dependent on soil particle size average, shape and distribution of pore, and degree of comple
16、tion 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 sufficient to permit development.Aquifers may be considered as falling into two categorise,confined and unco
17、nfined,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.For saturated,confined aquifer,pressure changes produce only slight changs in storage volume.In this cases, the weight of the overburden is supposed partl
18、y 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 increase, causing its compressional, with the result that some water is forced from its. Decreasing the hydrostat
19、ic 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. This strong coefficient may be defined as the volume of water that an aquifer takes in or relleases per uni
20、t 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 impermeable strata.An aquiclude is an impermeable stratum that may contain large quantities of water but whose trans
21、mission rates are ot high enough to permit effective development.An aquifuge is a formation that is impermeable and devoid of waters. Any circumstance that alters the pressure imposed on underground water will also cause a variation in the groundwater level.Seasonal factorshare, change in stream and
22、 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 water table or the piezometric surface,depending on whether the aquifer is free or confined.It is impor
23、tant 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 of a specific groundwater basin.The rate of movement of water through the ground is of an entirely diff
24、erent 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 the construction of wells or infiltration galleries.Numerous factors are involved in the numerical est
25、imation 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 graphical analysis or the use of various kinds of models.A well system may be considered to be compose
26、d of three elements:the well structure,the pump,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 groung surface.The open section is usually a perforated casing or a slotted metal screen that permit t
27、he 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 aquifer immediately adjacent to the screen.Flow then becomes established at locations some distance from t
28、he well in order to replenish this withdrawal.Owing to the resistance to flow offered by the soil,a head loss is encountered and the piezometric surface adjacent to the well is depressed.This is known as the cone of depression.The cone of depression spreads until a condition of equilibrium is reache
29、d 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 seriously impair the quality of the groundwater supply.Municipal and industrial wastes entering an aqu
30、ifer are major sources of organic and inorganic pollution.Large-scale organic pollution of groundwater is infrequent,however,since signficant quantities of organic wastes usually cannot be easily introduced underground.The problem is quite different with inordinance are removed only with great diffi
31、culty.In addition,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 passes through the soiled, a significant increase in the amounts of dissolved salt may occur.Theses s
32、alts 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 saline for satisfactory crop production.These saline contaminates are different to control because r
33、emoval 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 water produced by mixing will be suitable for use.Considerable care should be exercised to protect
34、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 slow rates of movement of groundwaters and the limited opportunity for surface waters to penetrate th
35、e earths surface.To supplement this natural recharge process,a recent toward artificial recharge has been developing.In California,for example,artificial recharge is presently a primary method of water conservation.Numerous methods are employed in artificial recharge operation.One of the most common
36、 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
37、streambed,which makes use of the natural water supply.The stream channel is widened, leveled,scarified,or treated by a combination of methods to increase its recharge capability.Ditches and furrows are also used.The basic types of arrangement are the ground;the lateral type,in which water is diverte
38、d into a number of small furrows from the main canal or channel;and the tree-shaped or branching type,where water is deferred from the primary channel into successively smeller canalis and ditcher.Where slopes are relatively flat and uniform,floodiing provides an economical means of recharge.Normal
39、practice is to spead the recharge water over the ground at relatively small depths so as not to disturb the soil or native vegetation.An additional method is the use of injection wells.Recharge rates are normally less than pimping rates for the same head condition,however,because of the clogging tha
40、t is often encountered in the area adjacent to the well casing.Clogging may result from the entrapment of fine aquifer particle, from suspended material in the recharge water which is subsequently strained out and deposited in the vicinity of the well screen,from air binding,from chemical reactions between recharge and natural water,and from bacteria.For best result the recharge water should be clear,contain little or no sodium,and be chlorinated.