地下水從地表下面提供水的水源叫做地下水。在美國地下水的儲量相當大，以至于超過了所有人工地上水與自然地上睡的儲量，一般來說，地下水的分布可分為通氣層和水飽和層兩類。在水飽和層，由于吸濕作用，所有的孔隙杯水充滿者。而在通氣層中，孔隙中充滿著部分空氣和部分水。通氣層可細分為三層。（1）濕土層。他從地表開始往下延伸到主干層。濕土層的總深度是不斷變化的并取決于土壤類型及其植被。在濕土層除了重滲透外，是不能達到飽和的。在這一層，水可以分為以下三類從空氣中吸收而來的濕水；由表面張力保持的毛細管水。還有從土壤中吸取的大量的重力水。（2）中間層從濕土層底部一直延伸到毛細管作用層的頂部。并且從不存在到幾百英尺的厚度不斷變化。中間層是近地面地區(qū)和近地下水位區(qū)的一個重要連接環(huán)。滲透水就是從這一環(huán)節(jié)通過。（3）毛細管作用層從地下水位延伸到一定的厚度，這一厚度取決于從土壤中吸收毛細管的高低。毛細管作用層的厚度隨土質的變化而變化，不僅因地域不同而變化還可能在同一地方其厚度也會不同。由于重力作用，從土壤中吸收的水叫給水度。給水度可以表示為由重力汲取水量與土壤總量之比。給水度的值取決于土壤顆粒的大小，形狀孔隙分布的密度及土壤的壓實程度。含水層的給水度的平均值在10到20之間。含水層是一個能保持水的巖層或者說是一個能夠傳輸足量的水以供水的流動和循環(huán)的形成層。含水層可以被看做兩種受限制的和不受限制的。這取決于在大氣壓下地下水或者自由水面是否存在。在含水層，水可能得到補給或者從含水層排出。每當這時含水層的儲水量的細微變化。在這種情況下，覆蓋層的重量部分是由流體靜壓力支撐，部分是由含水層的固體物質支撐。受限的含水層中的流體靜壓力由于用泵抽水或其他方法而降低，那么含水層的負荷就會增加。造成了含水層的壓縮。導致一些水從受限含水層中排出。降低流體靜壓力還會造成受限含水層微小的膨脹。其結果是導致另一些水分的流失。對于受限含水層來說，給誰的容積度是由儲水系數(shù)來表示的。這個存儲系數(shù)可以被定義為含水層所存儲的水的容積或含水層每單位表面積、垂直于地面的水頭的每單位變化釋放的水量。除了能夠提供讓人滿意的給水度率的持水巖層外，還有不持水巖層和不透水巖層。隔水層就是一種不透水巖層，可以容納大量的水，但是他沒有足夠的導水速率以使的水得到高效發(fā)展和利用。無水層是一種不透水并且不含水的底層。在任何情況下，只要改變壓在地下水上的壓力，就會導致地下水位的變化。季節(jié)因素，江河水位變化，蒸發(fā)，大氣壓變化，風力，潮汐，外部壓力，抽水及補水的各種形式以及地震，所有這些都可以引起地下水位的變化或測壓水面的變化。這取決于巖層的自由還是受限的。對于一個從事有關地下水供給的發(fā)展和利用工程師說，時刻注意這些因素非常重要，工程師還應該能夠判斷庫操作的重要關系。上訴因素與具體地下水。水在地下流動的速率與水在自然或人工制成餓水渠和水管中流動的速率具有完全不同的重要性。具體的速率值從每天5英尺到每年幾英寸不等。地下水的收集最初是通過修建水井和滲透坑來完成的。在這些收集工作能力的數(shù)字評估涉及到很多因素，有些適用于用相對簡單的數(shù)學表達式來解決的。系統(tǒng)考慮起來，可能要有三部分組成。井的結構，抽水機和排水管道系統(tǒng)。就井自身而言，它就包含一個敞開的部分進入井內并且通過管被輸送到地表，敞開的部分通常是一個表面有許多小孔餓管或者是開有溝槽餓鐵板，這塊鐵板允許水流進入的同時也阻止了小孔的破損。有時候一些沙礫在井管的底部堆積沒把鐵板圍住。當從井中抽水，水就立刻從鐵板附近的含水層中流出。然后水流就在離井有一段距離的地方變得固定起來，以確保對這次抽水的補給。由于土壤對水流的阻力，抽水會有很多損失并且附近抽水井的量壓面就會凹陷。這被稱作降混漏斗。降混漏斗會一直擴大直到均衡的狀況達到穩(wěn)定餓環(huán)境確定為止。地下水的質量受源地的影響特別大。水源的變化或是源地供水的降級質量都可能嚴重地損害地下供水的質量。進入含水層的城市和工業(yè)廢棄物是有機污染和無機污染餓主要來源。然而，由于大量的有機廢物通常不易進入到地下，所以大規(guī)模的地下水有機污染不會經常發(fā)生。而有關無機污染的問題就不同了，這是因為無機污染會很容易穿梭于土壤并且一旦進入土壤就會費很大勁才能被清除。另外，這種無機污染的不良影響會無限期地進行，因為稀釋非常慢，并且人工沖洗或處理往往是行不通或者是造價太高。當水流從土壤中流過，溶解鹽的數(shù)量會大大增加，降水和水流會造成土壤餓風蝕和侵蝕，然后形成可溶物，這些溶解鹽被這些可容物吸收。從大水漫灌的下游地區(qū)可以發(fā)現(xiàn)，澆灌的水太咸，而達不到令人滿意餓莊稼收成。這些含鹽的污染物很難控制，因為驅除方法過于昂貴。一個可能解決辦法就是用含鹽污染物較低的水來稀釋（例如，廢水處理工廠的污水）。這樣通過混合產出的平均鹽度的水適合于灌溉。消除廢棄物，保護地下水儲量免于不可治愈的傷害，還需要大量的努力。由于地下水流動速度相當緩慢以及極少有地表水入滲到地球內部，每年地下水中只有一部分才得以通過自然物理過程得以替換。為了彌補這種自然回灌過程的不足，近年興起一種人工回灌的新潮流。例如，在加利福尼亞，人工回灌是現(xiàn)在蓄水餓一種主要方式。人們在人工回灌操作時采用很多方法，其中最常見的方法之一就是使用蓄水池蓄水。通常的做法是把水攔截到一連串事先已建好的蓄水池中，因此水從一個蓄水池里流出來將會進入下一個蓄水池，以此類推。這些人工蓄水工程通常得挖建溝壕或者筑堤。另一種方式是改良河床，這種方式運用的是自然水供給。人們將河床拓寬，整平毀掉，或者是采用一系列方法去增加它的蓄水量。溝和槽在這里是被應用溝梁布設的基本類型有等高線型，即溝梁沿地表面等高線布設；側向型，即把水從側向進入,引入許多小溝梁中樹枝狀貨分支型，即把水從生河道引入連接在一起餓較小渠和溝中。當坡度相對平緩和一致是，洪水就是一種經濟的回灌方式。通常的做法是將相對較少的回灌水撒于地表上，這樣做就不會破壞土壤貨原始植被。另一種做法就是使水灌溉井。在基本條件相同時，灌溉井餓回灌效率通常要低于水泵的灌溉效率，這是因為在臨近水井的區(qū)域經常會形成堵塞，截留含水層顆粒在回灌井中被濾出的并且濾器附近沉淀下來的懸浮物，氣塞，會灌水于天然水體之間的化學反應均可造成阻塞。因此最好的回灌水應該是清潔的，不含或含少量而且已經消過毒的水。THEGROUNDWATERTHEOURCESOFWATERWHICHSUPPLYWATERFRONTBELOWTHEEARTHSSURFACEARECALLEDSUBSURFACESOURCESORGROUNDWATERSOURCEGROUNDWATERSTORAGEISCONSIDERABLYINEXCESSOFALLARTIFICIALANDNATURESURFACESTORAGEINTHEUNITEDSTATESGROUNDWATERDISTRIBUTIONMAYBEGENERALLYCATEGORIZEINTOZONESOFAERATIONANDASTERISKEDTHESATURATEDZONEIAONEINWHICHALLTHEVOIDSAREFILLEDWITHWATERUNDERHYDROSTATICPRESSURETHEAERATIONZONEINWHICHETHEINTERSTICESAREFILLEDPARTLYWITHAIRANDPARTLYWITHWATERS,MAYBESUBDIVIDEDINTOTHREESUBSONICTHESOILWATERZONEBEGINSATTHEGROUNDSURFACEANDEXTENDSDOWNWARDTHROUGHTHEMAJORROOTZONEOFFIREITSTOTALDEPTHISVARIABLEANDDEPENDENTONSOILTYPEANDVEGETATIONTHEZONEISUNSATURATEDEXCEPTDURINGPERIODOFHEAVYINFILTRATIONTHREECATEGORISEOFWATERCALSSIFICATIONMAYBEENCOUNTEREDINTHISREGIONALHYGROSCOPICWATERCONTENT,WHICHISADSORBEDFROMTHEAIRSEPARATIONCAPILLARYWATERRAT,WHICHEISHELDBYSUIFACETENSIONANDGRAVITATIONALWATERS,WHICHISEXCESSSOILWATERDRAININGTHROUGHTHESOILEDTHEINTERMEDIATEZONEEXTENDSFROMTHEBOTTOMOFTHESOILWATERZONETOTHETOPOFTHECAPILLARYFRINGEANDMAYVARYFROMNONEXISTENCETOSEVERALHUNTWEENTHENEARGROUNDSUIFACEREGIONANDTHENEARWATERWATERTABLEREGIONTHROUGHWHICHINFILTRATINGWATERSMUSTPASSEDTHECAPILLARYZONEEXTENDSFROMTHEWATERTABLETOAHEIGHTDETERMINEDBYZONETHICKNESSIAAFUNCTIONOFSOILTEXTUREANDMAYVARYNOTONGLYFROMREGIONTOREGIONBUTALSOWITHINALOCALAREANETWORKTHEWATERTHATCANBEDRAINEDFROMASOILBYGRAVITYISKNOWNASTHESPECIFICYIELDINGITISEXPRESSEDASTHERATIOOFTHEVOLUMEOFWATERTHATCANBEDRAINEDBYGRAVITYTOTHEGROSSVOLUMEOFTHESOILVALUESOFSPECIFICYIELDAREDEPENDENTONSOILPARTICLESIZEAVERAGE,SHAPEANDDISTRIBUTIONOFPORE,ANDDEGREEOFCOMPLETIONOFTHESOILEDAVERAGEVALUESOFSPECIFICYIELDFORALLUVIALAQUIFERSRANGEFROME10TO20ANAQUIFERISAWATERBEARINGSTRATUMORFORMATIONCAPBLEOFTRANSMITTINGWATERINQUANTITIESSUFFICIENTTOPERMITDEVELOPMENTAQUIFERSMAYBECONSIDEREDASFALLINGINTOTWOCATEGORISE,CONFINEDANDUNCONFINED,DEPENDINGONWHETHERORNOTAWATERTABLEORFREEWITHINANAQUIFERISCHANGEWHENEVERWATERISRECHARGEDTOORDISCHARGEDFROMANAQUIFERFORSATURATED,CONFINEDAQUIFER,PRESSURECHANGESPRODUCEONLYSLIGHTCHANGSINSTORAGEVOLUMEINTHISCASES,THEWEIGHTOFTHEOVERBURDENISSUPPOSEDPARTLYBYHYDROSTATICPRESSUREANDPATTLYBYTHESOILDMATERIALINTHEAQUIFERWHENTHEHYDROSTATICPRESSUREINACONFINEDAQUIFERISREDUCEDBYPUMPINGOROTHERMEANS,THELOADONTHEAQUIFERINCREASE,CAUSINGITSCOMPRESSIONAL,WITHTHE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