Well - Ido (English spelling) well

Japanese: 井戸 - いど（英語表記）well

A hole dug to draw water from underground. In ancient times, places where water was drawn, including springs and riverbanks, were generally called wells or wells, but nowadays it refers to dug wells. Depending on the resources sought underground, there are water wells, hot spring wells, oil wells, natural gas wells, etc., and depending on the purpose of use, there are production wells (to draw resources), return wells (to return uncontaminated air conditioning wastewater to the ground), observation wells (to measure changes in the groundwater level), etc., but here we will focus on Japanese water wells.

Groundwater is generally superior to river water or rainwater in terms of its quantitative stability and quality as a drinking water source, so people have long relied on springs that gush out from groundwater to make a living. Springs are natural outcrops of groundwater flow, and exist only in certain geographical locations, such as the ends of alluvial fans, terrace cliffs, mountainsides, and the foothills of volcanoes. Since springs are not necessarily found in plains suitable for farming, the idea of digging the earth to obtain water was born. Wells hollowed out from logs and buried underground have been found in ruins from the Yayoi period. During the Nara period, the knowledge of determining from the topography where to dig a well to find water, as well as the techniques for digging wells, were also spread by Buddhist monks.

[Miki Kobayashi]

Groundwater status and wells

Some rainwater seeps into the ground, but if there is a dense stratum (impermeable layer) underground that does not allow water to pass through easily or does not allow water to pass through, the seepage water is blocked by the layer and accumulates, filling the gaps in the gravel and other materials with water, forming a groundwater table. Water then moves underground from higher to lower elevations. This groundwater table can rise and fall freely depending on the relationship between the amount of infiltration and the amount of runoff, so it is called free groundwater (free groundwater) or unconfined groundwater. In contrast, groundwater in the stratum below the impermeable layer flows down the slope of the stratum after receiving inflow from upstream mountains, lakes, and rivers, and exists in a form that is held back by the impermeable layer. Therefore, if a hole (well) is drilled in the impermeable layer, this groundwater will rise into the well. This type of groundwater is called confined groundwater. As long as the water pressure and volume are sufficient, the water will flow out of the well, but if many wells are dug, the water pressure decreases and the water will eventually stop flowing out of the well.

The water table of free groundwater in plains is generally shallow, lying a few to a dozen meters below the ground surface, and can be pumped out relatively easily, so it has been widely used since ancient times. These are called shallow wells. In contrast, deep wells generally refer to wells that pump out confined groundwater.

[Miki Kobayashi]

Free surface groundwater intake

There are two types of shallow wells: vertical wells and horizontal wells. Vertical wells are dug vertically down to the groundwater level. Those with a large diameter are called tube wells, and those with a small diameter, such as one with an iron pipe driven into the well, are called tube wells. In open-ended wells, where groundwater flows in from the bottom of the well, gravel or sand is laid at the bottom for filtering purposes, and in closed-ended wells, where groundwater flows in from the periphery of the bottom of the well, gravel or sand is laid around the periphery. Water is pumped up using buckets, hand pumps, electric pumps, windmills, or animal power. Wells are excavated when the groundwater level is low, and the well walls are protected with stones, wood, concrete pipes, etc. to prevent soil collapse. This is called the well side (izutsu), and the above-ground enclosure is called igeta. The outside of the well and the surrounding area are compacted with clay or mortar to prevent wastewater from seeping in from the surface.

In sandy soil, alluvial fan, and terraces, the groundwater table is deep, and in some places water cannot be obtained unless one digs several tens of meters below the surface. In the Heian period, the method used was to dig a large area into the easily collapsible surface layer to create a depression, and then to dig a hole vertically from the bottom. In castles of the Sengoku period, deep wells were dug in areas that were slightly elevated and fortified, which suggests the difficulties of obtaining water.

Horizontal wells have been developed in arid regions such as the Middle East since before Christ. They collect water from aquifers in distant mountain foothills and run it through the desert underground to settlements, and can be several to tens of kilometers long. They are made by digging vertical holes every 30 to 50 meters and digging tunnels with a slight slope on both sides from the bottom to connect them to each other. They are called qanat or karez in the Middle East, foggara in North Africa, and qianzi wells or kansei wells in China. Underground waterways in the desert prevent water from evaporating during the water flow and being buried by sandstorms, and the vertical holes are used to maintain the waterway by raking up collapsing sand. There are few horizontal wells in Japan, but the "Manbo" (Meiji and Taisho periods), which led water to rice paddies from the terraces at the eastern foot of the Suzuka Mountains (Mie Prefecture), is a small-scale horizontal well.

On the other hand, there are buried collection conduits that take in large amounts of underground water that flows along rivers or in the gravel layers of the riverbed, and are used in water supply systems in local cities. These are concrete pipes with small holes in the pipe walls that are buried in gravel and connected horizontally to obtain large amounts of water. The original form is seen in the Sanuki Plain (Kagawa Prefecture)'s Dishui (a collection conduit that takes out underground water by digging a ditch in an old riverbed) and the Fusebi (a ditch that collects water by burying bundles of bamboo or brushwood) in the Hokusei Plain of Mie Prefecture. Both of these methods were devised as a way to take in water from rough rivers where the flow rate and water level fluctuate greatly. Radial wells (Manchuria wells) are vertical shallow wells with collection pipes (iron pipes with holes) drilled horizontally in a radial pattern around the bottom, and can be said to be a combination of a horizontal well for collecting water and a vertical well for pumping water.

[Miki Kobayashi]

Confined groundwater intake

In areas where groundwater could not be obtained stably and abundantly through shallow wells, efforts were made to obtain water by drilling through impermeable layers (hard fine sand or clay layers or bedrock). This is called an artesian well. The oldest well still in existence is Joseph's Well in Egypt, which dates back 4,000 years ago and was dug in two stages, 50 meters and 40 meters, for a total depth of 90 meters.

There are two methods for digging through hard layers of earth: the percussion method and the rotary method. The percussion method involves attaching a rope to an iron pipe or a cone-shaped weight with a hard tip, which is dropped repeatedly from an assembled tower to break up the layers of earth and dig further. In Japan during the Edo period, indigenous digging techniques such as dokkoisho digging were combined with techniques from the continent to develop the Kazusabori method, which uses the elasticity of bamboo, oak sticks, and bamboo tubes, and was widely used throughout the country until the early Showa period. The rotary method involves digging through a well by rotating an iron pipe with a special hard cutting edge (made of special steel or diamond) at high speed using a motor on the surface, cutting the well. In Japan, this method has been used to dig many full-scale deep wells since the Taisho period. In the past, cuttings were discharged by attaching a valve to the driving pipe and pushing the cuttings up into the pipe, but recently, it is done mainly by injecting mud water into the excavation point and circulating it with the surface. Mud water is a liquid with adjusted density and viscosity by mixing clay and chemicals such as barium salt, and is used for transporting cuttings, lubricating the rotating surface and cutting edge, cooling, solidifying the hole wall, and diagnosing the condition of the strata.

An external pipe (casing) is inserted into the excavated hole to prevent the collapse of sand and crushing due to earth pressure. Generally, there are many layers of rock underground, and the amount and quality of water that can be extracted varies depending on the permeable layer. A lattice (strainer) is installed in the external pipe at the surface of the layer to collect water, allowing water to flow in.

[Miki Kobayashi]

Problems associated with well use

Groundwater is water that seeps from the surface, accumulates in the gaps between sand and gravel, and flows, so if it is pumped up in excess of the amount of water that can seep or flow, various problems will arise. First, the groundwater level will fall, and then the shrinkage of the strata due to the decrease in moisture will cause the ground to subside, which will lead to flooding in urban areas due to rain, the risk of inundation and salt damage due to floods and high tides, and damage to structures and underground buried objects, among other public problems.

In Japan, the use of wells has been protected as part of land ownership under the interpretation of civil law, but in areas where subsidence is clearly a hindrance to public safety, the drilling and use of new or large-diameter wells is restricted. Secondary causes of the decline in groundwater levels include lowering of riverbeds, reduced flooding, pumping through underground construction, and a reduction in the area of ponds and green spaces in urban areas where rainwater can infiltrate.

In coastal areas, seawater seeps under the freshwater groundwater mass, and the boundary surface fluctuates daily depending on the amount of precipitation, the amount of infiltration from rivers, the amount of pumped groundwater, tides, etc., but if the pressure on the freshwater side weakens due to increased pumping, seawater will seep further inland, causing the groundwater to become saline. The only countermeasures are to restrict ground surface covering (pavement and buildings), regulate the amount of groundwater pumped, and take more active recharge measures such as injecting freshwater into the ground.

[Miki Kobayashi]

Well water quality

Groundwater is characterized by its constant temperature throughout the year. At a depth of 8-15 meters, groundwater temperature is constant (1-2°C higher than the annual mean air temperature), and at depths of 25-33 meters, the temperature increases by 1°C. Water quality is generally good due to filtration and adsorption within the strata and decomposition by soil microorganisms, but it varies depending on the speed and path of water flow and the geological conditions. Shallow groundwater is susceptible to contamination from the surface and may contain naturally occurring stable colored organic matter. Once the strata are contaminated, it takes a long time to recover, so particular care must be taken with chemical fertilizers and pesticides used on farmland and seepage from waste disposal sites. Deep groundwater with poor flow also has a low dissolved oxygen concentration, and may contain iron and manganese ions dissolved in anoxic conditions and underground bacteria, requiring water treatment.

[Miki Kobayashi]

The social position of wells

In areas where groundwater is distributed, groundwater is an excellent water resource because water can be extracted by simply digging a well. In arid regions, Southeast Asia, China, and other areas where river water is highly turbid or saline, groundwater is highly valued for drinking water and irrigation. In continental Europe, rivers and canals are constantly at risk of contamination accidents due to the large amount of ship traffic and the fact that they pass through multiple countries, so there is a strong perception that groundwater is the only safe source of drinking water. Therefore, groundwater is treated with care, with thorough measures taken to protect the environment in water resource recharge areas, prevent underground pollution, and conserve water. In contrast, in Japan, as society developed in the mid-20th century, large amounts of water were pumped for various purposes, which led to problems such as land subsidence. The Basic Environment Law lists land subsidence as one of the seven typical types of pollution. The right to dig a well on one's land and draw water is part of land ownership, but in cases where it is clear that the exercise of ownership rights is being restricted, such as in cases of land subsidence, the use of wells is currently restricted in large urban areas that have developed on alluvial lowlands, such as Tokyo, Nagoya, and Osaka. However, groundwater use in areas where the ground is solid and subsidence has not occurred is left unchecked, and there are some areas where the groundwater level continues to fall. On the other hand, many cities are trying to preserve groundwater for use only in emergencies, as it is the only urban water source that can be relied upon in the event of damage to water facilities due to earthquakes or other disasters. On the other hand, there are examples of small and medium-sized regional cities that are making full use of the advantages of groundwater resources by adjusting the total amount of pumping within the range of groundwater flow and actively cultivating water sources. Groundwater is a common asset unique to the region, and how it is positioned and what conservation measures and restrictions it should be used under are matters that the local community should decide as part of environmental water management.

[Miki Kobayashi]

Japanese Folklore

Today, wells are generally thought of as dug wells, but the "wi" in well (ido) means "gather" or "weir," and in the old days, a well was a place where water, whether it was a spring or a river, was dammed up and used for irrigation. This is evident from the names for wells that remain in various places. For example, wells are called "ka" in Okinawa, the Satsunan Islands (Kagoshima Prefecture), and Izu Oshima, "kawa" along the coast of Kyushu and the Seto Inland Sea, and "ike" in the Hokuriku region. Eventually, they were surrounded by wood or stone and called "izutsu" or "igeta." Picture scrolls such as "Senmen Hokekyo Satsuko" and "Shigisan Engi" depict water overflowing from these wells and wells.

However, the land where such naturally spring water or running water can be used is limited, so artificial methods of pumping groundwater were considered. There are two methods for this: horizontal wells and vertical wells. Horizontal wells are a method of building steps to the water source and using water buckets to carry the water out. The springs in caves in the Satsunan Islands and the spiral staircase-like "maimaizu" wells remaining on the Kanto Plateau, such as in Sayama City, Saitama Prefecture, are examples of this ancient well-digging technique. On the other hand, vertical wells are a method of digging a hole vertically to the groundwater source and pumping it up, a method of digging through a layer of hard soil, as seen in the Kazusa-bori well in Chiba Prefecture. Many such vertical wells have already been excavated at the Heijo Palace ruins in Nara and the Fujiwara Palace ruins in Kashihara, and it is thought that this excellent technology was introduced from the continent along with the capital city system. As such, dug wells have a long history, but their spread was slow. In many Japanese cities, up until the early Meiji period, when modern water supply systems began, people still obtained water for daily use from irrigation canals or bought river water from water vendors.

To pump water from a vertically dug well, a bucket was usually used, which could be a "flip bucket" that was lifted with a rope or pole, a "spring bucket" that used a lever, or a "wheel well" that used a pulley. However, after 1897 (Meiji 30), hand pumps were adopted, and after 1945 (Showa 20), with the advent of home electrification, they were replaced by electric pumps.

Water is also important to human life, and is one of the factors that lead to the formation of villages. The existence of a communal well is also one of the factors that create a special social bond, and the Edo well replacement ceremony held on Tanabata is considered a kind of ritual for the well god (water god). Furthermore, the mysticism of deep wells has given rise to various popular beliefs and customs, such as the idea that dragon gods and well masters reside in wells.

[Mizuo Miyamoto]

Folklore of the World

A typical Chinese village has a well in the middle, and the settlement is formed around it. The well supplies drinking water and is a social place where people do laundry. In German rural areas, community life is also centered around the village well, and when a village has many wells, well-groups are formed around them. The importance of wells to people's lives is true in other regions as well, but it is more pronounced in the arid regions from West Asia to North Africa. Here, wells belong to the group or individual who dug them, and are subject to inheritance. Many Muslim settlements also have a well in the middle, and a mosque is built nearby. When Muslims pray at a mosque, they are required to purify all orifices of the human body. The water of a well has the power to purify. The religious and mystical power of certain wells is also seen in other regions. In Bali, Indonesia, a well in a temple is said to contain sacred water and to be effective in curing illnesses.

In ancient Greece, there was a custom of divination by throwing offerings into sacred wells. If the offering sank, it was believed to bring favor to the gods, and if it did not, it was considered a bad omen. Among the Slavs, there was a custom in which young girls would gaze into a well at dawn on St. George's Day, waiting for their future husband to be reflected in the water. Furthermore, it is common in European folklore to worship the god of wells as a god of fertility and healing. A certain well in Ireland was sanctified by the power of a saint and is said to have the power to cure physical disabilities, and many believers gather there on the night of the summer solstice.

On the other hand, wells dug into the ground were also considered to be entrances to an underground world, and were thought to be the home of water spirits and water monsters. Among the Finns, it is said that a water monster called the Well Bull lives at the bottom of the well, and that if a child approaches a well, he or she will be dragged into the well. This popular belief is said to have originated from the idea of linking water gods and cattle and horses, which is widespread in the east and west of Eurasia.

[Tamura Katsumi]

"Groundwater Hydrology" by Sakai Gunjiro (1965, Asakura Publishing) ▽ "Groundwater Resources Science" by Shibasaki Tatsuo et al. (1973, Kyoritsu Publishing) ▽ "Development and Conservation of Groundwater Resources" edited by Kayane Isamu (1973, Hydraulic Science Institute) ▽ "Pumping Tests and Well Management" by Yamamoto Soki (1962, Shokodo Publishing) ▽ "Hydrogeology" by Kurata Nobuo (1955, Asakura Publishing) ▽ "Study of Wells" by Yamamoto Hiroshi (1973, Sogeisha Publishing) ▽ "The Story of Wells and Water Supply" by Horikoshi Masao (1981, Ronsosha Publishing)

Groundwater status and wells
©Shogakukan ">

Groundwater status and wells

Strata and well morphology (shallow and deep wells)
©Shogakukan ">

Strata and well morphology (shallow and deep wells)

Horizontal wells in the desert
©Shogakukan ">

Horizontal wells in the desert

Lowering of groundwater level due to pumping
When well A pumps, the surrounding groundwater flows in the direction of the well, and the groundwater level drops in a cone shape as shown in b. However, if the amount of water pumped exceeds a certain limit, the water level drops significantly, disturbing the strata around the well and allowing sand and other materials to flow in. If well B is now dug in the neighboring land and pumped, the groundwater level will change to c. This is called mutual interference between wells. If the amount of water pumped by B increases further, the water surface will change to d, and well A will run dry. In this way, when multiple wells pump water in excess of the amount of water to be replenished, the groundwater level will drop and cause land subsidence .

Lowering of groundwater level due to pumping

Groundwater conditions in coastal areas
©Shogakukan ">

Groundwater conditions in coastal areas

Kazusa digging
A large hineguruma (bamboo that has been split and shaved into thin strips) is wrapped around it in several layers, and the hineguruma is reeled out using a bow bamboo attached to the top of a tower to give it momentum, and the metal fittings on the tip of the bamboo are used to dig into the layer of soil. ©Shogakukan ">

Kazusa digging

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

地下から水などを汲(く)み出すために掘った穴。古語では泉や川辺なども含めて水を汲み取る所を井または井戸と総称したが、現在では掘(ほり)井戸のことをさす。地中に求める資源によって水井戸、温泉井戸、石油井戸、天然ガス井戸などがあり、また使用目的により生産井（資源を汲み出す）、還元井（未汚染の冷房排水などを地中に戻す）、観測井（地下水位の変化を測る）などがあるが、ここでは日本の水井戸を中心に記述する。

　地下水は飲用水源としての量的安定性と水質の良好さの点で河川水や天水よりも一般に優れているので、人々は古くから地下水が湧(わ)き出た泉に依存して生活を営んだ。泉は地下水流の自然露頭であり、扇状地の末端や段丘崖(がい)、山腹、火山の麓(ふもと)など特定の地形の所にしか存在しない。農耕に適した平野部にかならずしも泉はないので、土を掘って水を得る知恵が生まれた。丸太をくりぬいて地中に埋めた井戸が、弥生(やよい)時代の遺跡からみつかっている。奈良時代になると、どこに井戸を掘ったら水が出るかを地形から判断する知識や掘井戸の技術は仏教僧によっても広められた。

［小林三樹］

地下水の存在形態と井戸

雨水の一部は地中に浸透するが、地下に水を通しにくいか通さない緻密(ちみつ)な地層（不透水層）があると、浸透水はその層に遮られて水がたまり、砂礫(されき)などの間隙(かんげき)が水で満たされて地下水面が形成される。そして水は地中を高所から低所に向かって移動する。この地下水面は浸透量と流出量との関係によって自由に昇降しうるので、自由面地下水（自由地下水）または不圧地下水という。これに対し不透水層の下の地層内の地下水は、上流の山地や湖や川からの流入を受けて地層の傾斜に沿って流下し、不透水層に抑え込まれた形で存在している。したがって不透水層に孔(あな)（井戸）をあけると、この地下水は井戸内に上昇してくる。このような地下水を被圧地下水という。水圧と水量が十分なうちは自噴するが、多数の井戸が掘られると水圧が低下し、やがて自噴を停止する。

　自由面地下水の水面は平野部では一般に浅く、地表面下数メートル～十数メートルにあり、比較的容易に汲み出せるので古代から広く利用されてきた。これを浅井戸という。これに対し深井戸は、一般に被圧地下水を汲み出すものをさす。

［小林三樹］

自由面地下水の取水

浅井戸には垂直井戸と水平井戸とがある。垂直井戸は地下水面の下まで縦に穴を掘ったもので、口径の太いものを筒井戸、鉄管を打ち込むなど細いものを管井戸という。地下水を井の底面から流入させる開端井戸では底面に、また井の下部の周囲から流入させる閉端井戸では周囲に、濾過(ろか)の目的で砂利や砂を敷き詰める。水はつるべ、手押しや電動のポンプを用いて、また風車や畜力を利用して汲み上げる。井戸は地下水位の低下している時期に掘削し、井壁は土の崩落を防ぐため石または木材、コンクリート管などで保護する。これを井戸側(がわ)（井筒(いづつ)）といい、地上部の囲いを井桁(いげた)という。井筒の外側と井桁の周囲は、地表からの汚水の浸透を防ぐため粘土やモルタルで締め固める。

　砂地、扇状地、段丘などでは地下水面が深く、地表面から数十メートルも掘らなければ水を得られない所もある。平安時代には崩れやすい表層を広く掘り下げて窪地(くぼち)をつくり、その底から垂直に孔をうがつなどの方法が採られた。戦国時代の城も小高い要害の地ほど深い井戸が掘られており、水を得る苦労がしのばれる。

　中近東など乾燥地帯で紀元前から発達した井戸に横井戸（水平井戸）がある。これは、遠い山麓(さんろく)などの帯水層から水を集め、砂漠の地下を流して集落まで導き出すもので、長さ数キロメートル～十数キロメートルにも及ぶ。30～50メートル置きに縦穴を掘り、その底から両側へわずかに勾配(こうばい)をつけたトンネルを掘って相互につなげたもので、中近東ではカナート、カレーズ、北アフリカではフォガラ、中国では乾児井(かんにせい)、坎井(かんせい)などとよばれている。砂漠の地下水路は導水途中の蒸発と砂嵐(すなあらし)による埋没を防ぎ、縦穴は、崩れる砂をかき上げて水路を保守するためである。日本に横井戸は少ないが、鈴鹿(すずか)山地東麓（三重県）の段丘から水田に水を導いた「まんぼ」（明治・大正期）が小規模ながら横井戸である。

　一方、川沿いや川底の砂利層の中を流れる伏流水を集水トンネルで大量に取水するものに集水埋渠(まいきょ)があり、地方都市の水道で使用されている。これは、管壁に細孔をあけたコンクリート管を砂礫中に埋めて横につなぎ、大量の水を得るもので、原型は讃岐(さぬき)平野（香川県）の出水(ですい)（旧河道などに掘割(ほりわり)を設けて伏流水を取り出す集水渠）、三重県北勢平野の伏樋(ふせび)（竹や柴(しば)を束ねて埋め集水する樋(とい)）にみられる。いずれも流量と水位が大きく変動する荒れ川からの取水方法としてくふうされたものである。放射状井戸（満州井戸）は、縦の浅井戸の底部から集水管（孔をあけた鉄管）を水平放射状に周囲にうがったもので、集水目的の横井戸と汲み上げ目的の縦井戸を組み合わせたものといえる。

［小林三樹］

被圧地下水の取水

浅井戸で地下水を安定して豊かに得られない地域では、不透水層（堅い細砂や粘土の層か岩盤）を打ち抜いて水を得る努力が重ねられた。それが掘抜き井戸である。現存する最古の井戸は4000年前のエジプトのヨセフの井戸で、50メートルと40メートルの2段に掘られ、合計90メートルの深さがある。

　堅い地層を掘り抜く方法に打撃法と回転法とがある。打撃法は硬い先端部を付した鉄管とか円錐(えんすい)形のおもりに綱索をつけ、組み立てた櫓(やぐら)の上から何度も落下させて地層を打ち砕いて掘り進むものである。江戸時代の日本では、どっこいしょ掘りなど土着の掘抜き技術に、大陸伝来の技術が集大成され、竹の弾力と樫(かし)の棒、竹筒を用いた上総掘(かずさぼ)り方式として発展を遂げ、昭和初期まで全国で広く用いられた。回転法は、特殊な堅い刃先（特殊鋼やダイヤモンド製）を付した鉄管を地表のモーターから高速で回転させて削りながら掘り進むもので、日本では大正時代からこの方法により多数の本格的深井戸が掘られた。掘りくずの排出には、古くは打込み管に弁を付し管内に押し上げたが、最近はもっぱら泥水を掘進点に圧入し地上と循環させる方式でなされる。泥水は、粘土とバリウム塩などの化学薬品を調合して密度と粘度を調整した液で、削りくずの輸送、回転面と刃先の潤滑、冷却、孔壁の固化という目的のほかに、地層状態の診断のために用いられる。

　掘削した穴には外筒管（ケーシングcasing）を挿入して、砂の崩落と地圧による圧壊を防ぐ。地下には一般に幾層もの地層が重なっているが、透水層によって取水可能な水量と水質が異なっている。集水したい地層面の外筒管には水が流入する格子（ストレーナーstrainer）を設ける。

［小林三樹］

井戸利用に伴う障害

地下水は地表からの浸透水が砂礫の間隙(かんげき)に蓄えられて流動するものであるから、浸透量または流動量を超えて揚水すると、種々の障害を生ずる。まず地下水位の低下をきたし、続いて水分の減少による地層の収縮が地盤の沈下をもたらし、市街地内に降った雨による氾濫(はんらん)、洪水や高潮による浸水や塩害の危険、構造物や地下埋設物の損壊などの公共的障害の原因になる。

　日本では井戸の利用を土地所有権の一部として民法解釈上で保護してきたが、地盤沈下による公共の安全の阻害が明白な地域では、新規または大口径の井戸の掘削や使用を制限している。なお地下水位の低下には、河床の低下、洪水の減少、地下工事による揚水、市街地内での池や緑地など雨水が浸透できる面積の減少なども副次的に原因している。

　海岸部では淡水の地下水塊の下に海水が浸入しており、その境界面は降水量や河川からの浸透量、汲上げ地下水量、潮汐(ちょうせき)などによって日々変動しているが、揚水量増などで淡水側の圧力が弱まると、海水が内陸側にいっそう入り込み、地下水の塩水化を招く。対策としては、地表面被覆（舗装や建築物）の制限、地下水の揚水量規制のほか、地中への淡水圧入などさらに積極的な涵養(かんよう)策をとるしかない。

［小林三樹］

井戸水の水質

水温が年中ほぼ一定であることは地下水の特徴である。地下水の水温は8～15メートルの深さで恒温度（年平均気温の1～2℃高）となり、それ以深では25～33メートルについて1℃ずつ温度が高くなる。水質は、地層内での濾過や吸着、土壌微生物による分解などを受けているため一般に良好であるが、水の流動の速度や経路、地質状況によって異なる。浅い地下水は地表からの汚染を受けやすいほか、天然由来の安定な着色有機物を含む場合がある。地層を汚染すると回復に時間がかかるので、農地で散布する化学肥料や農薬、廃棄物処分場からの浸出水にはとくに注意を要する。また深層地下水は、流動の乏しいものほど溶存酸素濃度が低く、無酸素状態で溶出した鉄、マンガンイオンや地下細菌を含み、水処理を要する場合もある。

［小林三樹］

井戸の社会的位置づけ

地下水が分布する地域では、井戸さえ掘れば水を取り出せるので地下水は優れた水資源である。乾燥地帯や東南アジア、中国など河水が高濁もしくは高塩分の地域では、飲用と灌漑(かんがい)用に地下水は大いにたいせつにされている。またヨーロッパ大陸では河川や運河は船舶の航行が多いこと、複数の国を経由して流れていることなどから汚染事故の危険をつねに抱えており、地下水のみが安全な飲料水源であるとの認識が強い。そこで水源涵養地域の環境保全や地中汚染の防止、節水を徹底したうえで大切に扱われている。それにひきかえ日本では、20世紀中葉の社会の発展につれ、各種用途に大量に揚水され、地盤沈下などの障害を生ずるに至った。環境基本法では地盤沈下を典型7公害の一つに掲げている。自分の敷地に井戸を掘って水を汲む権利は土地所有権の一部であるが、地盤沈下など公共の福祉を侵害していることが明らかな場合には、所有権の行使を制限できるとの概念に基づいて、現在、東京、名古屋、大阪など沖積低地に発達した大都市域では、井戸の使用が制限されている。しかし地盤が強固で沈下を生じていない地域での地下水利用は放任されているので、地下水位が低下を続けている地域も散見される。その一方で、震災などで水道施設が損壊した場合に依存しうる唯一の都市内水源として、多くの都市で緊急時に限定して活用するための保存が図られている。他方、地方中小都市で、地下水流動量の範囲に揚水総量を調整するとともに、積極的に水源涵養を行って、地下水資源のもつ長所を十分に活用している例もある。地下水は地域固有の共同財産であり、地下水をどう位置づけてどのような保全策や制限のもとで活用するかは、地域社会が環境的水管理の一環として決めていくべき事柄である。

［小林三樹］

日本の民俗

井戸は今日一般には、掘井戸のことと考えられるが、井戸(ゐど)の「ゐ」は、集(ゐる)、堰(ゐ)の意味で、古くは泉でも川でも、水をせき止めて、用水として水を汲(く)み取る所を「井」といった。そのことは、各地に残っている井戸に対する呼び名からも知られる。たとえば、沖縄や薩南(さつなん)諸島（鹿児島県）、伊豆大島では「カー」、九州から瀬戸内海沿岸では「カワ」、北陸地方では「イケ」とよんでいる。やがて、これに木や石の囲いを構えて、「井筒(いづつ)」「井桁(いげた)」とよんだ。『扇面(せんめん)法華経冊子』や『信貴山(しぎさん)縁起』などの絵巻物をみると、この井筒、井桁から水があふれ出ているようすが描かれている。

　しかし、このような自然に湧(わ)く水や流水を利用できる土地は限られており、ここに人工的な地下水の汲み上げ方法が考えられた。これには横井戸と縦掘り井戸の二つの方法がある。横井戸は、水源まで階段をつくり、水桶(みずおけ)などで水を運び出す方法で、薩南諸島の洞窟(どうくつ)の泉や、埼玉県狭山(さやま)市など関東台地に残る螺旋(らせん)階段状の「まいまいず」などは、その古い掘井戸技術を示すものである。一方、縦掘り井戸は、地下水のところまで垂直に穴を掘り、地下水を汲み上げる方法で、千葉県の上総掘りなどにみられる堅い土の層を掘り抜く方法である。こうした縦掘り井戸は、すでに、奈良の平城宮跡や橿原(かしはら)の藤原宮跡などから数多く発掘されており、その優れた技術は、都城制などとともに大陸から伝えられたものと考えられている。このように掘井戸の歴史は古いが、しかし普及は遅く、日本の多くの都市では、近代的水道の始まる明治初年まで、用水路から日常の用水を求めたり、水売りから川の水を買った所も少なくない。

　なお、縦掘り井戸の水を汲み上げるには、普通、釣瓶(つるべ)が用いられたが、これには、縄や竿(さお)などでつり上げる「ふりつるべ」のほか、てこを応用した「はねつるべ」、滑車(かっしゃ)を用いた「車井戸」などの方法が行われた。しかし1897年（明治30）以後は、手押しポンプが採用され、1945年（昭和20）以後は、家庭電化に伴って、電動ポンプアップに変わってしまった。

　また、人間生活にとってたいせつな水は、村落形成の一要因ともなるが、共同井戸の存在は、特殊な社会結合を生む一要素となり、七夕(たなばた)に行われた江戸の井戸替え行事なども一種の井戸神（水神）の祭祀(さいし)と考えられる。さらに深井戸の神秘性から井戸に竜神や井戸の主(ぬし)がいるなど種々の俗信、習俗を生んでいる。

［宮本瑞夫］

世界の民俗

典型的な中国の村は真ん中に井戸をもち、それを中心に集落が形成される。井戸は飲み水を供給するとともに、洗濯なども行われる社交場である。ドイツの農村も村井戸を中心に共同体の生活が営まれており、一村に多くの井戸のある場合、それを中心に井戸仲間が形成される。こうした井戸がもつ人々の生活への重要性は他の地域でも当てはまるが、西アジアから北アフリカにかけての乾燥地帯ではいっそう顕著である。ここでは、井戸は掘った集団や個人の所有に帰し相続の対象となっている。イスラム教徒の集落も多くは中心に井戸をもち、その近くにモスクが建てられる。イスラム教徒は、モスクで礼拝するにあたって、人体のあらゆる穴を清めることが必要とされる。井戸の水はその清めの力をもつ。特定の井戸の水がこうした宗教的、神秘的力をもつことは、他の地域にもみられる。インドネシアのバリ島では、ある寺院の中にある井戸は、神聖な水をたたえており、病気治癒に効果があるとされている。

　古代ギリシアでは、聖なる井戸に供え物を投げ込むことによって占いを行う風習があった。供え物が沈むと神の好意が受けられるとし、そうでないと悪い兆しと判断された。スラブ人の間には、聖ジョージの日の明け方、娘が井戸を見つめ、水面に未来の夫の映るのを待つという風習があった。さらに、井戸の神が多産や治病の神として崇(あが)められることも、ヨーロッパの民俗に広くみられる。アイルランドのある井戸は、聖人の力で聖化され、身体障害を治す力があるといわれ、夏至の夜に多くの信者が集まる。

　他方、地底に向けて掘られた井戸は、地下の国への入口とも観念され、水の精霊や水怪の住む所と考えられた。フィン人の間では、井戸の雄牛とよばれる水怪が井底に住むといわれ、子供が井戸に近づくとこれに引きずり込まれるなどと伝える。この俗信は、ユーラシアの東西に広がる水神と牛馬とを結ぶ観念に由来するとされる。

［田村克己］

『酒井軍治郎著『地下水学』（1965・朝倉書店）』▽『柴崎達雄他著『地下水資源学』（1973・共立出版）』▽『榧根勇編『地下水資源の開発と保全』（1973・水利科学研究所）』▽『山本荘毅著『揚水試験と井戸管理』（1962・昭晃堂）』▽『蔵田延男著『水理地質学』（1955・朝倉書店）』▽『山本博著『井戸の研究』（1973・綜芸社）』▽『堀越正雄著『井戸と水道の話』（1981・論創社）』

地下水の存在形態と井戸

地層と井戸の形態（浅井戸と深井戸）

砂漠の横井戸（水平井戸）

揚水による地下水位の低下
井戸Aが揚水すると、周囲の地下水は井戸の方向に流動し、地下水面はbのような円錐形をして低下するが、揚水量がある限度を超えると水位は著しく低下し、井戸周辺の地層が乱され、砂などが流入するに至る。いま隣地に井戸Bが掘られて揚水されると、地下水面はcのように変化する。これを井戸群の相互干渉という。さらにBの揚水量が増すと水面形はdのようになり、井戸Aは水が枯れてしまう。このように、多数の井戸で補給量に比べて過剰に揚水されると、地下水位の低下をきたし、地盤沈下をもたらす©Shogakukan">

揚水による地下水位の低下

海岸部での地下水の状態

上総掘り

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