Water Supplies

INTRODUCTION

Water Supply and Waterworks, provision of a supply of water for domestic, industrial, and irrigation needs, and the engineering installations necessary to treat and pump the water to the consumer. Today, the average daily water consumption rate in populated areas ranges from to litres 400 to 450 litres per household per day, and extensive water-treatment plants are essential for providing water safe for human consumption.

SOURCES OF WATER

The ultimate source of all natural potable water on the earth is rain, which is rarely used as a direct source except on islands in salt water, such as Bermuda, where the rain is collected and led into cisterns to serve as the only available water supply. When rain falls, it runs off into streams, in the case of heavy rains, or soaks into the ground, percolating through porous strata until it reaches an impervious stratum, upon which it collects, forming groundwater. Groundwater is the source of wells and of the springs that feed streams, rivers, and lakes. In its course, groundwater dissolves soluble mineral matter, and often the surface waters of rivers and lakes are polluted by the influx of sewage or industrial wastes. In modern water-supply systems, an entire watershed is usually made into a reservation to control pollution. The waters are impounded by a system of dams, and flow by gravity, or are pumped, to the local distribution system.
The quality of water from these sources varies greatly. Surface waters generally contain larger quantities of turbidity and bacteria than groundwaters, but groundwater contains higher concentrations of dissolved chemicals. Seawater contains high concentrations of dissolved chemicals and some microscopic organisms as well. Because water quality does vary widely from source to source, the Local Authority has established certain standards for drinking water. Similar water quality standards have been adopted by the World Health Organization. In water being considered as a source for supply, these standards set the concentration levels of chemical compounds and bacteria that can be safely allowed in the treated water.

TREATMENT

Undesirable tastes and odours are removed from water by aeration. Bacteria are destroyed by the addition of a few parts per million of chlorine, and the taste of chlorine is then removed with sodium sulfite. Excessive hardness, which renders the water unsuitable for many industrial purposes, is reduced by the addition of slaked, or hydrated, lime or by an ion-exchange process using water softening compounds. Suspended organic matter, which supports bacterial life, and suspended mineral matter are removed by the addition of a flocculating and precipitating agent, such as alum, before settling or filtration. Artificial fluoridation of public water is done in many Water Authority areas as a measure for preventing dental caries.

HISTORY

Early peoples had no need of engineering works to supply their water. Hunters and nomads camped near natural sources of fresh water, and populations were so sparse that pollution of the water supply was not a serious problem. After community life developed and agricultural villages became urban centres, the problem of supplying water became important for inhabitants of a city, as well as for irrigation of the farms surrounding the city. Irrigation works were known in prehistoric times, and before 2000 BC the rulers of Babylonia and Egypt constructed systems of dams and canals to impound the flood waters of the Euphrates and Nile rivers, controlling floods and providing irrigation water throughout the dry season. Such irrigation canals also supplied water for domestic purposes. The first people to consider the sanitation of their water supply were the ancient Romans, who constructed a vast system of aqueducts to bring the clean waters of the Apennine Mountains into the city and built settling basins and filters along these mains to ensure the clarity of the water. The construction of such extensive water-supply systems declined when the Roman Empire disintegrated, and for several centuries local springs and wells formed the main source of domestic and industrial water.

I INTRODUCTION

Water Table, underground border between the ground in which all spaces are filled with water and the ground above in which the spaces contain some air. The level of the water table tends to follow the shape of the overlying ground surface, rising under hills and dipping in valleys, but with a gentler slope than the ground. The level of the water table also varies with the climate, rising during rainy periods and falling during dry spells. Wells dug down to below the water table will fill with water.

II WATER TABLE FORMATION

The water table forms when rainwater seeps into the soil or bedrock instead of evaporating back to the atmosphere or flowing directly into a stream as surface runoff. Most soils and many rocks are both porous and permeable. Porous materials have openings, such as cracks, voids, and spaces between particles, that can contain water. These openings are called pores. Permeable materials are materials that allow water to flow through them. At shallow depths, the pores are filled with a mixture of air and water. This region constitutes the zone of aeration, or unsaturated zone. Water percolating downward eventually fills all available pore space below a certain level, forming the saturated zone. The surface, or border, between the zone of aeration and the saturated zone is the water table. Surface tension can cause water to rise a short distance upward from the water table. This produces a transition zone between the saturated and unsaturated zone called a capillary fringe.

III GROUNDWATER

Water in the saturated zone is referred to as groundwater. Some soils and sedimentary rocks are so porous that water can occupy up to 40 percent of their volume. As depth increases, high pressures squeeze the pores shut. As a result, almost all groundwater is found in the top 8 km (5 mi) of the earth's crust.

Groundwater, water found below the surface of the land. Such water exists in pores between sedimentary particles and in the fissures of more solid rocks. In arctic regions, groundwater may be frozen. In general such water maintains a fairly even temperature very close to the mean annual temperature of the area. Very deep-lying groundwater can remain undisturbed for thousands or millions of years. Most groundwater lies at shallower depths, however, and plays a slow but steady part in the hydrologic cycle. Worldwide, groundwater accounts for about one-third of one percent of the earth's water, or about 20 times more than the total of surface waters on continents and islands. (The oceans account for 97 percent of the earth's water.)

Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live. Groundwater may appear at the surface in the form of springs, or it may be tapped by wells. During dry periods it can also sustain the flow of surface water, and even where the latter is readily available, groundwater is often preferable because it tends to be less contaminated by wastes and organisms.

The rate of movement of groundwater depends on the type of subsurface rock materials in a given area. Saturated permeable layers capable of providing a usable supply of water are known as aquifers. Typically, they consist of sands, gravels, limestones, or basalts. Layers that tend to slow down groundwater flow, such as clays, shales, glacial tills, and silts, are instead called aquitards. Impermeable rocks are known as aquitards, or basement rocks. In permeable zones, the upper surface of the zone of water saturation is called the water table

IV AQUIFERS

An aquifer is a body of rock or soil that is sufficiently porous and permeable to store and transfer significant amounts of groundwater. An aquitard is a body of relatively impermeable rock. An aquifer is called confined when it is bounded above and below by aquitards or unconfined when there is no aquitard above it. A perched aquifer is a body of groundwater that lies above the regional water table because it is underlain by a small aquitard. The top of this small zone of saturation is known as a perched water table.

V RECHARGE AND DISCHARGE

Water flows in and out of aquifers as part of the water cycle. The flow of water into aquifers is called recharge and the flow of water out of aquifers is called discharge. The places where recharge occurs are called recharge areas. Discharge occurs wherever the ground dips down to the level of the water table. For example, springs occur in valleys where the valley sides meet the water table. If an enclosed depression in the earth dips below the water table, water can flow out of the saturated zone and into the depression, forming a lake or pond.
When recharge is equal to discharge, the water table is stationary. Heavy rainfall or spring melt can cause recharge to temporarily exceed discharge and the water table will rise. A rising water table may produce temporary springs, streams, and ponds. These temporary discharge areas then drain water from the aquifer and lead to a restoration of the original level of the water table.

If a well is dug down below the level of the water table, it will start to fill with water. As water is removed from the well, the water table surrounding the well will drop, forming a cone-shaped depression in the water table. The depth and steepness of the cone of depression depend on how fast the water is being withdrawn, how porous and permeable the aquifer is, and how fast the aquifer is being recharged.

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