1

J.S. Laura

10.1 Introduction

Life originated on earth millions of years ago in water and water is essential for all kind of known living forms. So whenever there are talks of possibility of finding life on other planets the presence of water is taken as the first sign of existence of life there. Earth is known as blue planet because more than three fourth of its surface area is covered with water. Water on earth is found in all the three states of matter; solid (ice), liquid (water) and gas (water vapours) because of the presence of suitable combination of temperature and pressure. Water on earth is constantly converted from one form to the other through various processes.This makes the flow of water through the environment of earth possible and ensures the continuity and availability of water in almost all the parts of world. Spatially water on earth is distributed on the surface below the surface as well as in the atmosphere.

10.2 Classification of water on basis of salt quantity

The earths continental crust is composed of low density rocks containing easily erodable salts and over a period of billions of years these salts have been deposited into oceans and seas by rivers through the flow of water created by rain and snow melt. On basis of salt quantity, water can be classified into fresh water and salt water ( or ocean/sea water). Salinity of oceans is expressed as the amount of salt in 1000g of water ( referred to as ‘parts per thousand’ or ppt.) Most of the oceans have a salinity between 34ppt and 36ppt (35‰ – 36‰). Water with a salinity of less than 1 percent of that of the oceans ( 0.35‰) is defined as fresh water. Water with higher salinity but not rxceeding 1‰ is typically referred to as marginal water because it less suitable for many humans and animals uses. Water terms based on quantity of salts expressed as TDS is presented in the box.

10.3 Distribution and quantity of water

The worlds water resources distribution is depicted in the figure1. Of the total available water on the earth, about 97 % water is sea water, which is non-drinkable, unfit for human consumption and other uses because of high concentration of salts in it. The remaining 2.5 % of world’s water is fresh water which is present in the form of polar ice caps, glaciers, rivers, lakes, streams, reservoirs, ground water and as vapours in the atmosphere. Although these latter sources constitute just a tiny fraction of the total, they are much more important than their meagre percentage indicates. Of the fresh water fraction more than 68 % is frozen in Antarctica, the Arctic and glaciers, and not directly available for use to man, 30.1 % is stored as ground water remaining approximately 1.2 % is present as surface water. The largest fraction (69%) of the surface fresh water is present as ground ice and permafrost.

Table1. Break up of fresh water resources.

The total volume of water on Earth is estimated at 1.386 billion km³ of which 35,029,000 km³ is freshwater out of this the available freshwater equals 200,000 km³. Of this available fresh water approximately 10,000,000 BCM (Billion Cubic Meters) is stored in underground aquifers. 119,000 BCM is present as net rainfall falling on land after accounting for evaporation. 91,000 BCM is found in natural lakes. Over 5,000 BCM is contained in manmade storage facilities like reservoirs. 2,120 BCM is found in rivers (Table 1.)

10.4 Hydrological cycle

Water is technically considered a renewable resource because it can be used over and over again and the water sources can be replenished through the hydrological cycle. Water is constantly in motion and all the sources of water are interconnected through the movements of water molecules by way of the hydrologic cycle. Water evaporates as vapor from all sources as oceans, lakes, and rivers, it is transpired from plants, condenses in the air and falls as precipitation and then it moves over land surface and through the ground into waterbodies, where the cycle begins again. That’s why the volume of water in particular water reservoir remains more or less constant under natural conditions, unless over exploitation by man. This flow of water through the earth’s environment constitutes the water cycle also known as the hydrological cycle, which ensures the continuity and availability of water to the environment.The driving force for the cycle is the solar energy from the sun. The three basic process involved in the cycle are evaporation, condensation and precipitation. Various other processes involved in the water cycle include transpiration, convection, freezing, melting, runoff and groundwater flow. The hydrological cycle is depicted in Figure 2.

The cycle starts with the process of evaporation and transpiration. Solar energy from the sun transforms liquid water from surface of water bodies into water vapours. This process is termed as evaporation. The process of evaporation also takes place from the surface of leaves and stems of plants, and termed as evapotranspiration. Thus through these two phenomenons water  is converted from its liquid state to the gaseous state. Through this process the water leaves the Earth’s surface and enters the atmosphere as a gas. Evaporation is natures purifying process through which water is removed from various dissolved and suspended impurities present in the waters. About 80 % of water vapours in the atmosphere is contributed by oceans mainly from near the equator, which receives highest amount of sun’s energy. Another 10 % is contributed by evapotranspiration and the remaining 10% evaporation comes from inland water sources like streams, lakes, rivers and seas.

Figure 2. Global water cycle

As these water vapours rise in the atmosphere they get cool adiabatically, ultimately reaching the dew point at a certain height resulting in the formation of water droplets, this process is called condensation. Condensation is the process and that step of the water cycle in which water vapor is changed back into liquid water. Condensation is very important to our weather and climate because it is the process responsible for cloud formation. Clouds form when water vapor condenses around small particles, like bits of dust or smoke in the air. Thus condensed water vapours form the clouds. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition. Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening due to deposition. Without clouds, we would not get to the third phase, called precipitation, Warm air can holds more water vapours, so when rising air cools, it reaches saturation point and the growing water droplets falls down due to their weight and through precipitation gaseous water from atmosphere returns back to the earth’s surface. This process is called precipitation. Depending on the process and degree of cooling the precipitation is of different types and includes rain, snow, sleet, hail etc. In colder areas, like on poles and in high mountains, the precipitated water accumulates in its solid form as snow and ice. Melted snow and ice become the source of water for rivers in summer months. Rest of the precipitated water flows through lakes and rivers as runoff and eventually mix in the ocean. Water also moves in its solid form ice and snow as a glacier. Water faling on land surface moves as runoff into water bodies. Rivers carry water to lakes and oceans through flow of water in channels. Some of the precipitation water seeps into the permeable soil through the process of infiltration and contributes to the replenishment of the ground water. Thus water gets back into the respective reservoir through aforesaid processes in a cyclic way forming the hydrological/water cycle.

Residence time

Water cycle diagrams in figure 2 basicaly has two components, reservoirs and flows/ pathways. Reservoirs are places where water stays for some length of time or “stored” for some time. The oceans, lakes,rivers, the atmosphere glaciers and ice caps,are some of the major reservoirs. Flows or pathways are the routes water takes between reservoirs. Evaporation moves water from the oceans to the atmosphere. Precipitation moves liquid (rain) or solid (snow) water from the sky back to Earth’s surface. Snowmelt runoff turns solid water into a liquid that flows down rivers to the sea. The amount of time that water spends, on average, in a reservoir is termed as the residence/reservoir time. The average residence time of some of the major reservoirs is presented in table 2.

10.5 Sources of fresh water

On the basis of occurrence the fresh water sources can be classified into two categories, Surface water and Ground water.

10.5.1 Surface water

Water that is located on the surface, naturally in the form of streams, rivers, lakes, and other waterways, or in manmade reservoirs, and other containments that have been built. They recharge from direct precipitation and surface runoff, and it is naturally lost through evaporation, evapotranspiration, groundwater recharge and discharge to the oceans. Total water in the surface water system depend on many factors like storage capacity of water body, the permeability of the soil beneath these storage bodies, the characteristics of runoff in the watershed, timing of the precipitation and local evaporation rates.

The surface water can be put into two categories, flowing waters and static waters.

Static water: This includes lakes, ponds and other reservoirs, natural or manmade. Lake is an area filled with water and localised in a basin. Lake is surrounded by land, apart from any river or other outlet or inlet that serves to feed or drain it. Most man made dams are associated with a lake.

Flowing waters: River or stream is natural flowing freshwater watercourse which under force of gravity flows towards ocean, sea, lake or another river. In some areas which are relatively drier, river flows into the ground and disappear at the end of its course without joining another water body. Rivers may be perennial and flow throughout the year or seasonal and flow during rains or snowmelt only. Apart from surface flow some perennial streams are associated with base flow also, which are supported by ground water. When the flow is base flow only then streambed is below water table, and such streams are called effluent streams. Such streams are found in drier climates. But when excess surface runoff drain into the stream, it supplements base flow, and such stream are called influent stream. Smaller streams or water channels are variously named as creek, brook, rivulet and rill (Figure 3).

Figure 3. Influent streams- Water table below stream flow. Effluent streams- Water table above stream flow.

Watershed

Watershed is an area of land that drains all the streams and rain to a common outlet such as the outflow of a reservoir, mouth of a bay or any point along a stream channel. It is used interchangeably with drainage basin or catchment area. Ridges and hills that separate two watersheds are termed as the drainage divide . Watersheds are the basic practical units for the managment of water resources. The pictorial presentation of a watershed shown in Figure 4

Figure 3. A watershed, dipicting drainage basin witin the drainage divide (red dotted line)

10.5.2 Groundwater

It is the water that resides in the cracks and spaces in soil, sand and rock beneath the earth’s land surface. It is stored in and moves slowly through geologic formations of soil, sand and rocks. When precipitation occurs and water flows over the permeable soil, gravity draws water down ward through this permeable soil. This makes the pore spaces in soil and rocks partly filled with water and partly with air. This zone below the ground surface is called zone of aeration or vandose zone ( Figure 5). The water continues to infiltrate downward through vandose zone until impermeable material is reached and begins to accumulate over it. In this way this zone just above the impermeable layer becomes saturated with water and called zone of saturation or phreatic zone. This is the true ground water and top of this zone is known as water table. The underground water not only moves vertically but horizontally too. The sub surface three dimensional systems which contains the entire flow paths followed by all the water is called Ground water basin. The basin may contain one or more recharge and discharge areas.

The subsurface permeable geological structure, that holds and transmits significant quantities of water to wells and springs is called aquifer. Aquifers are of two types, confined aquifer and unconfined aquifer, on the basis of permeability of its upper layer. Unconfined aquifer is one in which overlying soils and rocks are permeable. Water in such aquifer is not under unusual pressure. When any well is drilled into unconfined aquifer, the water will rise to the same height as is the water table in the adjacent rocks. All the area over such aquifers recharges it by surface water because of presence of permeable rocks. Most of the hand pumps and wells we use for water withdrawal belong to unconfined aquifers. On the other hand confined aquifer is one in which overlying soils and rocks are impermeable. In such aquifer water may be under considerable pressure because of impermeable closed stricture above it and lateral elevation of aquifer. If well is drilled in confined aquifer the water can rise above its level in aquifer because of pressure. This is called artesian system. Related to the artesian system is piezometric surface, which is the imaginary level to which the pressurised water would rise if bore hole is drilled in artesian system. In contrast to the aquifer, the aquiclude is the formation which is porous and capable of absorbing water, but does not transmit water at rates sufficient to furnish an appreciable supply for a well or spring. Similarly aquitard term is used to describe the less permeable beds those may be permeable enough to transmit water in quantities that are significant in the study of regional ground water flow, but their permeability is not sufficient to allow the completion of production wells within them. Sometimes discontinuous impermeable or relative impermeable layers (aquiclude/aquitard) are found in permeable vandose zone. So the  water accumulates above the actual regional water table in unsaturated zone, this type of structure is known as perched aquifer, and its upper water layer is called perched water table.

10.6 Water quality

The quality of any body of surface or ground water is a function of either or both natural processes and human activities. The anthropogenic polluting activities, such as the discharge of domestic, industrial, urban and other wastewaters into the water sources contribute to the existing natural quality of the water. How ever even without human influence the natural water quality would vary due to the contribution from the weathering of bedrock minerals, by the evapotranspiration , the deposition of dust and salt by wind, by the natural leaching of organic matter and nutrients from soil, by hydrological factors that lead to runoff, and by biological processes within the aquatic environment can lead to change in physical and chemical composition of water.

Typically, water quality is determined by comparing the physical and chemical characteristics of a water sample with water quality guidelines or standards. Drinking water quality guidelines and standards are designed to enable the provision of clean and safe water for human consumption, thereby protecting human health. These are usually based on scientifically assessed acceptable levels of toxicity to either humans or aquatic organisms.

“Water quality” is a term used to express the suitability of water to sustain various uses or processes. Any particular use will have certain requirements for the physical, chemical or biological characteristics of water; for example limits on the concentrations of toxic substances for drinking water use, or restrictions on temperature and pH ranges for water supporting invertebrate communities. Consequently, water quality can be defined by a range

of variables which limit water use. Although many uses have some common requirements for certain variables, each use will have its own demands and influences on the water quality.

10.7 Water quality standards

Practicaly water quality is determined by comparing the physical and chemical characteristics of a water sample with water quality guidelines or standards. Water quality guidelines and standards are designed to enable the provision of clean and safe water for human consumption, thereby protecting human health and to safeguard the quality water sources. Certain criterias are selected to describe the water quality.The critarias are usually based on scientifically assessed acceptable levels of toxicity to either humans or aquatic organisms as well as other process uses. States, territories and authorized bodie adopt water quality criteria to protect the quality of a water body. Water quality criteria can be numeric (e.g., the maximum pollutant concentration levels permitted in a water body) or narrative (e.g., a criteria that describes the desired conditions of a water body being “free from” certain negative conditions).

To safeguard the water quality, countries and states inact legislations to give a legal control and enforcement of the acts and rule describing the standards. The Government of India has enacted, The Water (Prevention and Control of Pollution) Act was enacted in 1974 to provide for the prevention and control of water pollution, and for the maintaining or restoring of wholesomeness of water in the country.

Standards are the concentration to be maintained to achieve the stated objectives .The term standard applies to any define rule , principle or measurement established by the authority or that has official backing.Generaly standard is defined as “pollution level that cannot legally be exceeded during a specific time period in a specific geographical are.”The primary standards are related to human health and secondary standards are related to protect human welfare. Three types of standards are of interest in water quality moniotoring study.

Drinking water standards Effluent standards Stream standards

Drinking water standards

In 1975, a report prepared by the World Health Organization in cooperation with the World Bank showed that some 1230 million people were without safe water supplies. These appalling facts were central to the United Nations decision to declare an International Drinking Water Supply and Sanitation decade, beginning in 1981. Further, the VI Five-Year Plan of India had made a special provision for availability of safe Drinking water for the masses. Therefore, the standard was prepared with the objective of assessing the quality of water resources, and to check the effectiveness of water treatment and supply by the concerned authorities. The standard therefore categorized various characteristics as essential or desirable and has been adopted by Bureau of Indian Standards (BIS).

Tables 3 to 7 Compare the Indian drinking water standards with that of World Health Organization (WHO).

Table3. Organoleptic and Physical parameters

Effluent Standards

The effluents standards pertain to the quality of wastewater originating from community, agricultural operation and industry. In general, these standards restrict the quality and quantity of pollutants in the effluents that set the desired degree of treatment required befor discharge. They important discharge standards are given in Table 8. Table8. Discharge parameters and standards.

Stream Standards

The stream standards are set with the purpose of maintaining the quality of the various types of water bodies. In India, the Central Pollution Control Board (CPCB) has developed a concept of designated best use. According to this, out of the several uses of water of a particular body, the use which demands highest quality is termed its designated best use. Five designated best uses have been identified. This classification helps the water quality managers and planners to set water quality targets and design suitable restoration programs for various water bodies. Criteria based designated use of water and water classification given by Central Pollution Control Board is given in the table9.

Table9. Water Quality Criteria, Central Pollution Control Board (CPCB)

The water quality criterias given in the classification of water as per best use determines the level of treatment for use of the water as well of discharge of the waste into the various classes of water bodies.

1. For a water source of a particular class of water, if the quality critari of the class satisfy the intended use quality requirement ,then the water from the water source can be used with minor or no treatment.
2. If the water quality of the intended use is higher than the quality of the class of water available, then the level of treatment to be done befor use will be higher to bring the water to the desired quality level.
3. The quality of the wastewater to be discharged into a particular class of water source, has to be equal or higher than that class of that water body.

References

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