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Factors Controlling Water Quality



Factors Controlling Water Quality
Factors Controlling Water Quality

Pure water that contains only hydrogen and oxygen is rarely found in nature.Rainwater contains dissolved gases and traces of mineral and organic substances originating from gases, dust, and other substances in the atmosphere. When raindrops fall on the land, their impact dislodges soil particles and flowing watererodes and suspends soil particles. Water also dissolves mineral and organic matter from the soil and underlying formations. There is a continuous exchange of gases between water and air, and when water stands in contact with sediment in the bottoms of water bodies, there is an exchange of substances until equilibrium is reached. Biological activity has a tremendous effect upon pH and concentrations of dissolved gases, nutrients, and organic matter. In general, natural bodies of water approach an equilibrium state with regard to water quality that depends upon climatic, hydrologic, geologic, and biologic factors.
Human activities also strongly influence water quality, and they can upset the natural status quo. The most common human influence for many years was the introduction of disease organisms via disposal of human wastes into water supplies.
Until the past century, waterborne diseases were a leading cause of sickness and death throughout the world. We have greatly reduced the problems of waterborne diseases in most countries, but because of the growing population and increasing agricultural and industrial effort necessary to support mankind, surface waters and groundwaters are becoming increasingly contaminated. Contaminants include suspended soil particles from erosion that cause turbidity and sedimentation in water bodies; inputs of nutrients that promote eutrophication and depletion of dissolved oxygen; toxic substances such as heavy metals, pesticides, and industrial chemicals; and heated water from cooling of industrial processes.
Water quality is a complex topic of importance in many scientific and practical endeavors. It is a key issue in water supply, wastewater treatment, industry, agriculture, aquaculture, aquatic ecology, human and animal health, and many other areas. People in many different occupations need information on water quality. Although the principles of water quality are covered in specialized classes dealing with environmental sciences and engineering, many who need a general understanding of water quality are not exposed to the basic principles of water quality in a formal way. The purpose of this post is to present the basic aspects of water quality with emphasis on physical factors controlling the quality of surface waters. However, there will be brief discussions of groundwater and marine water quality as well as water pollution, water treatment, and water quality standards.
It is impossible to provide a meaningful discussion of water quality without considerable use of chemistry and physics. Many water quality posts in the internet are available,in which the level of chemistry and physics is far above the ability of the average readers to understand. In this post, I have attempted to use only first-year, collegelevel physics in a very basic way. Thus, most of the discussion of water quality hopefully will be understandable even to readers with only rudimentary formal training physics.
Water is a simple molecule with two hydrogen atoms and an oxygen atom. Its molecular weight is 18. Water has a unique feature that causes it to behave differently than other compounds of similar molecular weight. The water molecule is dipolar because it has a negatively charged side and a positively charged one.
This polarity leads to water having high freezing and boiling points, large latent heat requirements for phase changes between ice and liquid and between liquid and vapor, temperature dependent density, a large capacity to hold heat, and good solvent action. These physical properties influence the behavior of water in nature, and the student of water quality should be familiar with them.
Structure of Water Molecule
The water molecule consists of two hydrogen atoms covalently bonded to one oxygen atom . The angle formed by lines through the centers of the hydrogen nuclei and the oxygen nucleus in water is 105 _. The distance between hydrogen and oxygen nuclei is 0.96_10_8 cm. The oxygen nucleus is heavier than the hydrogen nucleus, so electrons are pulled relatively closer to the oxygen nucleus. This gives the oxygen atom a small negative charge and each of the hydrogen atoms a slight positive charge resulting in a separation of electrical charge on the water molecule or polarity of the molecule .
The molecules of any substance attract each other as well as molecules of other substances through van der Waals forces. Negatively charged electrons of one molecule and positively charged nuclei of another molecule attract. This attraction is almost, but not completely, neutralized by repulsion of electrons by electrons and nuclei by nuclei, causing van der Waals forces to be weak. Electrostatic van der Waals attractions between molecules increase in intensity with increasing molecular weight, because the number of electrons and nuclei increases as molecular weight increases.
The positively charged side of a water molecule attracts the negatively charged side of another to form hydrogen bonds. Hydrogen bonding is illustrated here in two dimensions, but hydrogen bonding actually is three-dimensional. One water molecule can be bonded to another with the axis of attraction extending in any direction.Hydrogen bonds are not as strong as covalent bonds or ionic bonds, but they are much stronger than van derWaals attractions. The degree of molecular attraction in water is much greater than in nonpolar substances. Because of hydrogen bonding, water actually has the structure (H2O)n rather than H2O. The number of associated molecules (n) is greatest in ice and decreases as temperature increases.All hydrogen bonds are broken in vapor, and each molecule exists as a separate entity (H2O).
Properties of Water
The physical properties of water have a profound influence on the behavior of water in nature. In explaining those properties, reference often will be made to standard atmospheric pressure. Thus, it is necessary to discuss the concept of standard atmospheric pressure before proceeding.
Atmospheric Pressure
Atmospheric pressure is the weight of the atmosphere acting down on a surface.The thickness and weight of the atmosphere above a surface varies with the elevation of the surface; mean sea level and a temperature of 0 _C is the reference point (standard) for atmospheric pressure.
The traditional method for measuring pressure is shown in Fig. 1.1. A tubeclosed at its upper end and evacuated of air is placed vertically in a dish of liquid.
If the liquid is water, the force of the atmosphere acting down on its surface at sea level would cause water to rise to a height of 10.331 m in the column. The device illustrated in Fig 1-1.  is called a barometer, and tmospheric pressure often is called barometric pressure. However, to avoid such a long glass column for the measurement of atmospheric pressure, water was replaced by mercury (Hg) that is 13.594 times denser than water. Standard atmospheric pressure measured with a mercury barometer is 760 mmHg. Today, there are alternatives to the mercury barometer for measuring atmospheric pressure. The most common—the aneroid barometer—is basically a box partially exhausted of air with an elastic top and a pointer to indicate the degree of compression of the top caused by the external air.
 A schematic view of a traditional mercury barometer
Fig. 1.1 A schematic view of a traditional mercury
barometer


Thermal Characteristics
Water is a liquid between 0 and 100 _C at standard atmospheric pressure. Freezing and boiling points of water, 0 and 100 _C, respectively, are much higher than those of other hydrogen compounds of low molecular weight, e.g., methane (CH4),ammonia (NH3), phosgene (PH3), and hydrogen sulfide (H2S), that are gases at ordinary temperatures on the earth’s surface. The aberrant behavior of water results from hydrogen bonding. Considerable thermal energy is required to break hydrogen bonds and convert ice to liquid water or to change liquid water to vapor. Molecules of other common hydrogen compounds do not form hydrogen bonds and are joined only by weaker van der Waals attractions.
Depending on its internal energy content, water exists in solid, liquid, or gaseous phase. In ice, all hydrogen atoms are bonded; in liquid phase, a portion of the hydrogen atoms is bonded; in vapor, there are no hydrogen bonds. An increase in the internal energy content of water agitates its molecules, causes hydrogen bonds to stretch and break, and temperature to rise. The opposite effect occurs when the energy content of water declines.The amount of energy (or heat) in calories (cal) required to raise the temperature of a substance by 1 _C is the specific heat. The specific heat of ice is about 0.50 cal/g/_C, the specific heat of water is 1.0 cal/g/_C, and steam has a specific heat of 0.48 cal/g/_C. Water has a high specific heat compared with most other substances.Thus, the specific heat of many common substances in nature depends on their water content. Dry mineral soils have specific heats of 0.18–0.24 cal/g/_C, but most soils may have specific heats of 0.5 cal/g/_C or more. Wood typically has a specific heat of 0.60 cal/g/_C or more, but the specific heat of succulent leaves is around 0.9cal/g/_C.
Water freezes when its energy content declines and molecular motion slows so that hydrogen bonds form to produce ice. Ice melts when its energy content rises and molecular motion increases and too few hydrogen bonds are present to maintain the crystalline structure of ice. If the temperature of water falls to 0 _C, 80 cal of heat must be removed from each gram of water to cause it to freeze with no change in temperature. It follows that to melt 1 g of ice at 0 _C with no change in temperature requires 80 cal. The energy necessary to cause the phase change between liquid water and ice (80 cal/g) is called the latent heat of fusion.

Water changes from liquid to vapor when it attains enough internal energy and molecular motion to break all hydrogen bonds. Water vapor condenses to form liquid water when it loses energy and molecular motion decreases to permit formation of hydrogen bonds. The amount of energy necessary to cause the liquid to vapor phase change is 540 cal/g. This quantity of energy is termed the latent heat of vaporization.











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