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PAPER No. 4: Environmental Chemistry MODULE No.4: Hydrosphere and Chemical composition of water bodies

Subject Chemistry

Paper No and Title Paper 4, Environmental Chemistry

Module No and Title Module 4 : Hydrosphere and Chemical Composition of water bodies

Module Tag CHE_P4_M4_e-text

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TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Chemical Composition of water bodies 3.1 Dissolved Gases 3.2 Metal Ions 4. Chemical Composition of specific water body 4.1 Ground Water 4.2 Surface Water

4.3 Wetlands 5. Summary

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1. Learning Outcomes

After studying this module, you shall be able to:

• Know about the hydrosphere and distribution of water on earth. • Learn the various components found in water bodies. • Know the origin of various metal ions present in rivers.

2. Introduction

Hydrosphere

Hydrosphere is a term used to describe the combined mass of water found on, under and over the surface of a planet. It refers to the discontinuous layer of water at or near the earth’s surface. It includes surface water (which may exist in the liquid state or solid state in form of ice), groundwater and water vapour present in the atmosphere. The hydrosphere covers about 70% of the surface of the earth. In his award winning book, ‘Water’, Marq de Villiers described the hydrosphere as a closed system in which water exists and says it has been built with the specific purpose of regulating life. The total amount of water, according to him, has not changed since geological times. Water can be polluted, abused and misused but certainly not created or destroyed, it can only migrate. Water is the basic necessity for life. It has been referred to as ‘the elixir of civilization’. All ancient civilizations have flourished on river plains. Several cities and civilizations have disappeared due to water scarcity resulting from climatic changes. Water is the most abundant substance present on the earth’s surface and is distributed in various forms like oceans, seas, rivers, lakes, streams, groundwater, ice caps, glaciers, soil moisture and water vapour in the atmosphere. The hydrosphere holds about 1.39 x 109 km3 of water. The oceans and seas hold about 97% of the hydrosphere in form of salt water. The dominant part of fresh water is stored in ice caps and glaciers. Only 0.3 to 0.4% of fresh water is found in lakes and streams while 0.05% is present in rivers. The water content of wetlands and swamps is about 0.03% while the atmosphere contains about 0.004%. This trivial amount plays an important role in the hydrological cycle. The earth’s water is not pure but contains dissolved and suspended impurities which may be inorganic or organic. The movement of water plays an important role in the transportation of these materials on the earth’s surface.

3. Chemical composition of water bodies

3.1 Introduction

Water is a unique liquid; without it, life is not possible on earth. Water bodies have a complex composition which includes various gases, minerals and organic substances. Due to the presence of metal ions, dissolved gases and other organic substances different types of chemical reactions like oxidation-reduction, complexation etc. occur in water. Atmospheric precipitation is the

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principal source of the water that makes up lakes and rivers on the earth’s surface. The water is not pure it contains atmospheric gases and also dissolved and suspended mineral matter. The major chemical components which are found in all water bodies are dissolved gases and different ions and these are discussed below.

3.1.1 Dissolved Gases

The dissolved gases owe their presence in the water bodies to the fact that the atmosphere and the water bodies are in contact with each other and the exchange of gases occur between surface of water body and atmosphere. The major gases which are present in water bodies are oxygen and carbon dioxide while methane and hydrogen sulfide are present in minor amounts. The amount of these gases in water is measured in ppm or mg/L. Aquatic plants (algae) release O2 (oxygen) in water by photosynthesis. This oxygen gets dissolved in water and is called dissolved oxygen (DO). DO is consumed in the process of decomposition of organic waste (pollutant) present in water. Due to depletion of DO the survival of aquatic organisms becomes difficult. The concentration of dissolved oxygen depends on temperature of water also the partial pressure and salt content of water. As temperature of the water increases, solubility of oxygen decreases. In tropical oceans and lakes, the level of oxygen at surface water is lower than at the depth. However, surface water does not become deprived of O2 as the gas diffuses into water from the atmosphere.

The second important gas present in water bodies is (carbon dioxide) CO2. The main sources of CO2 in water are (i) from the decomposition of organic waste and (ii) by the presence of CO2 in air. Dissolved CO2 is present as loosely associated carbonic acid. When falling as rain, this slowly erodes calcareous rocks and results in input of calcium slats to adjacent water bodies during drainage. The soluble atmospheric gases enter the water bodies during rain fall. Any atmospheric gases that are soluble in water will pass into water bodies during rainfall. Reducing conditions in sediments result in the production of (methane) CH4 and (hydrogen sulphide) H2S and these gases are also produced by volcanic process.

3.2. Metal Ions

Many chemical substances are soluble in water. The major sources of solutes are minerals and rocks with which water is in contact. Metal ions in water (Mn+) exist in many different forms (ions, hydrated metal ions, undissociated compounds and colloids). Some ions are present in large amount (Ca+2, Mg+2, Na+, K+, Cl-, SO4

-2, HCO3-1, CO3

-2) and are called macro component which are present in amount exceeding mg/L. other elements like borate and fluoride are present in small amounts (µg/L) and are called micro component.

Calcium ion: It is generally found in highest concentration. Minerals like gypsum, anhydrite, CaSO4, dolomite etc. are the major source of calcium in water bodies. Ca+2ion along with Mg+2 ion accounts for the total hardness of water. Chloride ion occur in water samples are due to dissolution of salt deposits, discharge of effluents, mixing of sewage wastes, sea water contamination and flow of irrigation drainage. Chloride ion is present in all types of water.

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Sulphate is present in water because sulphate bearing minerals are common in most sedimentary rocks. In the weathering process, many of them e.g., gypsum get dissolved in water. Most of the sulphate in water system originates from the oxidation of sulphide ores and their dissolution. The oxidation of pyrites FeS2 leads to the formation of sulphates of iron and H2SO4. Carbonate and bicarbonate: Dissolved CO2 reacts with water to form carbonic acid. H2CO3 a weak inorganic acid. H2CO3 further dissociates to form H+, HCO3

-and CO32-.

The relative amounts of CO32-, HCO3

-, and CO2 are dependent on pH. Figure 3 showed the various forms of inorganic carbon at different pH. At pH below 4.5 H2CO3 & CO2 species predominates and almost no CO3

2- and HCO3- found.

All components of natural water confer certain properties like salinity, alkalinity, acidity, hardness etc. Water contains dissolved matter (referred to as TDS which is total dissolved solids) and suspended matter. TDS in fresh water is expressed in terms of g/dm3 or mg/dm3(ppm) while the same for other types of water is expressed in form of PSU (practical salinity unit).

Figure 1 Forms of inorganic carbon at different pH levels

4. Chemical Composition of specific water bodies

4.1 Ground water

Groundwater and surface water have different characteristics. These are depicted in table 1.

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Table 1: Comparison of properties of surface and ground water

As water percolates through layers of soil it remains in contact with minerals present in soil and gets saturated with dissolved solids. In this process of seepage it gets depleted of microorganisms present in surface water. The types and relative concentrations of the chemical constituents depend on residence time, solubility and rate of movement.

The characteristics of groundwater are its weak turbidity, constant temperature, chemical composition and overall absence of oxygen. Circulation of ground water varies its composition due to pollutants and contaminants present in it. Furthermore, groundwater is often very pure microbiologically

The major mineral constituents come from soluble minerals in soils and sedimentary rocks. The common ions present are calcium, sodium, bicarbonate and sulphate. Ground water present in coastal areas contains chloride. Nitrate ion may be a natural component but a high level indicates pollution. The groundwater in the Ganga – Brahmaputra delta region in West Bengal and Bangladesh is severely contaminated with arsenic compounds which are highly toxic rendering the water unfit for consumption.

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4.2 Surface water

4.2.1 Rivers and Streams

The chemical composition of surface water is variable through time and sampling must be done periodically at different places along the stream and river course. During dry season ground water contributes more water to rivers while in rainy season the major contribution is from run offs. The composition of river and stream water changes due to pollution. Rivers are dominated by the presence of calcium, silica and hydrogen carbonate which mainly owe their origin from silicate rocks and carbonates. The level of potassium is low due to retention in clays. The dissolved inorganic components may be classified into macro components and micro components. Macro components constitute 90 – 95% of all components in fresh water and include calcium, sodium, magnesium, potassium, chloride, sulphate, hydrogen carbonate and carbonate. The concentration of micro components a generally up to 1ppm and includes lithium, rubidium, caesium, bromide, iodide, fluoride, copper, zinc, nickel, silver, lead and other heavy metals. Dissolved gases like nitrogen, oxygen, argon and carbon dioxide are also present. Surface water contains nutrients which are limiting factors for growth of aquatic plants. These include nitrogen (in form of ammonium, nitrite, nitrate and organic compounds) and phosphorus (in inorganic and organic forms).

Dissolved organic compounds generally enter as pollutants. They may increase mobility of heavy metals and influence dissolved oxygen content and flavor of water. The average salinity of river water is low – about 120 ppm by weight. Of this salt content, carbon as hydrogen carbonate and carbonate (referred to as dissolved inorganic carbon, DIC) constitutes 48% and silicon as silicic acid 33%. The remaining 19% consists mainly chloride, sodium and magnesium in descending order

The composition of river and stream water is controlled by various factors including water rock interaction (weathering), temperature and human activity. As water interacts with carbonate rocks, the DIC increases due to the following reaction.

(In this reaction 2 moles of hydrogen carbonate is formed – 1 mole from the rock and 1 mole from the atmosphere)

With silicate rocks the reaction (Urey reaction) is represented as

(In this reaction atmospheric carbon dioxide is converted into hydrogen carbonate)

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Weathering reaction may be congruent when the entire mineral goes into solution (as the reaction of CaCO3 with water and CO2) or incongruent when the mineral is leached and modified to give secondary minerals. The weathering of aluminosilicates to give clay minerals is an example of incongruent reaction.

In addition to weathering of carbonate and silicate rocks, weathering of salt deposits containing halite (NaCl) and gypsum (CaSO4), weathering of sulphide deposits and weathering of organic carbon affect river water composition.

Temperature is another crucial factor in determining river water composition. It affects rate of dissolution, elemental solubility and mineral stability. In tropical climates the weathering is more intense than that in temperate climate. The TDS in rivers and streams also depends on type of rock. Sedimentary rocks are more prone to weathering than volcanic rocks which in turn weather more than metamorphic rocks.

Apart from dissolved substances rivers contain suspended materials mainly silicon, aluminium, iron, calcium, sodium, potassium, titanium and organic compounds. The average chemical composition of some Indian rivers is given in Table 2.

Table 2: Average Chemical Composition (ppm) of Some Indian Rivers

4.2.2 Seas and Oceans

The main source of sea water is river discharge. Sea water has a more uniform composition than river water. It contains by weight about 3.5% dissolved salts while the same for river water is

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only 0.012%. The salinity of sea water is about 35 parts per thousand of which sodium and chloride make up about 30 parts while magnesium and sulphate contribute 4 parts. The remaining 1 part is made up by calcium and potassium (0.4 parts each) and the rest by carbon as hydrogen carbonate and carbonate. 99% of sea water is made up of eight elements (hydrogen, oxygen, carbon, sodium, potassium, chlorine, calcium and magnesium) though most of the naturally occurring elements have been detected in sea water. A comparison of the constituents of river and sea water is given in Table 3 and Table 4

Table 3 Average Composition of Sea water and River water

Table 4: Ratios of major Constituents In Sea Water and River Water

Sea water contains almost everything. By doing sensitive analysis of sea water it has been shown that every stable element is present in sea water although in very small amount e.g., gold is found to be present in 20 x 10-12mol/L. The Na+ and Cl- ions constitute about 85% of the ions found in sea water. Certain constituents in sea water, such as calcium, magnesium, hydrogen carbonate, and silica, are partly taken out of solution by biological organisms, chemical precipitation, or physical-chemical reactions. Ocean contains substantial quantities of many dissolved elements, complex solution of mineral salt and decayed biological matter. The chemical composition of sea water varies slightly from area to area due to addition of fresh water and evaporation. However this variation in small and in comparison to river water the composition is relatively constant.

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Most of salts found in ocean are obtained by weathering and erosion of cooled igneous rocks of earth’s crust and by the dissolving actions of rains and streams which transport their mineral washing to sea. Some of the ocean's salts have been dissolved from rocks and sediments below its floor. Other sources of salts include the solid and gaseous materials that escaped from the Earth's crust through volcanic eruptions or that originated in the atmosphere.

4.2.3 Lakes

A lake, in contrast to rivers and streams, is a body of relatively still water. Lakes are part of surface water bodies and are often the store house of water that supports the surrounding ecosystems.

The chemical composition of lakes is affected by complex chemical and biological processes occur in water body. There is lack of water circulation in such big reservoir and therefore the chemical reactions that occur on surface of water are different than that occurring at the bottom. Due to water’s unique temperature – density relationship distinct layers are formed in still water (Figure 4). This is called thermal stratification. . The surface of the lake is heated by the solar radiation while the lower part of the lake remains cooled and this result in the formation of layers in the lake. The upper warm layer is called epilimnion while the lower colder layer is called hypolimnion. Between these two layers there is a region in which the temperature changes rapidly called thermocline. Due to this thermal stratification and poor mixing between the layers (due to lack of water circulation) the epilimnion is well provided with oxygen and hypolimnion become depleted of oxygen. When the temperature difference between the surface layer (epilimnion) and bottom layer (hypolimnion) is significant, then these layers have different chemical and biological properties. The epilimnion has a higher concentration of dissolved oxygen due to photosynthetic activity of algae and is aerobic, while the hypolimnion has lower dissolved oxygen and more carbon dioxide. During autumn and spring, when the epilimnion cools, the temperature of both layers may become equal and then the lake behaves as one hydrologic unit. This is called overturn.

The chemical composition of lake water varies due to varying chemistry of erosion products of different lake basins but the main constituents are similar. The salinity is computed from the total concentration of ions present which are sodium, potassium, magnesium, calcium, carbonate, silicate and chloride. The order of abundance for cations and anions is:

The relative amount of these ions present is shown in Table 6. Other inorganic ions may be nitrate, phosphate (both are plant nutrients), metals like mercury, manganese, copper, lead and organic matter. Under anaerobic conditions hydrogen sulphide is also present. When aquatic plants take up carbon dioxide and hydrogen carbonate, then the pH of lake water may increase and this results in deposition of calcium carbonate and dolomite. In the absence of any living organism, a lake contains a wide array of molecules and ions from weathering of soils in the watershed, the atmosphere and the lake bottom. Therefore, the chemical composition

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of a lake is fundamentally a function of its climate. Each lake has an ion balance of the three major anion and four major cations.

Figure 2: Thermal stratification of lake

4.2.4 Wetlands

Wetlands are saturated with water permanently or seasonally. They cover 2.2% of the Earth’s surface and contain all elements of the biosphere. The water may be salt water (noted in wetlands long the coastal areas), fresh water or brackish (more salinity than fresh water but less than sea water).

Water Salinity based on Dissolved Salts

Fresh Water Brackish Saline Water

< 0.05% 0.05 – 3% 3 -5 %

The composition of water in wetlands is highly variable and no generalisation can be made. The composition depends on their location (adjacent to rivers, lakes and sea) and on the source of water. Wetlands receiving mainly water from the atmosphere generally have low mineral ionic composition while those acquiring water from precipitation as well as ground water are rich in minerals particularly calcium and magnesium The amount of dissolved oxygen and carbon dioxide is dependent on temperature and atmospheric pressure. The major ionic components in wetlands include ammonium, calcium, magnesium, sodium, potassium, sulphate and chloride. The minor components are cobalt, copper, nickel and zinc. The low concentration of the transition metal ions arise due to the fact that they are sorbed on particles or precipitates. Anions are less sorbed than cations. Many organic compounds are dissolved in the water and thus the TOC (total organic carbon) value is high. The pH of the water is variable and may range from acidic to alkaline. Arsenic, if present, is mobile at high pH. The water flowing out of wetlands is

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chemically distinct from inflow water because a range of physical and chemical reactions occur as water passes through organic materials.

5. Summary

• The combined mass of water found on, under and over the surface of the earth is known as the hydrosphere. • It includes surface water (oceans, seas, rivers, streams, lakes, glaciers, ice caps), ground water and water vapour. • 97% of the hydrosphere is in form of salt water in seas and oceans while the rest is in form of fresh water. • The major part of fresh water is stored in ice caps and glaciers. • The chemical composition of water bodies is variable but they all contain dissolved gases (mostly oxygen and carbon dioxide), inorganic ions (obtained from rocks and minerals) and organic compounds. • The composition of sea water is more uniform than that of fresh water. The major components are sodium and chloride. The salt content is quite high. • Rivers contain a high level of inorganic components like calcium, silica and hydrogen carbonate which are obtained by weathering of rocks. • Lakes exhibit thermal stratification where different layers are formed differing in temperature. These layers may have different chemical composition. • Wetlands may have fresh water, salt water or brackish water. The composition depends on location and source of water. • Human activities result in altering the composition of water bodies.

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