Unit 33: Water — Long Questions

Acid rain is produced when sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) dissolve in rainwater, increasing the concentration of hydronium ions (H₃O⁺) and lowering the pH of the water that reaches aquatic ecosystems. Water is amphoteric and its self-ionisation (Kw = 1.0 × 10⁻¹⁴ at 25°C) sets the baseline pH scale, to which aquatic life is highly sensitive. This increased acidity mobilises toxic heavy metal compounds, such as aluminium hydroxide and lead salts, from bottom sediments into the water column — either by increasing their solubility through protonation or by displacing them from mineral surfaces. As a result, acid rain does not introduce heavy metals directly, but it increases their bioavailability and toxicity to aquatic organisms that are already present in sediments.

Agricultural runoff is a classic example of Non-Point Source (NPS) pollution: rainfall washes ammonium nitrate (NH₄NO₃) fertilizers and pesticides/herbicides (e.g. DDT, glyphosate) off thousands of hectares of farmland into rivers, at lower concentrations but over a very wide area, dependent on rain events. The excess nitrogen and phosphorus cause eutrophication: rapid algal blooms that block sunlight, and whose decomposition consumes dissolved oxygen, creating hypoxic "dead zones" where aquatic life cannot survive. Pesticides such as DDT are non-polar, persistent organic pollutants that dissolve into fatty tissues (not water) and bioaccumulate; they act as endocrine disruptors, causing reproductive failure in wildlife (e.g. eggshell thinning in birds) and are linked to hormonal cancers in humans. High nitrate levels in drinking water sourced from such runoff can also cause "Blue Baby Syndrome" (methaemoglobinaemia) in infants. Overall, agricultural runoff degrades water quality, reduces biodiversity, and disrupts aquatic ecosystems.

Agricultural runoff contributes to several distinct types of water pollution: (1) Nutrient pollution/Eutrophication — fertilizers like ammonium nitrate cause algal blooms, oxygen depletion, and dead zones; (2) Chemical pollution — pesticides and herbicides (DDT, glyphosate) and phosphate-fertilizer impurities (a source of cadmium) contaminate water, bioaccumulate, and harm human and aquatic health; (3) Microbiological pollution — runoff from agricultural land carrying animal waste introduces pathogens such as E. coli, Salmonella, and Giardia, causing water-borne diseases; (4) Sediment pollution — poor irrigation practices increase soil erosion, washing sediment into rivers and streams, which clogs waterways and harms aquatic habitats. As a Non-Point Source, all of this enters water bodies at low concentration but over a wide area, heavily dependent on rainfall.

Heavy Metals • Mercury — bacteria convert it to methylmercury ([CH₃Hg]⁺), which is highly fat-soluble, crosses biological membranes and the blood-brain barrier, and biomagnifies. It is a potent neurotoxin causing Minamata disease, characterized by ataxia, numbness in the hands and feet, and general muscle weakness.
• Lead (Pb²⁺) — interferes with the production of haemoglobin, leading to anaemia. In children it acts as a cumulative poison causing irreversible neurological damage and developmental delays.
• Cadmium (Cd) — Cd²⁺ has a higher charge density and can replace Ca²⁺ in bones, causing "Itai-itai" disease, where bones become extremely brittle; it also causes severe kidney damage (nephrotoxicity).

Pesticides and related organics • Pesticides/herbicides (DDT, glyphosate) are non-polar, accumulate in fatty tissues, and act as endocrine disruptors, mimicking hormones like estrogen. This causes reproductive failure in wildlife (e.g. eggshell thinning in birds) and is linked to hormonal cancers in humans.
• PCBs (used as coolants in electrical transformers) are probable human carcinogens; they suppress the immune system and cause skin conditions such as chloracne.

Sources of Heavy Metals • Mercury — coal combustion, gold mining, and industrial waste from thermometer or battery manufacturing.
• Lead — leaded petrol (now banned worldwide), old plumbing (lead pipes), lead-acid batteries, and paints; lead ions (Pb²⁺) enter water mainly through corrosion of pipes.
• Cadmium — metal plating, plastic stabilisers, and impurities in phosphate fertilizers.
In general, heavy metals enter water systems through industrial discharge, mining operations, and improper waste disposal.

Sources of Organic Compounds • Pesticides and herbicides (e.g. DDT, glyphosate) — mainly from agricultural runoff and domestic gardening.
• PCBs (polychlorinated biphenyls) — used as coolants in electrical transformers; they leak into water from hazardous waste sites.

Water treatment proceeds through primary (removal of physical debris), secondary (biological processing), and tertiary (chemical purification) stages, using the following technologies:

1. Screening and Sedimentation — water is passed through metal screens to remove large insoluble particles, then held in large tanks (sedimentation) so suspended solids settle under gravity.
2. Coagulation and Flocculation — chemical coagulants such as aluminium sulphate (Al₂(SO₄)₃) are added to precipitate dissolved contaminants (heavy metals, phosphates) and cause fine colloidal particles to clump into larger masses called "floc", which are then removed.
3. Filtration — water passes through layers of graded sand and gravel, which act as a physical barrier trapping fine suspended matter and some microorganisms; activated carbon is used for more advanced purification.
4. Disinfection — chlorination is most common: chlorine gas or sodium hypochlorite (NaOCl) forms hypochlorous acid (HOCl), which penetrates the cell walls of bacteria and viruses and disrupts their functions. Ozone (O₃) and sometimes hydrogen peroxide (H₂O₂) are alternative disinfectants.
5. Reverse Osmosis (RO) — water is forced under pressure through a semi-permeable membrane that filters out dissolved salts, bacteria, and other impurities, producing high-purity water.
6. Ion Exchange — used to "soften" hard water; as water passes through an ion-exchange column, Ca²⁺ and Mg²⁺ ions are swapped for Na⁺ or H⁺ ions.

Dissolved Oxygen (DO) is the amount of oxygen available in water for aquatic organisms to breathe. High DO generally indicates clean, unpolluted water; low DO suggests pollution or heavy organic matter decomposition.

Biochemical Oxygen Demand (BOD) is the amount of oxygen required by microorganisms to break down organic matter in water over a specific period, usually five days at 20°C. It indicates the level of organic pollution: high BOD means a large amount of organic matter (heavy pollution), which depletes dissolved oxygen and harms aquatic life.

Chemical Oxygen Demand (COD) is the amount of oxygen required to chemically oxidise both organic and inorganic substances in water using oxidising agents (KMnO₄, K₂Cr₂O₇, etc.). High COD indicates a high concentration of oxidisable chemical pollutants, suggesting poor water quality and possible industrial or domestic contamination; it assesses the total pollution load.

Relation to water pollution as pollution increases, DO falls while BOD and COD rise. For example, very pure water has DO 7–10 mg/L, BOD < 1 mg/L, COD < 2 mg/L, whereas industrial waste can have DO 0–2 mg/L but BOD of 500–5000+ mg/L and COD of 1000–20,000+ mg/L. Used together, DO, BOD, and COD give a comprehensive picture of organic and chemical pollution levels and overall water quality.