Groundwater Chemistry | Geology Optional for UPSC PDF Download

Composition of Ground Water

Importance of Water on Earth

• Essential Uses: Water is vital for drinking, irrigation, and industrial processes.
• Geologic Cycle: Transforms rocks into clay, sand, and solutes.
• Source: Most water on land originates from rainwater.

Chemical Composition of Ground Water

• Rainwater Chemistry: Rainwater carries dissolved substances from the atmosphere.
• Interaction with Earth: As rainwater percolates through soil and rocks, its composition changes significantly, increasing dissolved minerals.
• Groundwater Characteristics: Higher levels of dissolved mineral constituents compared to surface and rainwater.

Ground Water Quality and Management

Dissolved Constituents in Ground Water

• Major Cations: Sodium (Na), Calcium (Ca), Silica (Si).
• Major Anions: Chloride (Cl), Phosphate (PO4³⁻), Bicarbonates (HCO₃⁻, CO₃²⁻), Nitrate (NO₃⁻).
• Minor Elements: Potassium (K⁺), Magnesium (Mg²⁺), Lead, Copper, Sulfate, Fluorine, Mercury, Arsenic.
• Suspended Materials: Clays, silts, and microorganisms, including both harmless and pathogenic bacteria.

Water Classification by Dissolved Solids

• Freshwater: <1,000 mg/kg.
• Brackish Water: 1,000-10,000 mg/kg.
• Saline Water: 10,000-100,000 mg/kg.
• Brine: >100,000 mg/kg.

Factors Influencing Ground Water Composition

Natural Factors

• Hydrogeological Factors: Climate, porosity, soil characteristics, and groundwater flow patterns.
• Mineralogical Factors: Composition of rocks and minerals through which water flows.
• Contact Area and Time: Surface area and exposure duration between water and rocks.

Geochemical Processes

• Weathering Reactions: Hydrolysis, carbonation, hydration, oxidation, and dissolution of minerals.
• Leaching Effects: Long contact times increase solute content in groundwater.

Chemical Constituents in Ground Water

Major Elements

• Calcium (Ca): Derived from igneous, metamorphic, and sedimentary rocks through weathering.
• Magnesium (Mg): Sourced from minerals like olivine, biotite, and serpentine.
• Sodium (Na): Found in minerals like albite and zeolites; highly soluble.
• Potassium (K): Sourced from silicate minerals but present in low concentrations in groundwater.

Carbonates and Bicarbonates

• Formation: CO2 dissolves in water, forming carbonic acid and carbonate species.
• Weathering: Contributes significantly to bicarbonate ions in groundwater.

Sulfates

• Sources: Gypsum, sulfur minerals, and oxidation of sulfides.
• Reactions: Include reduction, precipitation, and solution processes.

Trace Metals

• Common Trace Elements: Lead (Pb), Zinc (Zn), Copper (Cu), Mercury (Hg).
• Health Implications: Some, like nitrate and fluoride, can be harmful at high concentrations.

Ground Water Movement

• Slow Movement: Subsurface water moves slowly through porous media.
• Fissure Channels: Allow faster movement with less contact surface.

Ground Water Quality Assessment

Physical Quality Tests

• Parameters: Color, odor, taste, turbidity, and temperature.
• Turbidity: Suspended matter affecting water clarity.
• Color: Caused by natural colloidal particles or suspended matter.
• Other Indicators: Odor from gases (e.g., H2S) and taste changes due to chloride concentrations.

Chemical Quality Tests

• Major Cations and Anions: Include Ca, Mg, Na, K, HCO3, SO4, Cl.

Testing Methods:

• Titration: For bicarbonates and carbonates.
• Gravimetric/Colorimetric: For sulfates.
• Spectrophotometry: For trace metals.

Impact of Human Activities

Agricultural Impact

• Irrigation: Alters groundwater composition due to evapotranspiration and soil leaching.
• Fertilizers and Additives: Introduce nitrates, chlorides, and boron into groundwater.

Urban and Industrial Influences

• Contaminants: Sewage effluents, stormwater runoff, and leachates from solid waste.
• Pollution Sources: Include urban wastes, soil amendments, and pesticides.

Ground Water Management

• Monitoring Programs: Initiated by international organizations like WHO and UNEP.
• Key Objectives: Classifying water resources, identifying pollution sources, and proposing management strategies.

Summary

Understanding the composition and quality of groundwater involves analyzing natural and human-induced factors. Proper management ensures sustainable use of this vital resource while minimizing pollution and health risks.