Groundwater Chemistry | Geology Optional Notes for UPSC PDF Download

Composition of Ground Water

Importance of Water on Earth

• Water is crucial for various activities like drinking, irrigation, and industrial processes.
• It plays a vital role in the geologic cycle by transforming rocks into clay, sand, and solutes.
• Most of the water found on land originates from rainwater.

Chemical Composition of Ground Water

• Rainwater is not just pure H₂O; it carries a diverse range of dissolved substances from the atmosphere.
• When rainwater interacts with the Earth's surface, its chemistry undergoes significant changes.
• As water moves through the soil and into the groundwater system, the concentration of dissolved substances in groundwater rises rapidly.
• Groundwaters contain higher levels of dissolved mineral constituents compared to surface and rainwaters.

Ground Water Quality and Management

• Dissolved Constituents in Ground Water

  Groundwater is a natural reservoir for various forms of matter, including dissolved, suspended, and colloidal elements. It serves as a habitat for numerous microorganisms. Dissolved constituents in groundwater consist of dissociated and undissociated substances, primarily originating from interactions with aquifer rocks.

  • Major constituents:
    • Positively charged cations: sodium, calcium, silica
    • Negatively charged anions: chloride (CI), phosphate (PO4³-), bicarbonate carbonates (HCO₃⁻ CO₃²⁻), nitrate (NO₃⁻)
  • Minor constituents:
    • Potassium (K⁺), magnesium (Mg²⁺), lead, copper, brines, sulfate, fluorine, mercury, arsenic, and other trace elements

• Suspended Materials and Micro-organisms

  Suspended materials in groundwater mainly consist of clays and fine silts. Microorganisms found in groundwater systems include bacteria and viruses, some of which are harmless while others, known as pathogenic bacteria, can cause diseases.

• Composition of Various Types of Waters

  The concentration of dissolved ions in groundwater is commonly measured in parts per million (ppm), where one ppm represents one part of dissolved matter in one million parts by weight of the solution.

  • Freshwater: 1,000 mg/kg
  • Brackish water: 1,000-10,000 mg/kg
  • Saline water: 10,000-100,000 mg/kg
  • Others: >100,000 mg/kg

Factors Governing the Composition of Ground Water

• Water's Solvent Properties:
  • Water has the ability to dissolve a wide range of substances due to its unique chemical structure.
  • The water molecule is polar, meaning it has a positive and negative end, allowing it to surround and dissolve charged ions from minerals.
• Natural Factors:
  • Hydrogeological Factors:
    • The composition of ground water is influenced by factors like regional structure, degree of fissuring, climate, porosity, soil characteristics, and groundwater flow patterns.
  • Mineralogical Factors:
    • The minerals present in the rocks through which water flows impact the composition of groundwater.

Understanding Ground Water Quality and Management

Factors Influencing Ground Water Composition

• Rock Texture: The texture of rocks plays a significant role in determining the composition of ground water.
• Rock Temperature: The temperature of rocks affects the characteristics of the water passing through them.
• Mineralogical Composition: Different minerals in rocks contribute to the chemical composition of ground water.
• Contact Area: The surface area of contact between water and rocks influences water composition.
• Exposure Time of Rock: The duration for which water is in contact with rocks impacts its quality.
• Purity and Crystal Size of Minerals: The purity and crystal size of minerals in rocks affect water quality.
• Chemical Composition of Rocks and Minerals: The chemical makeup of rocks and minerals influences the dissolved ions in ground water.

Source of Dissolved Ions

• The mineral matter in rocks serves as the source of dissolved ions in ground water.

Characteristics of Different Rock Types

• Low Permeability Rocks (Igneous and Metamorphic): These rocks mainly store ground water in fracture planes.
• Sedimentary Rocks: They can become impermeable through diagenesis, containing water mainly in fracture planes.
• Porous Sandstones: These rocks can be completely saturated by water, with water contact surface increasing as grain size decreases.

Ground Water Movement

• Subsurface water movement is slower in large contact surfaces, particularly in porous media.
• Fissure Channels: Water movement is faster along fissure channels with less contact surface.
• Leaching Effects: Large contact surfaces with long periods of contact lead to powerful leaching effects.

Effects of Depth and Temperature

• Concentration of Dissolved Solids: Concentrations increase downward in water from deep ground water zones due to increased contact time with rocks.
• Temperature Impact: Rock temperatures rise with depth, affecting water solubility and dissolution rates of minerals.
• Saturation Deficit: The rate of solution is proportional to the saturation deficit, which increases with temperature.

Ground Water Quality Overview

• Saturation Deficit and Solute Content: As groundwater depth increases, the saturation deficit also rises, leading to higher solute content. This relationship influences the quality of groundwater.
• Impact of Climatic Patterns: Different climatic patterns result in varied plant communities and soil types, which, in turn, affect the composition of water. For instance, areas with abundant vegetation tend to have a higher presence of HCO3 ions.
• Metals and Plant Accumulation: Certain metals accumulate in plants, reaching peak concentrations. When these plants decompose, the metals enter the circulating groundwater, affecting its composition.
• Weathering Reactions: Alternating wet and dry seasons trigger weathering reactions that produce significant amounts of soluble inorganic matter, influencing the composition of streams and groundwater.
• Soil Formation and Water Composition: Some processes involved in soil formation are similar to the factors that control water composition, highlighting the interconnectedness of soil and water quality.
• Role of CO2 and Carbonates: Soil air, rich in CO2, influences water pH by producing carbonate ions through interactions with water, affecting the dissolution of rock minerals.
• Composition of Soluble Salts: Salts in soils and rocks consist of cations (Na, Ca, Mg), anions (SO4, Cl), and minor constituents (Pb, Hg), originating primarily from the earth's crust minerals.
• Process of Weathering: Weathering involves various geochemical reactions such as hydrolysis, carbonation, hydration, solution, and oxidation, gradually releasing constituents into soluble forms.

Key Geochemical Reactions

• Hydrolysis: Involves the reaction between water and minerals, leading to the alteration of mineral compositions. For example, orthoclase undergoes hydrolysis to form clay minerals.
• Carbonation: Speeds up hydrolysis when CO2 and SO2 are dissolved in water, affecting the transformation of alumino silicates into clay minerals and carbonates.
• Hydration: The process of adding water molecules to minerals, changing their composition.
• Oxidation: Involves the loss of electrons, commonly occurring through oxygen dissolution. For instance, iron minerals can oxidize to form different compounds.
• Solution: Some minerals are directly soluble in water, dissociating into component ions.

Ground Water Quality and Management

• Geochemical Processes
  • Various salts and geochemical processes interact with rocks, altering groundwater compositions significantly.
  • For instance, the presence of ferric oxide precipitate (Fe2O3) can impact water quality.
• Man-made Factors
  • Agricultural Impact
    • Water moving past the root zone in irrigated areas differs in composition due to factors like evapo-transpiration and leaching of soil salts and additives.
    • Mobile constituents like Na, Cl, NO3, and B are prone to concentration through evapo-transpiration.
    • Salinity of return flow increases due to dissolution of soil salts, fertilizers, and additives, impacting groundwater quality.
  • Urban and Industrial Influences
    • Urban sources introduce contaminants like sewage effluents, stormwater runoff, and solid wastes, potentially polluting groundwater.
    • Leachates from solid waste disposal sites can lead to groundwater pollution, determined by waste type, volume, soil conditions, and aquifer depth.
• Surface Contaminants
  • Soil amendments, pesticides, herbicides, and animal wastes can introduce salts, organic matter, and bacteria into the groundwater, affecting its quality.
  • Chemicals in soil amendments like gypsum and lime can alter pH levels and bicarbonate content, influencing groundwater chemistry.

Determination of Ground Water Quality

• Over the past few decades, there has been a growing awareness of the deterioration of water resources due to human activities.
• This degradation is particularly severe in urban and industrialized regions, leading to widespread water pollution.
• Water pollution is easily noticeable due to its extensive spread and high mobility within the hydrological system.
• Various international organizations, such as WHO, UNESCO, WMO, and UNEP, have developed monitoring programs to address this issue.
• In 1974, WHO initiated a freshwater quality program with support from UNEP, leading to the establishment of a joint project in 1976 involving UNEP, WHO, UNESCO, and WMO.
• The goals of this project include classifying water resources, collecting baseline data on natural water quality, conducting routine water quality surveillance, identifying and quantifying pollution sources, aiding in the selection of pollution control measures, and proposing water resource management strategies.

Usage of Water and Quality Assessment

• Water serves various purposes in households, agriculture, industries, and recreational activities, each with different water quantity and quality requirements.
• Assessing water quality for drinking, domestic use, industrial processes, and agriculture involves evaluating the physical, chemical, and bacteriological properties of water.
• Understanding water quality is crucial for managing water and land resources effectively.

Composition of Ground Water

• Factors influencing the composition of ground water play a significant role in determining its quality.
• Weathering reactions contribute to changes in the composition of ground water, affecting its properties and constituents.
• The inorganic components of sewage, such as Na, Cl, Ca, SO4, HCO3, NO3, TDS, and trace metals, can impact ground water quality.
• Ground Water Quality and Management
  • Physical Quality of Ground Water
    • Physical Quality Tests
      • Tests such as color, odor, taste, turbidity, and temperature are conducted to determine the physical quality of water.
    • Turbidity
      • Turbidity refers to the presence of suspended matter in water that affects the passage of light through it.
      • It can be caused by various suspended materials of different sizes, ranging from colloidal to coarse dispersions.
      • Turbidity is crucial in public water supplies due to aesthetic, filterability, and disinfection reasons.
    • Colour
      • Natural color in water is primarily from negatively charged colloidal particles.
      • Surface waters may appear colored due to suspended matter, categorized as apparent or true color.
      • Color is measured in units, typically using a standard solution of K2PtCl6 for comparison.
    • Other Parameters
      • Odor can be produced by gases like H2S and CH4, while high chloride concentrations can make water taste salty.
      • Temperature can influence the amount of total dissolved solids in water.
      • Overall, variations in these parameters in groundwater are usually minimal.
  • Chemical Quality of Ground Water
    • Groundwater quality in terms of dissolved substances varies based on hydro-geochemical conditions and seasonal changes.
    • The concentrations of cations (Ca, Mg, Na, K) and anions (HCO3, SO4, Cl) are key indicators of groundwater composition.

Chemical Quality Variation in Ground Water

• Ground water quality varies due to the presence of different chemical constituents.
• Calcium
  • Major source: igneous, metamorphic, and sedimentary rocks
  • Principal sources in rocks: silicate group minerals
  • Formation in water: through weathering of minerals like plagioclase, feldspar, pyroxene, etc.
  • Carbonic acid impact: breakdown of minerals into soluble calcium products
• Magnesium
  • Abundance in rocks: basic igneous rocks, volcanic rocks, metamorphic rocks, sedimentary rocks
  • Major sources: olivine, augite, biotite, hornblende, serpentine, talc
  • Conversion in water: insoluble silicates to soluble carbonates through weathering
• Sodium
  • Occurrence in minerals: albite, plagioclase feldspars, nepheline, sodalite, glaucophane, aegerine
  • Important sources: precipitates in soils, certain clay minerals, zeolites
  • Solubility in water: readily soluble in ground water

Factors Influencing Chemical Composition

• Factors influencing chemical composition in ground water include:
• Weathering
  • Impact on mineral breakdown and solubility
  • Conversion of insoluble silicates to soluble carbonates
• Carbonic Acid Presence
  • Effect on mineral solubility and water composition
  • Conversion of minerals into more soluble forms

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Ground Water Quality and Management

Potassium

• The primary sources of potassium (K) in ground water include silicate minerals found in igneous and metamorphic rocks, as well as sedimentary rocks.
• Despite being abundant in rocks, the concentration of potassium in ground water is typically much lower compared to sodium (Na).
• Potassium concentrations in potable ground waters usually range from 1 ppm to 10 ppm, while in brines, they can range from 100 ppm to several thousand ppm.

Carbonates and Bicarbonates

• One of the most significant dissolved constituents in ground water is bicarbonate (HCO3), which is derived from various sources, including the atmosphere, hydrosphere, and lithosphere.
• Carbon dioxide (CO2) dissolves in water, leading to the formation of carbonic acid and the subsequent production of carbonate species.
• Chemical weathering of rocks by CO2-saturated water also contributes to the presence of carbonate species in natural waters.

Weathering Reactions

• Weathering reactions involving carbonate minerals and silicates play a crucial role in the accumulation of bicarbonate ions in ground water.
• Examples of weathering reactions include the congruent dissolution of carbonate minerals like calcite and the incongruent weathering of silicates to form clays.

Sulphate

• Ground water's sulphate content varies due to geochemical reactions such as reduction, precipitation, solution, and concentration.
• Sources of sulphate in ground water include sulphur minerals, sulphides from rocks, and gypsum from sedimentary rocks.
• Oxidation of sulphide minerals results in the production of sulphates, leading to variations in sulphate concentrations in ground water.

Ground Water Quality

• Sulfuric Acid and Calcium Sulfate Formation: - Sulfuric acid reacts with calcium carbonate in weathered zones to produce soluble calcium sulfate.
• Concentration of Calcium Sulfate: - Calcium sulfate can dissolve in water up to about 1500 ppm at room temperature.
• Chloride Sources and Concentration: - Chloride in groundwater comes from minerals like sodalite and chlorapatite as well as evaporite minerals. - Concentrations can vary widely from a few ppm to several thousand ppm based on different sources.
• Total Hardness of Water: - Water hardness is defined by the concentration of calcium and magnesium ions as calcium carbonate. - It is categorized into carbonate hardness (temporary) and non-carbonate hardness (permanent).
• Minor Constituents in Ground Water: - Fluoride, nitrate, and boron are essential but occur in minor quantities. - High concentrations of nitrate and fluoride can be harmful to human health.

Understanding Groundwater Quality and Management

Trace Metallic Constituents in Groundwater

• Certain metallic elements in groundwater exist in trace amounts, such as Pb, Zn, Cu, Ni, Cd, and Hg, with concentrations below 1 ppm.

Determination of Dissolved Chemical Constituents

• Anionic constituents like HCO3 and CO3 in groundwater are typically identified through volumetric methods.
• For HCO3 and CO3 ions, a titration is conducted using 0.02 M H2SO4 as a titrant, with methyl orange and phenolphthalein as indicators for different pH ranges.
• The titration results in total alkalinity or phenolphthalein alkalinity, with the endpoint signaled by an orange color.
• Chloride (Cl) is determined using silver nitrate as a titrant and potassium chromate as an indicator, leading to the formation of silver chloride (AgCl) precipitate at the endpoint.

Quantifying Concentrations

• Concentrations of CO3, HCO3, and Cl are calculated based on the quantities of H2SO4 or AgNO3 consumed and their respective normalities.

Sulfate Determination

• Sulfate (SO4) levels are determined through gravimetric or colorimetric methods, involving precipitation with barium chloride to form barium sulfate (BaSO4).

Cationic Constituents Analysis

• Major cations and minor constituents are typically analyzed using colorimetric, flame photometric, and atomic absorption spectrophotometric methods.