Unit 3: Environmental Monitoring: Air, Water, Soil, and Biological Assessment

1. Ambient Air Monitoring

Ambient air monitoring is the systematic measurement of pollutants in the outdoor atmosphere. It is essential for assessing general air quality, understanding health impacts, and enforcing regulatory standards. Air pollutants are broadly classified into gases and particulate matter (PM), each requiring distinct collection and measurement techniques.

Methods of Collection of Gases

Gaseous pollutants like sulfur dioxide (SO₂), nitrogen oxides (NO_x), carbon monoxide (CO), and volatile organic compounds (VOCs) are collected using several specialized methods:

• Passive Sampling: Relies on the natural diffusion of gas molecules onto an adsorbent material without active mechanical air movement. Diffusion tubes (such as Palmes diffusion tubes) are commonly used for long-term mapping of gases like NO₂.
• Active Sampling: Uses a mechanical pump to draw a known volume of air through a collection medium. This is categorized into:
  • Absorption (Impingers): The air stream is bubbled through a liquid chemical absorbent. For example, SO₂ is collected in a potassium tetrachloromercurate solution using a midget impinger.
  • Adsorption: Air passes through a solid adsorbent (e.g., activated charcoal, silica gel, or Tenax polymer) that traps gas molecules on its surface. These are later desorbed thermally or via solvent extraction for analysis.
• Grab Sampling: Captures a whole sample of air instantaneously using evacuated canisters, glass flasks, or inert plastic bags (e.g., Tedlar bags). This is ideal for volatile organic compounds.
• Condensation / Cold Trapping: Air is drawn through a collection tube immersed in a cooling agent (such as liquid nitrogen or dry ice) to freeze, liquefy, and isolate volatile components.

Methods of Collection of Particulate Pollutants

Particulate matter (such as PM₁₀ and PM_2.5) consists of solid or liquid suspended particles. Various mechanisms separate and collect these particles based on their aerodynamic diameter:

• Sedimentation: Employs dustfall jars to collect large, settleable particles (typically >10 µm) under the influence of gravity over a designated period, usually 30 days.
• Filtration: Air is drawn through a pre-weighed filter paper (such as glass fiber or PTFE) using a High-Volume Air Sampler (HVAS). The mass of collected particles is determined gravimetrically.
• Impaction: Uses cascade impactors where air passes through progressively smaller nozzles directed at collection plates. Larger particles with higher inertia impact first, allowing size-fractionated collection of particulates.
• Electrostatic Precipitation: Particles are ionized by passing through a high-voltage corona discharge and subsequently collected on oppositely charged plates. This method is highly efficient for capturing fine particles.
• Centrifugal Separation: Cyclone samplers spin incoming air inside a conical chamber. Centrifugal forces push larger particles to the outer walls where they drop into a collector, while smaller particles exit through the top.

2. Water Sampling and Physicochemical Analysis

Water quality monitoring requires representative sampling followed by systematic laboratory analysis of physical and chemical attributes to assess the health of aquatic ecosystems or water suitability for human consumption.

Methods of Collection of Water Samples

Water samples must be collected in a manner that preserves their integrity from the field to the laboratory:

• Grab Sampling: A single sample collected at a specific spot and time. It represents the water composition only at that exact instant.
• Composite Sampling: Formed by mixing multiple grab samples collected at different times or locations in proportion to the water flow. This provides an average profile of water quality over time.
• Integrated Sampling: Prepared by mixing samples collected simultaneously from different depths or across a stream cross-section to account for spatial variation.

Specialized equipment, such as Kemmerer or Van Dorn samplers, is used to collect water at specific depths without contamination. Samples must be stored in high-density polyethylene (HDPE) or borosilicate glass containers, chilled to 4°C, and sometimes preserved with chemical reagents (e.g., nitric acid for metals) to prevent biological or chemical alterations.

Analysis of Physicochemical Characteristics of Water

Physicochemical variables determine the ecological, domestic, and industrial usability of water resources:

pH Value: Represents the hydrogen ion activity in water.
pH = -log[H⁺]
Measured electrometrically using a pH meter calibrated with standard buffer solutions.

Dissolved Oxygen (DO): The concentration of free, non-compound oxygen dissolved in water. It is essential for aquatic respiration and is measured using either the Winkler titration method (iodometric method) or electrochemical membrane probes.

Biochemical Oxygen Demand (BOD): The amount of dissolved oxygen required by aerobic microorganisms to decompose organic matter present in water over a specific period (typically 5 days at 20°C).
BOD₅ (mg/L) = (D₁ - D₂) / P
Where D₁ is the initial DO, D₂ is the DO after 5 days of incubation, and P is the decimal dilution fraction of the sample.

Chemical Oxygen Demand (COD): The measure of the oxygen equivalent of the organic matter content susceptible to oxidation by a strong chemical oxidant (typically potassium dichromate, K₂Cr₂O₇, under acidic conditions). Unlike BOD, COD measures both biodegradable and non-biodegradable organic matter.

3. Soil Sampling and Physicochemical Analysis

Soil monitoring involves obtaining representative soil samples from agricultural, industrial, or natural landscapes and analyzing their physical and chemical state to assess soil health, fertility, and contamination levels.

Methods of Collection of Soil Samples

To acquire representative soil samples, systematic strategies and tools must be utilized:

• Sampling Designs:
  • Simple Random Sampling: Chosen when the study area is homogeneous.
  • Systematic Grid Sampling: Samples are collected at regular, predetermined spatial intervals. This is ideal for mapping contaminants.
  • Stratified Random Sampling: The area is split into distinct sub-units (strata) based on topography, crop history, or soil type, and random samples are taken from each stratum.
• Sampling Tools: Soil augers (screw, bucket, or tube types) are used to extract core samples from specific depths (e.g., 0-15 cm for agricultural surface soil, deeper for root zone profiles). Spades are used for digging test pits.
• Sample Preparation: Samples are composite-mixed in a clean plastic container, air-dried at room temperature (never oven-dried for volatile components), ground gently with a pestle and mortar, and passed through a 2 mm sieve to isolate the fine earth fraction for analysis.

Analysis of Physicochemical Characteristics of Soil

Understanding soil properties helps manage agricultural productivity and track soil pollution:

• Soil Texture: The relative proportion of sand, silt, and clay particles. It is measured in the laboratory using the pipette method or the hydrometer method based on Stokes' Law, which governs the settling velocity of particles in a liquid medium.
• Bulk Density: The mass of dry soil divided by its total volume (including pore space). High bulk density indicates compaction, which restricts root growth and water infiltration.
• Moisture Content: Determined gravimetrically by weighing a fresh soil sample, drying it in an oven at 105°C for 24 hours, and reweighing to calculate water loss.
• Soil pH: Measured potentiometrically in a 1:1 or 1:2.5 soil-to-water suspension using a glass electrode pH meter. It dictates nutrient availability and microbial activity.
• Cation Exchange Capacity (CEC): The total capacity of a soil to hold and exchange exchangeable cations (e.g., Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺). It is typically determined by leaching the soil with an ammonium acetate solution at pH 7.0.
• Soil Organic Carbon (SOC): Commonly determined using the Walkley-Black wet oxidation method, where organic carbon is oxidized with a mixture of potassium dichromate (K₂Cr₂O₇) and concentrated sulfuric acid (H₂SO₄).

4. Bio-monitoring and Bio-indication

Physical and chemical monitoring provide precise snapshots of environmental conditions, but biological monitoring evaluates the integrated, long-term impacts of pollution on living systems.

Concepts and Definitions

Bio-monitoring: The systematic use of biological responses (at the molecular, cellular, physiological, organismal, or community level) to evaluate environmental changes, typically caused by anthropogenic activities.

Bio-indication: The use of an organism, a part of an organism, or a community of organisms (referred to as a bio-indicator) to detect qualitative changes in the environment, indicating the presence of pollutants or ecological degradation.

Types of Bio-indicators

• Accumulation Indicators (Bio-accumulators): Organisms that accumulate pollutants within their tissues without being immediately killed. This allows quantitative measurement of environmental contamination over time (e.g., heavy metal accumulation in mosses).
• Impact Indicators (Response Indicators): Organisms that show highly visible physiological, morphological, or behavioral symptoms when exposed to specific stressors (e.g., leaf injury in plants exposed to ozone).

Common Biological Indicators

Active vs. Passive Bio-monitoring

• Passive Bio-monitoring: Monitoring naturally occurring native organisms already present in the target environment.
• Active Bio-monitoring: Relocating healthy organisms of known physiological states from a clean environment into the monitored area for a specific exposure period, then observing their physical and chemical responses.