In Situ Reactive Zones (IRZ) Notes

In Situ Reactive Zones (IRZ)

Introduction to In Situ Reactive Zones

  • In Situ Reactive Zones (IRZs) are a technology used for environmental remediation.

Background of IRZ

  • Injection-based cleanup has evolved significantly in the last 30 years.
  • IRZs offer faster, cheaper, and less invasive methods.
  • They leverage natural degradation processes.
  • In situ extraction techniques like soil vapor extraction and air sparging have paved the way for non-extractive methods.
  • The core concept involves intercepting or degrading contaminants in saturated soil or groundwater.

Goal of IRZ

  • The primary goal is to create a unique subsurface zone that intercepts, immobilizes, or degrades contaminants into harmless end products.

Success Factors for IRZ

  • Success depends on reactions:
    • Between injected reagents and contaminants.
    • Between injected reagents and the subsurface environment.
  • It requires manipulating biogeochemistry to optimize these reactions.
  • Target reaction rates must equal or exceed the movement of contaminants.
  • Injected reagents must be distributed and sustained throughout the IRZ.

IRZ Reactions – Processes

  • Transport Mechanisms: These mechanisms leave the chemical structure intact.
  • Transformational Mechanisms: These mechanisms alter the chemical makeup or molecular structure.
  • Reactions can be chemical, biochemical, or biological.

IRZ Reactions – Types

  • Heterogeneous:
    • Occur at liquid-solid-gas interfaces.
    • Most subsurface reactions are heterogeneous.
  • Homogeneous:
    • Occur with dissolved reactants.
    • Rare in groundwater.
  • Chemical/Biological:
    • Involve the breaking and forming of bonds.
    • May involve the replacement of atoms.
  • The ultimate goal is to transform contaminants into harmless or less harmful end products.
  • Ideally, contaminants are mineralized.

Mineralization

  • Mineralization is the complete conversion of an organic chemical to stable inorganic forms of natural elements like C, H, N, O, and P.
  • Examples include conversion to CO<em>2CO<em>2, H</em>2OH</em>2O, NO<em>32−NO<em>3^{2-}, NH</em>4+NH</em>4^+, PO43−PO_4^{3-}.

Contaminant Properties to Consider

  • Biological:
    • Can microorganisms use the contaminant?
    • Is it an electron donor or receptor?
    • Can microorganisms metabolize it?
    • Is Monitored Natural Attenuation feasible?
  • Chemical:
    • Does it sorb to soil?
    • Does it form precipitates?
    • What is its volatility and solubility?

Chemical vs. Biological Processes

  • Oxidation: Electrons are transferred from the contaminant.
  • Reduction: Electrons are transferred to the contaminant.
  • OILRIG: Oxidation Is Loss, Reduction Is Gain of electrons.
  • REDOX REACTIONS: Net electron transfer between contaminant and reactant.

Advection and Dispersion

  • Old concept: Advection and dispersion account for contaminant transport.
  • New concept: Advection and Diffusion.
    • In more permeable soils, advection dominates.
    • In less permeable soils, diffusion dominates.
  • Permeability also affects reagent injection.
  • Water is incompressible and must go somewhere.

Volume–Radius Relationship

  • Only a small fraction of aquifer pore spaces actively participate in advective groundwater and contaminant flow.
  • Reagents will:
    • Migrate preferentially within the mobile fraction.
    • Move via slower advection through permeable soils with lower hydraulic conductivity values.
    • Diffuse between these zones and into the surface of low-permeability soils.

Reagent Types / Factors to Consider

  • Dissolved reagents migrate with advective flow.
  • Reagent Injectability: Is the reagent soluble in water (easier) or a solid?
  • Longevity: How long does the reagent last in the environment? (working life)
  • Reagent-related by-products: Consider any intermediary products.
  • Reagent Handling: Safety, storage, and risk considerations.

Reagent Selection

  • Reagent solubility dictates the maximum dosing concentration.
  • Reagent injectability depends on solubility or particle size (less than 1 micron for delivery via a fixed well).
  • Solid materials may require elevated injection pressures to induce soil fractures.
  • Reagent longevity is the duration the reagent remains at sufficient strength.
  • Consider potential chemical intermediates generated during reactions.
  • Ensure proper health and safety precautions during remediation.

Injection Frequency

  • Consider:
    • Half-life/working strength of the reagent.
    • Migration rate by advective transport.
  • Use tracer tests (dye, salt, isotopic) to assess.

Injection Well Application

  • Well-based injections typically involve significant volumes of fluid (1000s of gallons).

Environmental Fracturing

  • Fracturing alone is not a remediation technique.
  • It enhances other technologies by improving accessibility to contaminants.
  • Can be used in both the vadose (unsaturated) and saturated zones.
  • Goal: Improve flow of air and water, as well as delivery of treatment reagents (increasing soil permeability).

Fracturing Augments Other Technologies

  • In situ biodegradation (increase oxygen/nutrients).
  • In situ electrokinetics (fluid flow due to applied charge - electroosmosis).
  • In situ vitrification (through rapid heating/cooling - glassification!).
  • In situ air sparging (fracture pathways/air flow).
  • SVE (Soil Vapor Extraction) - Fracture pathways/airflow.

Fracture Propagation

  • The primary determinant of fracture orientation is in situ stresses.
  • Pressure exceeding natural strength and in situ stresses leads to fracture.
  • Fractures propagate normal (perpendicular) to the least principal stress, following the path of least resistance.

Hydraulic and Pneumatic Fracturing

  • K0K_0 = horizontal stress : vertical stress.
  • Ratio > 1 = flat fractures (desirable).
  • Larger the K0K_0, more horizontal.
  • Potential problem with vertical fractures: short circuiting.

Fracture Application

  • Considerably smaller reagent volumes (10s to 100s of gallons) are used.
  • Fractures move horizontally through narrow openings.
  • Designs utilize multiple discrete intervals to allow vertical coverage across the interval of contamination.

Hydraulic Fracturing

  • How do you know it's working? Monitor injection pressure as a function of time.

Environmental Fracturing vs. Oil/Gas Fracking

  • Oil/gas fracking:
    • Is a much larger operation.
    • Involves much greater depths.
    • Uses different chemicals and time frames.
  • Environmental fracturing:
    • Typically treats less than 100 ft depth (usually 20-35ft).
    • Limited radius of influence requiring multiple wells.

Oil/Gas Fracking Details

  • Horizontal drilling from a single pad site.
  • More extensive fracturing for well production.
  • 2-4 million gallons of water + are used; 3 million gallons is a common figure.
  • Much higher injection pressures: 8,000 psi.

IRZ Injection Configurations

  • Various configurations are used, including grid patterns and continuous curtains, to ensure comprehensive treatment of the contaminant plume.

Key Ideas Recap

  • Overall Goal of IRZ: Degrade or immobilize contaminants.
  • Rate of reaction:
    • Injected reagents vs contaminants and subsurface environment.
    • In the IRZ vs flow of contaminants (contact time).
  • Distribution of injected reagents for a sustained reaction.
  • Reaction types: chemical, physical, biochemical, microbial.
  • Reagent considerations (injectability, longevity, safety).
  • Env. Fracturing vs Fracking (scale, depth, materials).
  • Fracture propagation (stress, direction).