Reclamation of polluted sites

Reclamation of Polluted Sites

1. Flow and Transport

1.1 Different Kind of Models

• Black box model = Data driven, no physical based, it is a statistical model, null prediction capability.
• White box model = Physical driven, no rely only on data but we know the physics behind the model, there is physical and mathematical knowledge, maximum prediction capability.
• Direct model = It is a predictive modelling, we know the parameters of the system and we are changing boundary condition in order to understand the system under different conditions.
• Inverse modelling = We want to determine and derive the parameters, it is a parameter estimation.
• Analytical
• Numerical

Key aspects of modelling:

• Models are a simplification of the reality; they cannot represent everything. The first step in model development is the identification of the model objectives.
• Model must be based on the experimental data; the more complex the model, the more data you need.
• Models must not be too complex if complexity is not strictly needed.

1.2 Step 0 - Conceptual Model

It is a descriptive representation of a groundwater system that incorporates an interpretation of the geological and hydrogeological conditions.

1. Identification of modelling objectives
2. Collection of existing data
3. Preliminary conceptual model
4. Characterization plan
5. Field data collection and analysis if it is insufficient I have to come back to step 3
6. Final conceptual model

Ex: Hydrogeological conceptual model

Mathematical Model vs Computer Code

Model = Representation of the nature

• Incorporates physical features of a natural system in a numerical context
• Geology
• Source
• Observation

Code = Tool to develop a model

• Uses mathematical expression to solve groundwater flow or/and transport
• Governing equation for groundwater flow (Darcy)
• Transport equation (advection, dispersion)

1.2 Groundwater Problems

They are physically based. The governing equations are partial differential equations + state equations.

• The initial conditions define the initial state within the domain.
• Boundary conditions define the exchanges through the boundaries of the domain.

1.3 Calibration

Adapting model parameters to better represent experimental data. The calibration is composed by:

• Comparison of general flow characteristics
• Somewhat useful but could lead to errors because contouring programs don't account for physicality of groundwater flow.

The quantitative calibration is composed by:

• Automatic procedures using specific software that find the solution minimizing least squares errors
• Visualization of results

2. Soil Water Air Contaminant Interaction

2.1 Soil

Soil properties:

• Particle size and grain size distribution soil texture triangle = Silt, clay, sand with different composition of these 3 elements in percentage we can define different types of soil, it is a visual representation of soil in order to give a name to a specific type of soil.
• Porosity ratio of the total volume of empty space and total volume.
• Effective porosity = Ratio between the interconnected voids in which the motion of the fluid is allowed and the total volume, it is lower than the porosity.
• Hydraulic conductivity = Volumetric flow rate of water flowing through a porous medium per unit cross-section under the effect of a hydraulic gradient of unit value at temperature of 20°C, depends on the fluid properties.
• Permeability (Intrinsic) is an intrinsic property of the porous medium.
• Soil composition important parameter is the natural organic matter (NOM), it is not a compound but it is a mixture of different compounds originated from the decomposition of plants, animals, and microorganisms. It is divided into fulvic acid, humic acid, humin. From left to right they are hydrophilic, smaller molecules, less aromatic rings, less carbon content, more oxygen, more functional groups.
• Inorganic fraction is composed by the primary mineral coming from rock mechanical decomposition (like quartz), they form the structure of the soil and secondary mineral coming from the chemical decomposition of the rock, they contribute to the fine fraction, they include clay. Both of them have a negative charge, they are hydrophilic, they have a surface charge.
• The fine particle interacts more with the contaminant because of the more specific area.
• Cation exchange capacity (CEC) amount of the cations adsorbed in a reversible manner per unit mass of soil (meq/100g or mol/kg). Most of the surfaces that we considered have negative surfaces so they are able to release positive charge. It depends on soil type, specific surface area, and pH. It is relevant for retention of inorganic/charge contaminants. High cation exchange capacity is crucial in determining the extent of adsorption phenomena.
• Competitive sorption due to CEC = Strength of the binding increases with valence number and mass of the cations. Major cation in order of decreasing exchangeability. Multivalent ions and with larger diameter tend to replace the other and they adsorb more.
• pH affects the chemical equilibrium considering the acid dissociation constant.
• Redox potential is the tendency for a chemical species to be reduced. The more positive the potential is the more likely the species will be reduced. The redox potential is defined with respect to a reference electrode (Hydrogen electrode) in standard conditions (T=25°C, P=1atm, C=1M). If the species stays at the bottom of the table (negative value), it has a very low tendency to be oxidized.
• Nernst equation = The reduction potential of a given reduction semi-reaction in non-standard conditions, it calculates the redox potential.

Measurement: Use a probe that measures the potential with respect to a standard porous electrode (usually composed by silver and it is put in contact with potassium chloride).

Pourbaix diagram = It is related with a specific solution. On x axis pH and on the y axis the redox potential. We divide the space in areas which correspond to a given species. Pourbaix diagrams are useful in predicting the spontaneous direction of electrochemical reactions, identifying the corrosion products, and predicting the changes in the environment in terms of potential and pH that result in high or low corrosive attack.

Considering iron and oxygen in water different type of iron in nature Fe, Fe⁺, Fe⁺⁺. The green region is the one of metallic iron, the brown area we have iron in the form of oxides, the pink area iron is in dissolved form.

2.2 Contaminant Properties

1. Solubility

Solubility of solid increases with T, of liquids remains almost constant and of gases decrease with T.

2. Henry’s Law

At a constant T the liquid phase concentration of a poorly soluble substances is proportional to its pressure in the gas phase.

3. Sorption

Sorption = Accumulation of a compound at the soil/liquid interface. Different type of sorption:

• Adsorption = Physical sorption, it is reversible and rapid.
• Chemisorption = Chemical sorption, it is irreversible and due to covalent bonds.
• Adsorption = If the grains are porous themselves, the contaminant may migrate within the grains, it is usually not a fast process.
• Linear isotherm applicable for low solute concentrations. Used for apolar and weakly polar contaminant.
• Freundlich isotherm the slope of the isotherm decreases with the increasing of C. used for polar, neutral and anionic contaminant.
• Langmuir isotherm for high C, S=beta. Beta is the maximum sorption capacity. Used for polar, neutral, cationic, anionic contaminant.

4. Multiphase Flow in the Subsurface

• Pancake model = Model it’s OK from qualitative point of view but it’s not representative of the real configuration.
• Shark Fin model = More accurate configuration (in real case), it represents the distribution of the saturation of the oil along the vertical, it’s not just floating but there is the penetration of the product, there is a decrease under the water table.
• The DNAPL pushes the front downwards of a partially saturated material and it finds a strong front of water DNAPL spreading laterally for this reason it’s a drainage process.
• Different distribution of a wetting phase, there is the injection of water and PER. This results in different distribution in the unsaturated zone, the bigger voids are usually occupied by the wetting phase, the surface of the solid attraction of the fluid.
• If we consider both wetting and not wetting phase (water and PER), it occupies the corners of the pore spaces, beads then saturated from below with water and PER thereby forced into the smaller spaces.

5. Preliminary Concepts

• Saturation = The fraction of the pore volume which is occupied by given fluid (g=gas, o=oil, w=water, p=pore volume).
• Volumetric content = The fraction of the total volume which is occupied by a given fluid (n=total porosity, Vt= total volume).
• Interfacial tension = Used in systems with two fluids or not miscible fluids, the interface between the two fluids behaves like an elastic membrane. The membrane can sustain a pressure difference.