Groundwater Contaminants Flashcards
4 methods that affect the chemical composition of groundwater
- Natural Processes
o Leaching of chemical deposits leading to increased concentrations of chlorides, sulphates, nitrates, iron, and inorganic chemicals
o Evapotranspiration from shallow water tables can lead to increase salt concentration in the groundwater - Non-point agricultural and urban runoff
o Runoff waters carry high concentrations of nutrients, metals, pesticides, microorganisms and organic materials - Waste disposal practices
o Contamination commonly found long after the original disposal due to the slow movement of groundwater - Spills, leaks and accidents
o Agriculture, mining, salt-water intrusion and leaking underground storage tanks containing mainly petroleum products
Forms of contaminants
- Pathogenic
- Inorganic
- Organic
- Medical drugs
- Plastic particles
Point source vs distributed source
Point Source= A small areal extent relative to the area of groundwater contamination
Distributed= Source extends to a large area relative to the contaminated area
NAPL
LNAPL
DNAPL
NAPL= Non-aqueous phase liquid
LNAPL= Light NAPL- less dense than water- floats on the surface
DNAPL= Dense NAPL- sink to the bottom of groundwater source, and forms pools
Why is it difficult to know the flow paths in aquifers
The presence of many different layers, with fissures, cracks and pores. It’s impossible to predict the exact path water will take through these
Conservative vs Non-conservative contaminant transport
Conservative= Where contaminant is passively transported- it doesn’t react with the soil or undergo transformations or decay- therefore mass is always constant
Non-conservative= Contaminant reacts with soil, it can chemically bind to the soil particles, react with the chemicals or microbes in the soil, precipitate or undergo radioactive decay- there is a loss is mass over time
3 types of groundwater fluxes
-Diffusive Flux
-Dispersive Flux
-Advective Flux
What does diffusive flux refer to
Molecular Diffusion
Molecular motions as opposed to bulk fluid movement
Diffusion doesn’t result in mass transport over larger distances
Important in low permeability soils and volatilisation of chemicals through soil air
What is a Brownian motion
Random movement of particles in a fluid
What is Fick’s Law
the spread is proportional to the concentration gradient
What does dispersive flux refer to
The spreading of contamination in groundwater due to pore scale flow velocities
i. Fluid in the centre of a pore flows faster than that near the edges due to friction
ii. Fluid travels faster through larger pores
iii. Pores are connected by multiple tortuous paths with some flow paths longer than other flow paths- therefore longer travel time to reach the same point downstream
Longitudinal Dispersion vs Transverse Dispersion
Longitudinal Dispersion= mixing along principal direction of flow
Transverse dispersion= spreading in directions normal to the flow due to divergence of flow paths at the pore scale
What does the adjective flux refer to
Groundwater moves with a bulk velocity, and advection results from the contaminant being carried along by this bulk flow
Most significant mass transport process and results from large scale gradients in fluid energy
Following a release of contaminants what 3 things are we interested in
-Where the plume of max concentration is located
-What the max concentration is
-What the extent of contamination is
What does the ratio of longitudinal to transverse dispersivity (αL/αT) effect
The shape of the plume
The lower the ratio the broader the plume will become through time
What are the two types of velocity and how do they compare
Groundwater Flow and Contaminant Flow
Equal in conservative flow, but not in non-conservative
Contaminants dissolved in groundwater can undergo a range of processes which can lead to their removal from solution, these are (x6)
(i) sorbed onto the surfaces of soil particles in the aquifer,
(ii) sorbed by organic carbon that may be present in the aquifer,
(iii) undergo chemical precipitation,
(iv) be subjected to abiotic as well as biodegradation,
(v) participate in oxidation reduction reactions and
(vi) for radioactive compounds the is radioactive decay.
How does sorption impact velocity
contaminants will move more slowly through the aquifer than the actual groundwater velocity. This is referred to as retardation.
How does Biodegradation, radioactive decay and precipitation effect concentration and velocity
decline in the solute concentration in the plume
do not necessarily slow down the plume movement
Why does a solute cling onto a solid
- Electrical attraction
- Van der Waals Attraction- intermolecular forces of attraction between molecules of the solids and absorbed solutes
- Chemisorption- chemical reaction between solute and solid surface
Adsorption vs Absorption
Adsorption- binds to the surface of the soil particle
Absorption- gets absorbed into the interior of the soil particle
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Equilibrium adsorption
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Kinematic adsorption
Sources of Pathogenic Microbes
- Septic Tanks
- Leaking Sewer Lines
- Waste Dumps
- Landfills
- Manure for Fertilisers
- Livestock Wastes
- Wildlife Faecal and Carcass Remains
Principle Contaminant Microbes
- Parasitic Protozoa- Cryptosporidium, Giardia
- Virus
- Bacteria- E.coli
- Algae and Fungi
Characteristics of parasitic protozoa
o 2-7mm
o Resistant to Chlorination
o Difficult to monitor Severe pathogenic effects on humans
o Can’t reproduce in groundwater
o Migration depends on the lifespan of original cysts or oocysts
Characteristics of viruses
o 0.002-0.007mm
o Can only reproduce in living cell
o Loos their ability to infect outside of organic matter-inactivation rate
o Can be killed by chlorination
o Can travel up to 1km in groundwater
Characteristic of bacteria
o 0.2-1mm
o Don’t need organic matter to remain infective- don’t inactivate
Factors affecting microbe transport (x6)
- Size
- Inactivation
- Surface electrostatic properties
- Hydrophobicity
- Shape
- Density
Aquifer properties that affect microbe transport (x8)
- Flow velocity
- Grain size- affects pore size and size of connecting channels
- Porosity
- Soil organic content
- Temperature- colder temperatures improve transport
- pH- lower pH improves transport
- Mineral composition
- Presence of colloids
Define Isoelectric Point (IEP)
the pH at which the surface charge of the virus changes from negative to positive
What is the impact of IEP
- Typical range is 3.5<IEP<7
- Viruses tend to have a negative surface charge, as do clay particles
- Where surface charge is the same repulsion occurs, resulting in being transported further
- Where the surface charge is opposite the attract, and adsorption and binding occur, therefore less distance
A model of virus and bacteria contains…
- Dispersion and Advection
- Equilibrium Adsorption
- Kinetic Reversible Adsorption
- Inactivation of viruses on both equilibrium and kinetic adsorption sites- modelled as radioactive decay
Reversible adsorption vs irreversible
Reversible adsorption involves both the attachment of viruses to the soil particle and their subsequent detachment.
For irreversible adsorption there is no detachment.
With reversible adsorption there are two types of sites that the particles bind to, equilibrium and kinetic.
For equilibrium sites, attachment and detachment are fast relative to the flow velocity, while for kinetic adsorption is kinetically limited relative to the water flow velocity.
pfu
Plaque Forming Units- describe the number of virus particles capable of forming plaques per unit volume.
Bacteriophages
Viruses that infect bacteria. In lab experiments on virus transport real viruses often can’t be used, therefore bacteriophages are used instead.
