Groundwater Contaminants Flashcards

1
Q

4 methods that affect the chemical composition of groundwater

A
  • 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
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2
Q

Forms of contaminants

A
  • Pathogenic
  • Inorganic
  • Organic
  • Medical drugs
  • Plastic particles
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3
Q

Point source vs distributed source

A

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

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4
Q

NAPL

LNAPL

DNAPL

A

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

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5
Q

Why is it difficult to know the flow paths in aquifers

A

The presence of many different layers, with fissures, cracks and pores. It’s impossible to predict the exact path water will take through these

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6
Q

Conservative vs Non-conservative contaminant transport

A

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

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7
Q

3 types of groundwater fluxes

A

-Diffusive Flux
-Dispersive Flux
-Advective Flux

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8
Q

What does diffusive flux refer to

A

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

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9
Q

What is a Brownian motion

A

Random movement of particles in a fluid

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10
Q

What is Fick’s Law

A

the spread is proportional to the concentration gradient

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11
Q

What does dispersive flux refer to

A

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

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12
Q

Longitudinal Dispersion vs Transverse Dispersion

A

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

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13
Q

What does the adjective flux refer to

A

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

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14
Q

Following a release of contaminants what 3 things are we interested in

A

-Where the plume of max concentration is located

-What the max concentration is

-What the extent of contamination is

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15
Q

What does the ratio of longitudinal to transverse dispersivity (αL/αT) effect

A

The shape of the plume

The lower the ratio the broader the plume will become through time

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16
Q

What are the two types of velocity and how do they compare

A

Groundwater Flow and Contaminant Flow

Equal in conservative flow, but not in non-conservative

17
Q

Contaminants dissolved in groundwater can undergo a range of processes which can lead to their removal from solution, these are (x6)

A

(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.

18
Q

How does sorption impact velocity

A

contaminants will move more slowly through the aquifer than the actual groundwater velocity. This is referred to as retardation.

19
Q

How does Biodegradation, radioactive decay and precipitation effect concentration and velocity

A

decline in the solute concentration in the plume

do not necessarily slow down the plume movement

20
Q

Why does a solute cling onto a solid

A
  • 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
21
Q

Adsorption vs Absorption

A

Adsorption- binds to the surface of the soil particle

Absorption- gets absorbed into the interior of the soil particle

22
Q

tr<td

A

Equilibrium adsorption

23
Q

tr>td

A

Kinematic adsorption

24
Q

Sources of Pathogenic Microbes

A
  • Septic Tanks
  • Leaking Sewer Lines
  • Waste Dumps
  • Landfills
  • Manure for Fertilisers
  • Livestock Wastes
  • Wildlife Faecal and Carcass Remains
25
Q

Principle Contaminant Microbes

A
  • Parasitic Protozoa- Cryptosporidium, Giardia
  • Virus
  • Bacteria- E.coli
  • Algae and Fungi
26
Q

Characteristics of parasitic protozoa

A

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

27
Q

Characteristics of viruses

A

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

28
Q

Characteristic of bacteria

A

o 0.2-1mm
o Don’t need organic matter to remain infective- don’t inactivate

29
Q

Factors affecting microbe transport (x6)

A
  • Size
  • Inactivation
  • Surface electrostatic properties
  • Hydrophobicity
  • Shape
  • Density
30
Q

Aquifer properties that affect microbe transport (x8)

A
  • 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
31
Q

Define Isoelectric Point (IEP)

A

the pH at which the surface charge of the virus changes from negative to positive

32
Q

What is the impact of IEP

A
  • 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
33
Q

A model of virus and bacteria contains…

A
  • Dispersion and Advection
  • Equilibrium Adsorption
  • Kinetic Reversible Adsorption
  • Inactivation of viruses on both equilibrium and kinetic adsorption sites- modelled as radioactive decay
34
Q

Reversible adsorption vs irreversible

A

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.

35
Q

pfu

A

Plaque Forming Units- describe the number of virus particles capable of forming plaques per unit volume.

36
Q

Bacteriophages

A

Viruses that infect bacteria. In lab experiments on virus transport real viruses often can’t be used, therefore bacteriophages are used instead.