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
