Groundwater Pollution Flashcards
Groundwater filtration
Groundwater could be good quality water because of the natural filtration that
occurs in the layers of soil.
● Fine sand may remove suspended solids or bacteria from the water in a
short distance.
● Coarse gravel could allow those pollutants travel long distances.
● Soluble pollutants are not affected by the filtering action of the soil.
Since 1970 many groundwater are reported that are contaminated.
If an aquifer that supplies drinking water is polluted:
● Abandon the contaminated well and drill new one some distance away.
● You can install special treatment units (aerators, activated carbon filters) to
remove contaminants, but these measures are expensive.
Sources of contamination
Sources of Contamination:
1. Soil and rocks
2. Industrial wastes
3. Subsurfaces sewage disposal systems
4. Municipal landfills
5. Mining and petroleum production
6. Agriculture
7. Urban areas
8. Saltwater intruction
Industrial wasters
There are two sources of groundwater pollution from industrial waste disposal:
● A leaky surface of landfills or lagoons
● A deep well wastewater injection
Industrial wastes are disposed in unlined landfills or lagoons.
Contaminated liquids leaked out of these landfills and lagoons, percolated through the soil, and eventually reach the groundwater.
Industrial wastes sometimes are pumped into the ground under pressure through deep wells (deep well injection) at about 300 m.
At this depth the groundwater is often saline and is not used.
The deep well injection is an acceptable method for industrial waste disposal when the geological conditions are suitable. However, accidental contamination can be happened.
Surface sewage disposal systems
Surface sewage disposal systems: are septic tanks and leaching field systems.
The septic tanks trap and store solids while the liquid effluent from the tank flows into a network of buried perforated pipes.
Leaching or absorption field: is a large area that serves to spread out the sewage, effluent through the perforated pipes. The effluent is percolated slowly into the soil and degraded naturally.
Septic contaminants
Septic disposal systems are the pollutant sources of
fecal bacteria
viruses
organic solvents
detergents
nitrates
chlorides
Mining and petroleum production. Agriculture. Urban.
Mining and petroleum production
● Surface water flowing in the vicinity of mines, pick up dissolved metals,
other solids, acidity, radioactive substances.
● The polluted water carries all these contaminants into groundwater.
● Leaky gasoline tanks at filling stations have been reported.
Agriculture:
Fertilizers and pesticides are sources of groundwater
contamination.
Urban areas:
The spread salt on the roads to keep them ice-free in the winter
is a source of chloride.
Saltwater intrustion/solution.
Intrusion of salty seawater into wells is considered a groundwater pollution.
This problem occurs in many coastal cities and towns when the elevation of
groundwater table is dropping.
Solution to the problem:
● Abandon the wells with salty water
● Artificial recharge of the groundwater from freshwater
● Desalinate the water
Prevent groundwater pollution
Measures to prevent groundwater pollution
● Natural purification can take centuries
● Cleanup efforts are very expensive
● Prevent it from occurring in the first place
● Laws related to solid and hazardous waste disposal
Toxic chemicals are the principal pollutants in groundwater contamination
NAPLS (Nonaqueous-phase liquids)
DNAPLs: they are more dense than water
(i.e. PCBs, pesticides)
LNAPLs: they are less dense than water
(i.e. crude oil, gasoline, benzene)
LNAPLS
LNAPLs: in the unsaturated zone some may dissolve into water in the pores,
some may volatilize and become mixed with air in other pore spaces
and some may adsorb onto soil particles.
In the water table, they do not dissolve well and instead spread out to
form a layer of contaminant floating on top of the saturated zone.
DNAPLs:
DNAPLS: in the unsaturated zone sink, dissolve into water in the pores,
some may volatilize and become mixed with air in other pore spaces
and some may adsorb onto soil particles.
In the water table, they keep sinking until they reach a layer of
relatively impermeable material.
There they form pools that can overflow and sink to the next
impermeable layer.
Remediation Technologies
- Conventional pump-and-treat systems
- Soil vapor extraction
- In situ bioremediation
- Permeable reactive barriers
- Conventional pump-and-treat systems
Pump-and–treat technology for aquifer clean up is based on extracting contaminated groundwater and then treating it above ground.
Treated effluent can then be used for beneficial purposes or returned to the aquifer.
The cost of cleaning up an aquifer and the length of time required to do so increases dramatically as the level of cleanup desired increases.
To achieve groundwater cleanup to drinking-water standards using conventional pump and treat technology is a futile goal for most contaminated aquifers.
- Soil Vapor Extraction
It removes organic vapors from the unsaturated zone.
The suction side of a blower pulls soil vapors up extraction wells and sends them to a vapor treatment unit.
The extraction wells consist of slotted, plastic pipe set into a permeable packing material.
They may be placed vertically or horizontally depending on circumstances.
Horizontal systems are often used when the contamination is near the surface.
The system may include an air injection system that helps push vapors toward
collection wells.
This technology is quite effective at removing volatile organic compounds that have leaked from underground storage tanks, such as leaky gasoline storage tanks at service stations. It may also help remove some NAPLs in the unsaturated zone.
The performance of SVE systems highly depends on the characteristics of the
subsurface and the contaminants.
Highly volatile substances located in permeable soils, SVE systems perform well.
Low-permeability zones in the subsurface are not easily flushed, so contaminants located there will be difficult to remove
Air sparging:
A compressor drives air through an injection well into the saturation zone.
The injected air rises through the contaminant plume and captures volatile
compounds as it moves.
The system is very effective at removing substantial quantities of volatile organic compounds, such as benzene and gasoline.
In situ bioremediation
This technology is designed to degrade subsurface pollution in place without the need to capture and deliver contaminants to an above-ground treatment system.
Advantages: No need for excavation
No need for above-ground treatment
No need for transportation to a disposal site
Elimination of the risk of human exposure to hazardous chemicals
This technology is based on stimulating the growth of microorganisms that are
indigeneous to the subsurface and that can biodegrade contaminants.
Oxygen and nutrients are provided to microorganisms.
They can degrade a number of common soil and groundwater contaminants,
especially petroleum-based hydrocarbons to carbon dioxide and water.
