Groundwater pollution and remediation

Definition: Water quality is the suitability of water for human and ecological uses.

It can be measured by physical, chemical, and biological characteristics.

Physical parameters: temperature, acidity (pH), dissolved oxygen, electrical conductance and turbidity.

Chemical and biological characteristics such as bod are done in labs.

Water quality

PesticidesOrganic PollutantsInorganic Pollutant agentsBacteriological contaminationSurfactants

Principal groundwater contaminant types and processes

Pesticides can reach water-bearing aquifers below ground from applications onto crop fields, seepage of contaminated surface water, accidental spills and leaks, improper disposal, and even through injection waste material into wells.

Contamination by pesticides

Inorganic pollutants are usually substances of mineral origin and elements that do not degrade easily.

Inorganic pollutants: metal, salts, and acids.

ExamplesMetals: Mercury poisoning, Lead

poisoning.Nonmetallic Salts: Arsenic, Sodium

ChlorideAcids: Sulfur and nitrogen compounds

Inorganic Pollutant agents

Organic pollutants are chemicals that contain carbon atoms.

Organic pollutants are divided into two categories: natural pollutants, and human made pollutants.

Natural pollutants: Sugars, amino acids, and oils.

human made pollutants: pesticides, solvents, industrial chemicals, and plastics.

Organic Pollutants

Organic pollutants: Dioxin, PCB, and DDT.Dioxin: is produced during the combustion

of municipal and medical wastes, backyard burning of trash, and industrial processes.

PCBs: is also produced during combustion in industries.

DDT: are insecticide used on agricultural crops, primarily cotton, and insects that carry diseases such as malaria and typhus, stable and slow to degrade.

Bacteriological contamination are caused due to the poor sanitary condition in some areas, the closeness of some hand dug wells to pit-latrines, and soakaways and dumpsites.

Bacteriological contamination of drinking water is responsible for the occurrence of waterborne diseases such as typhoid fever, dysentery, cholera, meningitis and diarrhea.

Faecal contamination of the wells comes from the presence of human and animal faeces.

Bacteriological ground water contamination

Surfactants are chemical compounds that surface tension when dissolved in water.

Surfactants have high biodegradability and moderate toxicity for fish.

Surfactants affect oxygen-turnover of water and reduce sedimentation process, and thus delaying water purification, particularly during aeration of waste water.

Contamination by surfactants

Groundwater remediation is the process used to remove pollution from groundwater.

Groundwater remediation are mainly divided into Ex-Situ Technology and In-Situ Technology

Technologies for groundwater remediation

Ex-Situ Technology is achieved by de-watering the polluted aquifer (pumping out), treating the water on surface, and finally re-injecting the treated water to the aquifer.

Ex-Situ Technology

Solidification is to solidify (immobilize) the recovered contaminated water in a stable matrix with cement.

Solidification is hard due to large quantity of water to be solidified.

Solidification is often used after vaporization process where the water loss is allowed to occur in an open system, leaving the contaminant behind in a more concentrated state.

These process are effective on radioactive contaminants

Collection and immobilization

Precipitation processesAdsorption and ion exchange systemsPhytoremediationComplementary technologies

Collection and treatment technologies

Precipitation is a physicochemical process where dissolved species are transformed into less soluble or insoluble compounds under changed chemical conditions.

These dissolved species facilitates the physical removal of the insoluble compounds using sedimentation or filtration process.

Precipitation is facilitated in three ways: PH modification, Redox potential change, Chemical composition change.

Precipitation processes

Ion exchange is the exchange of anions or cautions with the wastewater.

Adsorption is a similar process, which does not include the sorping material losing a counter.

Ion exchange resins can be recharged once they have become saturated.

Sorption and ion exchange materials: clays; by- products such as fly ash; organic materials such as peat; biological materials such as fungal and algal biomass; and commercial products such as zeolites.

Adsorption and ion exchange systems

Phytoremediation is the use of plants to remove contaminants from the subsurface and produce a harvestable biomass.

Phytoremediation is a relatively inexpensive operation.

It can be accomplished with minimal effort.Phytoremediation risks the inadvertent transfer of

contaminants down the food chain through the ingestion of plant material (the biomass) by animals.

There is also the problem that the biomass which is harvested has to be disposed of.

Phytoremediation

Some Complementary technologies include sedimentation, filtration, coagulation, flotation, and magnetic separation.

Complementary technologies

In-Situ Technology involves treatment of groundwater within the aquifer (in the sub-surface) by using thermal, chemical and biological treatment technology.

In situ technologies are divided into active and passive systems

Active containment systems are long term operation that are neither practical nor economical.

Passive containment systems involves the installation of a permanent, vertical barrier to intercept or contain contaminated groundwater. They do not need active operation.

In situ remediation

Interception barriers (active)Hydraulic containment (active)Downhole and shaft barriers (passive)

Treatment technologies:

Interception barriers is conducted by constructing barriers through the excavation of a dyke or channel to intercept contaminated groundwater flow.

The interception barriers can either surround a contaminated region or they can be placed directly in the flow path of the groundwater.

Intercepted water can be removed for treatment, stored in storage ponds, or re-injected in a region which poses less of an environmental or radiation threat.

Interception barriers (active)

Hydraulic containment is applied by the injection of uncontaminated water at strategic points within a contaminated aquifer such as at the leading edge of the affected body.

This injection changes the local groundwater flow pattern and thus prevents the contaminant from migrating in its original direction.

This method is used to prevent further migration of the radionuclides.

Hydraulic containment (active)

Uranium mining leaves unplugged drill holes and mine workings, permitting contaminated water to mix with previously uncontaminated aquifers.

This method would separate the aquifers by aquitards before the mining process is conducted.

To prevent the contamination of clean aquifers barriers are placed in the shafts and bore holes at a point where the aquitard has been breached in a mining site.

Downhole and shaft barriers (passive)

Soil flushing involves pumping flushing solution into groundwater via injection wells.

The solution then flows down gradient through the region of contamination where it desorbs, solubilizes, and/or flushes the contaminants from the soil and/or groundwater.

the solution is then pumped out via extraction wells located further down gradient.

In-situ Flushing

At the surface, the contaminated solution is treated using typical wastewater treatment methods, and then recycled by pumping it back to the injection wells.

Plain water or carefully developed solutions are used as flushing solutions.