Groundwater remediation is the process that is used to treatpolluted groundwater by removing the pollutants or converting them into harmless products.Groundwater is water present below the ground surface that saturates the pore space in the subsurface. Globally, between 25 percent and 40 percent of the world'sdrinking water is drawn from boreholes and dugwells.[1] Groundwater is also used by farmers toirrigate crops and by industries to produce everyday goods. Most groundwater is clean, but groundwater can become polluted, or contaminated as aresult of human activities or as a result of natural conditions.
The many and diverse activities of humans produce innumerablewaste materials and by-products. Historically, the disposal of such waste have not been subject to many regulatory controls. Consequently, waste materials have often been disposed of or stored on land surfaces where they percolate into the underlying groundwater. As a result, the contaminated groundwater is unsuitable for use.
Current practices can still impact groundwater, such as the over application offertilizer orpesticides,spills fromindustrial operations, infiltration fromurban runoff, and leaking fromlandfills. Using contaminated groundwater causes hazards topublic health throughpoisoning or the spread of disease, and the practice of groundwater remediation has been developed to address these issues. Contaminants found in groundwater cover a broad range of physical, inorganic chemical, organic chemical, bacteriological, and radioactive parameters. Pollutants and contaminants can be removed from groundwater by applying various techniques, thereby bringing the water to a standard that is commensurate with various intended uses.
Ground water remediation techniques span biological, chemical, and physicaltreatment technologies. Most ground water treatment techniques utilize a combination of technologies. Some of the biological treatment techniques includebioaugmentation,bioventing,biosparging,bioslurping, andphytoremediation. Some chemical treatment techniques includeozone andoxygen gas injection,chemical precipitation,membrane separation,ion exchange,carbon absorption, aqueous chemicaloxidation, andsurfactant enhanced recovery. Some chemical techniques may be implemented usingnanomaterials. Physical treatment techniques include, but are not limited to,pump and treat,air sparging, anddual phase extraction.[citation needed]
If a treatability study shows no degradation (or an extended lab period before significant degradation is achieved) in contamination contained in the groundwater, then inoculation with strains known to be capable of degrading the contaminants may be helpful. This process increases the reactive enzyme concentration within thebioremediation system and subsequently may increase contaminant degradation rates over the nonaugmented rates, at least initially after inoculation.[2]
Bioventing is an on site remediation technology that usesmicroorganisms to biodegradeorganic constituents in the groundwater system. Bioventing enhances the activity of indigenous bacteria and archaea and stimulates the natural in situ biodegradation ofhydrocarbons by inducing air oroxygen flow into the unsaturated zone and, if necessary, by adding nutrients.[3] During bioventing, oxygen may be supplied through direct air injection into residual contamination in soil. Bioventing primarily assists in the degradation of adsorbed fuel residuals, but also assists in the degradation ofvolatile organic compounds (VOCs) as vapors move slowly through biologically active soil.[4]
Biosparging is anin situ remediation technology that uses indigenous microorganisms to biodegrade organic constituents in the saturated zone. In biosparging, air (or oxygen) and nutrients (if needed) are injected into thesaturated zone to increase the biological activity of the indigenous microorganisms. Biosparging can be used to reduce concentrations ofpetroleum constituents that are dissolved in groundwater, adsorbed tosoil below thewater table, and within thecapillary fringe.[citation needed]
Bioslurping combines elements of bioventing and vacuum-enhanced pumping of free-product that is lighter than water (light non-aqueous phase liquid or LNAPL) to recover free-product from the groundwater and soil, and to bioremediate soils. The bioslurper system uses a “slurp” tube that extends into the free-product layer. Much like a straw in a glass draws liquid, the pump draws liquid (including free-product) and soil gas up the tube in the same process stream. Pumping lifts LNAPLs, such as oil, off the top of the water table and from the capillary fringe (i.e., an area just above the saturated zone, where water is held in place by capillary forces). The LNAPL is brought to the surface, where it is separated from water and air. The biological processes in the term “bioslurping” refer to aerobic biological degradation of the hydrocarbons when air is introduced into the unsaturated zone contaminated soil.[5]
In thephytoremediation processcertain plants and trees are planted, whose roots absorb contaminants from ground water over time. This process can be carried out in areas where the roots can tap the ground water. Few examples of plants that are used in this process areChinese Ladder fern Pteris vittata, also known as the brake fern, is a highly efficient accumulator ofarsenic. Genetically alteredcottonwood trees are good absorbers ofmercury and transgenic Indian mustard plants soak upselenium well.[6]
Certain types ofpermeable reactive barriers utilize biological organisms in order to remediate groundwater.[citation needed]
Chemical precipitation is commonly used inwastewater treatment to removehardness andheavy metals. In general, the process involves addition of agent to an aqueous waste stream in a stirred reaction vessel, either batchwise or with steady flow. Most metals can be converted to insoluble compounds by chemical reactions between the agent and the dissolved metal ions. The insoluble compounds (precipitates) are removed by settling and/or filtering.[citation needed]
Ion exchange for ground water remediation is virtually always carried out by passing the water downward under pressure through a fixed bed of granular medium (either cation exchange media and anion exchange media) or spherical beads.Cations are displaced by certain cations from the solutions andions are displaced by certain anions from the solution. Ion exchange media most often used for remediation arezeolites (both natural and synthetic) and synthetic resins.[2]
The most common activated carbon used for remediation is derived frombituminous coal.Activated carbon adsorbs volatile organic compounds from ground water; the compounds attach to the graphite-like surface of the activated carbon.[citation needed]
In this process, calledIn Situ Chemical Oxidation or ISCO, chemicaloxidants are delivered in the subsurface to destroy (converted to water and carbon dioxide or to nontoxic substances) the organics molecules. The oxidants are introduced as either liquids or gasses. Oxidants include air or oxygen,ozone, and certain liquid chemicals such ashydrogen peroxide,permanganate andpersulfate.Ozone andoxygen gas can be generated on site from air and electricity and directly injected into soil and groundwater contamination. The process has the potential to oxidize and/or enhance naturally occurring aerobic degradation. Chemical oxidation has proven to be an effective technique fordense non-aqueous phase liquid or DNAPL when it is present.[citation needed]
Surfactant enhanced recovery increases the mobility and solubility of the contaminants absorbed to the saturated soil matrix or present asdense non-aqueous phase liquid. Surfactant-enhanced recovery injectssurfactants (surface-active agents that are primary ingredient in soap and detergent) into contaminated groundwater. A typical system uses an extraction pump to remove groundwater downstream from the injection point. The extracted groundwater is treated aboveground to separate the injected surfactants from the contaminants and groundwater. Once the surfactants have separated from the groundwater they are re-used. The surfactants used are non-toxic, food-grade, and biodegradable. Surfactant enhanced recovery is used most often when the groundwater is contaminated bydense non-aqueous phase liquids (DNAPLs). These dense compounds, such astrichloroethylene (TCE), sink in groundwater because they have a higher density than water. They then act as a continuous source for contaminantplumes that can stretch for miles within an aquifer. These compounds may biodegrade very slowly. They are commonly found in the vicinity of the original spill or leak where capillary forces have trapped them.[7]
Some permeable reactive barriers utilize chemical processes to achieve groundwater remediation.[citation needed]
Pump and treat is one of the most widely used ground water remediation technologies. In this process ground water is pumped to the surface and is coupled with either biological or chemical treatments to remove the impurities.[8]
Air sparging is the process of blowing air directly into the ground water. As the bubbles rise, the contaminants are removed from the groundwater by physical contact with the air (i.e., stripping) and are carried up into the unsaturated zone (i.e., soil). As the contaminants move into the soil, asoil vapor extraction system is usually used to remove vapors.[9]
Dual-phase vacuum extraction (DPVE), also known as multi-phase extraction, is a technology that uses a high-vacuum system to remove both contaminated groundwater and soil vapor. In DPVE systems, a high-vacuum extraction well is installed with its screened section in the zone of contaminated soils and groundwater. Fluid/vapor extraction systems depress the water table and water flows faster to the extraction well. DPVE removes contaminants from above and below the water table. As the water table around the well is lowered from pumping, unsaturated soil is exposed. This area, called thecapillary fringe, is often highly contaminated, as it holds undissolved chemicals, chemicals that are lighter than water, and vapors that have escaped from the dissolved groundwater below. Contaminants in the newly exposed zone can be removed by vapor extraction. Once above ground, the extracted vapors and liquid-phase organics and groundwater are separated and treated. Use of dual-phase vacuum extraction with these technologies can shorten the cleanup time at a site, because the capillary fringe is often the most contaminated area.[10]
Monitoring-wells are often drilled for the purpose of collecting ground water samples for analysis. These wells, which are usually six inches or less in diameter, can also be used to remove hydrocarbons from the contaminant plume within a groundwater aquifer by using a belt-style oil skimmer. Belt oil skimmers, which are simple in design, are commonly used to remove oil and other floating hydrocarbon contaminants from industrial water systems.[citation needed]
A monitoring-well oil skimmer remediates various oils, ranging from light fuel oils such as petrol, light diesel or kerosene to heavy products such as No. 6 oil, creosote and coal tar. It consists of a continuously moving belt that runs on a pulley system driven by an electric motor. The belt material has a strong affinity forhydrocarbon liquids and for shedding water. The belt, which can have a vertical drop of 100+ feet, is lowered into the monitoring well past the LNAPL/water interface. As the belt moves through this interface, it picks up liquid hydrocarbon contaminant which is removed and collected at ground level as the belt passes through a wiper mechanism. To the extent thatDNAPL hydrocarbons settle at the bottom of a monitoring well, and the lower pulley of the belt skimmer reaches them, these contaminants can also be removed by a monitoring-well oil skimmer.[citation needed]
Typically, belt skimmers remove very little water with the contaminant, so simple weir-type separators can be used to collect any remaining hydrocarbon liquid, which often makes the water suitable for its return to the aquifer. Because the small electric motor uses little electricity, it can be powered fromsolar panels or awind turbine, making the system self-sufficient and eliminating the cost of running electricity to a remote location.[11]
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