Groundwater pollution is imminent in most developing countries as a result of increased anthropogenic activities apart from possible natural pollutants. This study reviewed groundwater pollution and discussed possible remediation measures. Sources of pollution can be categorized into two major types: point source pollution and non-point source pollution. Point source pollution (e.g. municipal sewage treatment plant and industrial plant, intense evaporation in shallow aquifers, degradation of water sources in areas located in geothermal/volcanic fields, and rock oxidation) is a single identify localized source while non-point source pollution (diffuse sources such as human land use, land use changes, chemical reactions of elements in the air or in the water and pollutes runoff from agricultural areas draining into a river) is characterized by multiple discharge point. Point source is relatively easy to identify, quantify and control. On the other hand, non-point source is difficult to monitor and control because the pollution cannot be traced to a single point of discharge. Pollution occurrence depends on the level of contaminant transported. Contaminants can be transported through filtration, sorption, chemical processes, microbiological decomposition and dilution. Groundwater pollution may cause ecosystem imbalance apart from severe sickness which may lead to death. Prevention of groundwater pollution is more appropriate than remediation. Such preventive measures include proper waste disposal, monitoring of hazardous materials, conducting environmental audit periodically and intensifying health education while remediation includes stream stripping, oxygen sparging, bioremediation, chemical oxidation and thermal treatment. This study revealed two main sources (point source and non-point source) of pollution with non-point pollution more difficult to remediate due to extent of spread. In addition, most pollution of groundwater is anthropocentric and can be prevented through intensive health education.
Water covers over 70% of the earth surface and is undoubtedly the most precious natural resource that exists on the earth. Without the seemingly invaluable compound comprised of hydrogen and oxygen, life on the earth would be non-existent. Water is a key ingredient surporting food production, sanitation and rural livelihoods, as well as ensuring continuity and functioning of ecosystem. It dictates the pace of settlement and agricultural and industrial development of any society and even in recent time, establishment of any human settlement is usually centered on available source of water supply and in modern time, issue of water has equally taken prominences in global matters [
The distribution of world’s water indicates that only 2.5% and 97.5% constitute fresh water and saline water respectively. 2.5% of the world’s freshwater, surface water and groundwater have 0.4% and 30.1% representations respectively [
The term quality of groundwater refers to its physical, chemical, and biological characteristics as they relate to the intended use of water. Groundwater quality is threatened mainly by human activities, although harmful substances are sometimes introduced by natural processes. Sustainable groundwater management must be based not only on prevention of the over exploitation of groundwater resources but also on prevention of contamination because unlike treatment at the point of use, prevention protects all of the resource. However, economic activities (primary activities) which produce commodities (mining, agriculture) and secondary or industrial activities (energy production, manufacturing, building, etc.) in addition to services (including transport) and households activities result into generation of enormous waste products which threatens the environment and leads to increasing groundwater pollution [
Understanding the physical environment of groundwater is important in its pollution evaluation. Groundwater is the great “unseen” salient element of the hydrologic cycle. Because it is out of sight it is frequently out of mind [
Water is the most precious resource that humans utilize from nature. It is a life support system not only for humans but “is the living environment for a large proportion of earth’s animals and plants” [
corrosion or ecological change”. [
To begin to address pollution prevention or remediation, we must understand how surface water and groundwater interrelate. Groundwater and surface water are interconnected and can be fully understood and intelligently managed only when that fact is acknowledged. If there is a water supply well near a source of contamination, that well runs the risk of becoming contaminated (
aquifer in the same manner that groundwater moves. Some contaminants do not always follow groundwater flow. It is possible to predict, to some degree, the transport within an aquifer of those substances that move along with groundwater flow.
For example, both water and certain contaminants flow in the direction of the topography from recharge areas to discharge areas. Soils that are porous and permeable tend to transmit water and certain types of contaminants with relative ease to an aquifer below. Just as groundwater generally moves slowly, so do contaminants in groundwater. Because of this slow movement, contaminants tend to remain concentrated in the form of a plume (
Groundwater and contaminants can move rapidly through fractures in rocks. Fractured rock presents a unique problem in locating and controlling contaminants because the fractures are generally randomly spaced and do not follow the contours of the land surface or the hydraulic gradient. Contaminants can also move into the groundwater system through macropores-root systems, animal burrows, abandoned wells, and other systems of holes and cracks that supply pathways for contaminants. Some drinking water wells actually draw water from nearby streams, lakes or rivers. Contaminants present in these surface waters can contribute contamination to the ground water system. Some wells rely on artificial recharge to increase the amount of water infiltrating an aquifer, often using water from storm runoff, irrigation, industrial processes or treated sewage. In several cases, this practice has resulted in increased concentrations of nitrates, metals, microbes or synthetic chemicals in the water.
The contaminant introduced into the soil-rock-groundwater system will spread within the system only if a transport mechanism is available, for example, a
flowing liquid. As soon as the contaminant reaches the subsurface water in the unsaturated or saturated zone, various processes (physical, geochemical and biochemical) determine its fate [
∂ C ∂ t − v ( ∂ C ∂ t ) + D ( ∂ 2 C ∂ x 2 ) − ∂ q ∂ t (1)
where C is the concentration of the contaminant, t is time, x is the distance in the flow path, v is the average water flow velocity in the porous media, D is the hydrodynamic dispersion coefficient and q is a coefficient regarding to processes affecting the concentration.
This equation considers mass conservation over an elemental volume of the aquifer and it is also known as the advection-dispersion equation as these are the two main processes controlling solute transport in a porous media. It includes (from left to right) an advection term, a dispersion term and a source/sink term that describes different processes affecting the concentration. Thus this equation considers the multiple ways in which a contaminant could move within an aquifer. Overall, advection is by far more relevant for contaminant transport than diffusion in sedimentary aquifers, whereas diffusion is the most relevant process in aquitards (i.e., geological materials of low hydraulic conductivity).
Water resources pollution and the respective groundwater quality degradation are caused by various human activities which result in the change of water
physicochemical characteristics. Groundwater pollution sources can be classified based on different parameters (origin, geometry of the source and transmission rate).
Pollution sources as a result of origin could be natural or anthropogenic, geometry of the source could be point source (landfills, waste dumps, septic tanks, underground tanks), linear (roads) and diffused (nitro pollution, acid rain, uranium decay) while the transmission rate represents continuous and recurrent transmission of pollutant.
The most common pollution sources are the anthropogenic ones. This category generally includes: disposal of wastewater and solid waste, disposal of industrial wastewater, use of fertilizers, pesticides and insecticides, disposal of by-products and waste from mining activities and nuclear energy waste. Anthropogenic sources can be caused by different activities such as over-pumping, unlimited use of fertilizers, mining activities, waste dumps, extended urban development, change of climatic conditions (microclimate), misuse of chemicals, disposal of organic and inorganic elements, heaps and sewage storage, river network interruption, mineral processing of radioactive minerals and cemeteries. Pollutants such as trace elements are concentrated in the discharge material/water from various anthropogenic activities and can be toxic and lethal to humans. Other contaminants that have been detected in groundwater and are related to human activities are hydrocarbons, halogens, biphenyls and biological contamination (bacteria; viruses; parasites). Groundwater can also be polluted through natural causes, i.e., the results of cycles or of natural phenomena. This category includes the following sources: easy to dissolve rocks (gypsum, mineral salt etc.), intense evaporation especially in shallow aquifers which causes elevation of groundwater and salt deposition, degradation of water sources in areas located in geothermal/volcanic fields, rock oxidation, seawater intrusion, decay of radioisotopes from uranium-rich bedrock, and chemical reactions of elements in the air or in the water.
Possible water pollutant sources are numerous and can also be classified into point sources and diffuse pollution sources. Pollution of surface waters is directly related with groundwater pollution. Thus, pollution of surface water usually results in respective groundwater pollution. Groundwater pollution sources are summarized in
Domestic wastewater from residences, institutions or other facilities such as confectioneries, restaurants, laundries etc. is a major pollution point source. It mainly consists of water containing organic and inorganic compounds, as well as bacteria and viruses. Organic matter in domestic wastewater can be faeces, urine, paper, soaps, detergents, food residuals, fats and oils while inorganic materials include ammonia, ammonium salts, clay, phosphorus etc. [
Industrial wastewater refers to wastewater from various industries, excluding the wastewater from the staff facilities and is another point source of groundwater pollution. This type of wastewater is a result of the water used in the industrial processes, which contains various compounds in low or high concentrations, both biological (e.g., wastewater of the food industry, paper industry and textile treatment) and non-biological (wastewater of the chemical industry containing pollutants such as acids, bases, chlorine, metals, cyanide, salts, hydrocarbons, phosphates) [
Hospital wastewater represents a major environmental threat and creates serious issues for the natural environment and human health. It is reported that hospital wastewater is 5 to 15 times more toxic than typical urban outflows [
Category | Types of contaminants | Source of contamination |
---|---|---|
Residential | Air pollution | Septic tank |
Household waste water | Sewer network | |
Household waste | Fuel oil | |
Furniture stripping/refinishing | Paints | |
Municipal | Municipal sludge spreading in land | Air pollution |
Salt for street de-icing | Streets and packing lots | |
Municipal incinerators | Municipal landfills | |
Sewer lines | Road maintenance depot | |
Wastewater treatment plants effluents | ||
Commercial | Airports | Metal plating |
Construction areas | Medical institutions | |
Car washes | Research laboratories | |
Cemeteries | Rail road tracks | |
Dry cleaners | Laundromats | |
Gas stations | Scap/junk yards | |
Golf courses | Recycling facilities | |
Industrial | Chemical industry/storage | Metal fabricators |
Electronics manufacture | Petroleum production | |
Mining and mine drainage | Pipelines | |
Metal working shops | Storage tanks | |
Toxic/hazardous spills | Wells | |
Agriculture | Animal feeds | Fertilizer storage/use |
Irrigation sites | Manure spreading areas | |
Sludge reuse | Chemical spills | |
Live stocks waste | Pesticides | |
Tanks | Wells |
any previous treatment [
Landfills are also considered as an important pollution point source of groundwater. Leachates from landfills are rich in nitrogen, chloride, mercury, iron etc. Contamination of aquifers in landfill sites has been detected in many cases around the world over the last decades. Usually, parameters that result in groundwater contamination are bad design or construction of the landfill, inappropriate selection of the installation site or false characterization of the hydro
geologic regime and soil permeability, among others. Landfill projects usually include groundwater monitoring programs (frequent samplings and analyses, groundwater elevations measurements), while in cases when there are nearby water bodies, monitoring of surface water is also required [
Mining waste are produced during the mining of mineral resources in mines and coal mines, and are a mixture of water and powdered mineral and possibly of heavy metals as by-products [
This type of sources includes natural, chemical infiltration and anthropogenic sources, such as storm water and urban runoff, highway runoff, and agricultural land (use of fertilizers). The use of fertilizers, pesticides, herbicides and animal waste in agriculture is a major non-point source of groundwater contamination [
Natural groundwater contamination also falls within the category of non-point sources. The presence of various elements and impurities in groundwater may not be originated from anthropogenic sources. Seawater intrusion is an issue with global dimensions since it contributes to the degradation of coastal freshwater aquifers [
The impacts/effects of pollution on the Earth include environmental deterioration, impacts on the health of living organisms, global warming, depletion of the ozone layer and reduced efficiency or infertility of farm lands and crop fields. Few of the impacts/effects of pollution are discussed below.
The effect of groundwater pollution on human health can be devastating. It could cause severe sickness which may lead to death in some cases. Such diseases include nausea, vomiting, diarrhea, headache, respiratory diseases, eye irritation and nose irritation. Chronic diseases that could be caused by groundwater pollution include cancer, liver damage, kidney damage, anemia, nervous system problems, circulatory system problems, bone diseases, hair loss and reproductive difficulties. Annual averages of 7 million people were sick in the USA from drinking polluted water [
Nature has designed the ecosystem to be balanced with one group of organisms depending on the other while over population of a group is controlled by their predators. Any reduction or increase in the population of a group could lead to an imbalance in the ecosystem. Such imbalance may have ripple effect on the whole ecosystem. For example, crocodiles feed on fishes, if all the fishes in a riverine environment die as a result of pollution, this will affect the crocodiles which may lead to them being starved to death or migration.
Pollution of groundwater can lead to scarcity of water. A very large percentage of the world’s population depends on groundwater for their daily use. When pollution occurs on a large scale, the entire consumers in a community are forced to abandon the consumption of the affected water. Alternative water supply has to be sought. This is always a problem as it is not usually easy to find alternative water supply for a large population.
Groundwater pollution threatens the safety usage of water necessitating prevention/remediation where the need arises.
Investing in safe drinking water is not only good for personal health and hygiene as well as the environment. It also promotes economic growth. It is therefore necessary to have effective means of preventing groundwater pollution. Prevention of groundwater pollution is the cheapest and most effective solution vis-à-vis remediation [
Wastes should be properly disposed. The disposal plan must ensure proper waste water discharge connections and if possible, floor drains should be eliminated. The plan must provide for proper use and maintenance of on-site septic systems. Plug and cover waste dumpsters must be provided. As for the hazardous materials, chemicals and fuels must be safely stored and handled; underground fuels and chemical tanks should be well monitored. Preference should be given to surface tanks which can be monitored more easily. Storage and loading areas are to be controlled while use of chemicals could be reduced or substituted. Prevention of groundwater from pollution through storm water is to protect chemicals and waste from rain and flood penetrating into the aquifers. Drains could be isolated from storage and loading areas while deicing salt and particles should be sparingly used. It is important to conduct environmental audit periodically. This would lead to the development of pollution prevention plan. High risk areas must be regularly inspected while an emergency response plan is developed. Land use plans would need to protect important aquifers and well fields. Residents and businesses must be educated and well informed. Hazardous waste collection must be conducted in all households. All pipelines and storage tanks must be monitored. The right quality of these materials should be procured. Their life span must be noted and they must be replaced at the appropriate time. Those that contain corrosive chemicals should receive special attention. Unused water wells should be properly disposed of. Special attention must be paid to individual, small and medium scale ventures that their activities produce chemical wastes. The level of their financial capability may not enable them to provide adequate preventive measures. The government should come to their aid. Use of insecticides and pesticides should be eliminated or reduced to the barest minimum. This could be done by relying more on waste from livestock. Monitoring wells should be installed at intervals on pipeline network and the vicinity of storage tanks. These would be inspected periodically to ensure early detection of leakages. In addition, the Water Policy formulation should clearly spell out all prevention plans and a body must be tasked to implement the Policy. Awareness measures and penalty should be spelt out and the citizenry should be well informed.
Groundwater remediation is the process that can be used to remove pollution from groundwater. Because each groundwater aquifer is unique, mineralization of organic contaminants by either biotic or abiotic mechanisms (e.g., complete conversion of the chemical to CO2 and water) is a function of the spatial heterogeneity of subsurface properties such as pore structure, hydraulic conductivity and microorganism populations as well as carbon and energy sources [
The Physical-chemical methods of aquifer remediation/restoration usually include drilling a well into the contaminant plume to remove contaminated groundwater to the surface. Then a variety of separation methods are available such as air stripping, activated charcoal and reverse osmosis to remove contaminants from groundwater aquifers [
In the subsurface, biological degradation occurs naturally. However, for some refractory chemicals such as organochlorine pesticides and some organochlorine solvents (e.g., trichloroethylene and tetrachloroethylene), the rates of degradation in many soil environments are slow. Because of this, these chemicals travel great distances from their point of entry without much attenuation in initial entry concentration [
Biotransformation was enhanced by injecting methane and oxygen-containing groundwater in alternating pulses into the zone of contamination in the subsurface. These field experiments have demonstrated that microbial transformations observed in the laboratory can be promoted in-situ in the field when proper conditions are observed.
There are many ways to remove pollution. These methods can be broadly divided into two groups. There are ex-situ and in-situ technologies. Whatever method is employed, groundwater clean-up is a very expensive venture. The cost however depends on extent, potential health effects and the alternatives available. The aquifer may be capped to prevent more pollution. By dewatering the polluted aquifer (pumping out), the water can then be treated on the surface by physical, chemical or biological technology. It is then finally re-injected back into the aquifer. In-situ technology involves treatment of groundwater within the aquifer by using thermal, chemical or biological treatment technology. On the other hand, ex-situ technology may involve the use of the following methods; Steam Stripping in which the water is treated by introducing steam which extracts the contaminants from the pumped out groundwater. The extracted steam can be recovered from the condensate or treated further by incineration. Oxygen sparging that involves the introduction of oxidizing and reducing agents. Example includes O3, H2O2 and hypochlorite. These will chemically convert the toxic contaminants to less toxic compounds. Bio Remediation which is achieved through treating pumped up groundwater by air with careful control of moisture, heat nutrients, oxygen and pH and Carbon Adsorption that involves the passing of contaminated pumped up groundwater through activated carbon column in which contaminants get adsorbed.
The in-situ technology involves treatment of groundwater in place without extracting the water from the aquifer. These may come employing Air Sparging. Air Sparging is the injection of contaminant free air into the sub-surface saturated zone which enables a phase transfer of hydrocarbons from a dissolved state to a vapor phase. Other in-situ technology methods are Bio Remediation, In-Well Air Stripping, Chemical Oxidation, Thermal Treatment and Phyto Remediation. Bio Remediation is the injection of oxygen to enhance biodegradation. It also combines the injection of degrading bacteria and nutrients into the aquifer to stimulate biodegradation. In-Well Air Stripping is the injection of air into a double screened well, lifting the water up the well and forcing it out in upper screen. As for the chemical oxidation, it involves reduction-oxidation reactions that chemically convert hazardous contaminants to less toxic compounds. Typical examples of this are cyanide oxidation and dechlorination. Thermal treatment involves increase in temperature of the source zone to increase the mobility of the pollutants. This mobility facilitates removal of pollutants and can also result in in-situ destruction of contaminants. Finally, Phyto remediation involves the use of macroscopic plants to destroy, remove, immobilize and treat contaminants. This process does not use microorganisms.
Groundwater resources are vital for human life and health, societal development and the preservation of the natural ecosystems. Quality degradation of groundwater bodies as a result of pollution represents a major worldwide issue. Various and multiple sources of groundwater pollution have been identified which include a series of both natural and anthropogenic sources. Human activities such as over abstraction, insufficient wastewater treatment, disposal and industrial activities and use of fertilizers in agriculture represent the main sources of groundwater contaminants. Seawater intrusion is another source of groundwater pollution which results in the salinity increase in coastal groundwater bodies and makes them inappropriate for use as drinking water. Prevention of groundwater from pollution is considered more appropriate to remediation. Prevention methods include proper waste disposal, monitoring of hazardous materials, conducting environmental audit periodically and intensifying health education while remediation measures are stream stripping, oxygen sparging, bioremediation, air stripping, chemical oxidation and thermal treatment. This study revealed two main sources of groundwater pollution (point and non-point sources), both of which may be natural or anthropogenic. However, non-point pollution sources are more difficult to remediate due to the extent of spread. In addition, most pollution of groundwater is anthropocentric and can be prevented through intensive health education.
This research did not receive any specific grant from funding agencies in the public, commercial, or non-profit sectors.
The authors declare no conflicts of interest regarding the publication of this paper.
Talabi, A.O. and Kayode, T.J. (2019) Groundwater Pollution and Remediation. Journal of Water Resource and Protection, 11, 1-19. https://doi.org/10.4236/jwarp.2019.111001