are often close to surface waters and in permeable soils. closed landfills can still pose a threat to ground water if they are not capped by an impermeable material before closure to prevent leaking of contaminants. love canal was one of the most widely known examples of groundwater pollution. in 1978, residents of the love canal neighborhood in upstate new york noticed high rates of cancer and an alarming number of birth defects. this was eventually traced to organic solvents and dioxins from an industrial landfill that the neighborhood had been built over and around, which had then infiltrated into the water supply and evaporated in basements to further contaminate the air. eight hundred families were reimbursed for their homes and moved, after extensive legal battles and media coverage. over-pumping satellite data in the mekong delta in vietnam have provided evidence that over-pumping of groundwater leads to land subsidence as well as consequential release of arsenic and possibly other heavy metals. arsenic is found in clay strata due to their high surface area to volume ratio relative to sand-sized particles. most pumped groundwater travels through sands and gravels with low arsenic concentration. however, during over-pumping, a high vertical gradient pulls water from less-permeable clays, thus promoting arsenic release into the water. other groundwater pollution can be caused by chemical spills from commercial or industrial operations, chemical spills occurring during transport (e.g. spillage of diesel fuels), illegal waste dumping, infiltration from urban runoff or mining operations, road salts, de-icing chemicals from airports and even atmospheric contaminants since groundwater is part of the hydrologic cycle. herbicide use can contribute to groundwater contamination through arsenic infiltration. herbicides contribute to arsenic desorption through mobilization and transportation of the contaminant. chlorinated herbicides exhibit a lower impact on arsenic desorption than phosphate type herbicides. this can help to prevent arsenic contamination through choosing herbicides appropriate for different concentrations of arsenic present in certain soils. the burial of corpses and their subsequent degradation may also pose a risk of pollution to groundwater. mechanisms the passage of water through the subsurface can provide a reliable natural barrier to contamination but it only works under favorable conditions. the stratigraphy of the area plays an important role in the transport of pollutants. an area can have layers of sandy soil, fractured bedrock, clay, or hardpan. areas of karst topography on limestone bedrock are sometimes vulnerable to surface pollution from groundwater. earthquake faults can also be entry routes for downward contaminant entry. water table conditions are of great importance for drinking water supplies, agricultural irrigation, waste disposal (including nuclear waste), wildlife habitat, and other ecological issues. many chemicals undergo reactive decay or chemical change, especially over long periods of time in groundwater reservoirs. a noteworthy class of such chemicals is the chlorinated hydrocarbons such as trichloroethylene (used in industrial metal degreasing and electronics manufacturing) and tetrachloroethylene used in the dry cleaning industry. both of these chemicals, which are carcinogens themselves, undergo partial decomposition reactions, leading to new hazardous chemicals (including dichloroethylene and vinyl chloride). interactions with surface water although interrelated, surface water and groundwater have often been studied and managed as separate resources. interactions between groundwater and surface water are complex. surface water seeps through the soil and becomes groundwater. conversely, groundwater can also feed surface water sources. for example, many rivers and lakes are fed by groundwater. this means that damage to groundwater aquifers e.g. by fracking or over abstraction, could therefore affect the rivers and lakes that rely on it. saltwater intrusion into coastal aquifers is an example of such interactions. a spill or ongoing release of chemical or radionuclide contaminants into soil (located away from a surface water body) may not create point or non-point source pollution but can contaminate the aquifer below, creating a toxic plume. the movement of the plume, may be analyzed through a hydrological transport model or groundwater model. prevention schematic showing that there is a lower risk of groundwater pollution with greater depth of the water well precautionary principle further information: precautionary principlethe precautionary principle, evolved from principle 15 of the rio declaration on environment and development, is important in protecting groundwater resources from pollution. the precautionary principle provides that "where there are threats of irreversible damage, lack of full scientific certainty shall not be used as reason for postponing cost-effective measures to prevent environmental degradation." one of the six basic principles of the european union (eu) water policy is the application of the precautionary principle. groundwater quality monitoring groundwater quality monitoring programs have been implemented regularly in many countries around the world. they are important components to understand the hydrogeological system, and for the development of conceptual models and aquifer vulnerability maps. groundwater quality must be regularly monitored across the aquifer to determine trends. effective groundwater monitoring should be driven by a specific objective, for example, a specific contaminant of concern. contaminant levels can be compared to the world health organization (who) guidelines for drinking-water quality. it is not rare that limits of contaminants are reduced as more medical experience is gained. sufficient investment should be given to continue monitoring over the long term. when a problem is found, action should be taken to correct it. waterborne outbreaks in the united states decreased with the introduction of more stringent monitoring (and treatment) requirements in the early 90s. the community can also help monitor the groundwater quality. scientists have developed methods by which hazard maps could be produced for geogenic toxic substances in groundwater. this provides an efficient way of determining which wells should be tested. land zoning for groundwater protection the development of land-use zoning maps has been implemented by several water authorities at different scales around the world. there are two types of zoning maps: aquifer vulnerability maps and source protection maps. aquifer vulnerability map it refers to the intrinsic (or natural) vulnerability of a groundwater system to pollution. intrinsically, some aquifers are more vulnerable to pollution than other aquifers. shallow unconfined aquifers are more at risk of pollution because there are fewer layers to filter out contaminants. the unsaturated zone can play an important role in retarding (and in some cases eliminating) pathogens and so must be considered when assessing aquifer vulnerability. the biological activity is greatest in the top soil layers where the attenuation of pathogens is generally most effective. preparation of the vulnerability maps typically involves overlaying several thematic maps of physical factors that have been selected to describe the aquifer vulnerability. the index-based parametric mapping method god developed by foster and hirata (1988) uses three generally available or readily estimated parameters, the degree of groundwater hydraulic confinement, geological nature of the overlying strata and depth to groundwater. a further approach developed by epa, a rating system named "drastic", employs seven hydrogeological factors to develop an index of vulnerability: depth to water table, net recharge, aquifer media, soil media, topography (slope), impact on the vadose zone, and hydraulic conductivity. there is a particular debate among hydrogeologists as to whether aquifer vulnerability should be established in a general (intrinsic) way for all contaminants, or specifically for each pollutant. source protection map it refers to the capture areas around an individual groundwater source, such as a water well or a spring, to especially protect them from pollution. thus, potential sources of degradable pollutants, such as pathogens, can be located at distances which travel times along the flowpaths are long enough for the pollutant to be eliminated through filtration or adsorption. analytical methods using equations to define groundwater flow and contaminant transport are the most widely used. the whpa is a semi-analytical groundwater flow simulation program developed by the us epa for delineating capture zones in a wellhead protection area. the simplest form of zoning employs fixed-distance methods where activities are excluded within a uniformly applied specified distance around abstraction points. locating on-site sanitation systems as the health effects of most toxic chemicals arise after prolonged exposure, risk to health from chemicals is generally lower than that from pathogens. thus, the quality of the source protection measures is an important component in controlling whether pathogens may be present in the final drinking-water. on-site sanitation systems can be designed in such a way that groundwater pollution from these sanitation systems is prevented from occurring. detailed guidelines have been developed to estimate safe distances to protect groundwater sources from pollution from on-site sanitation. the following criteria have been proposed for safe siting (i.e. deciding on the location) of on-site sanitation systems: horizontal distance between the drinking water source and the sanitation system guideline values for horizontal separation distances between on-site sanitation systems and water sources vary widely (e.g. 15 to 100 m horizontal distance between pit latrine and groundwater wells) vertical distance between drinking water well and sanitation system aquifer type groundwater flow direction impermeable layers slope and surface drainage volume of leaking wastewater superposition, i.e. the need to consider a larger planning area as a very general guideline it is recommended that the bottom of the pit should be at least 2 m above groundwater level, and a minimum horizontal distance of 30 m between a pit and a water source is normally recommended to limit exposure to microbial contamination. however, no general statement should be made regarding the minimum lateral separation distances required to prevent contamination of a well from a pit latrine. for example, even 50 m lateral separation distance might not be sufficient in a strongly karstified system with a downgradient supply well or spring, while 10 m lateral separation distance is completely sufficient if there is a well developed clay cover layer and the annular space of the groundwater well is well sealed. legislation institutional and legal issues are critical in determining the success or failure of groundwater protection policies and strategies. in the united states the resource conservation and recovery act protects groundwater by regulating the disposal of solid waste and hazardous waste, and the comprehensive environmental response, compensation, and liability act, also known as "superfund", requires remediation of abandoned hazardous waste sites. sign near mannheim, germany, indicating a zone as a dedicated "groundwater protection zone" management options for remediation of contaminated groundwater can be grouped into the following categories: containing the pollutants to prevent them from migrating further removing the pollutants from the aquifer remediating the aquifer by either immobilizing or detoxifying the contaminants while they are still in the aquifer (in-situ) treating the groundwater at its point of use abandoning the use of this aquifer's groundwater and finding an alternative source of water. point-of-use treatment main article: portable water purification portable water purification devices or "point-of-use" (pou) water treatment systems and field water disinfection techniques can be used to remove some forms of groundwater pollution prior to drinking, namely any fecal pollution. many commercial portable water purification systems or chemical additives are available which can remove pathogens, chlorine, bad taste, odors, and heavy metals like lead and mercury. techniques include boiling, filtration, activated charcoal absorption, chemical disinfection, ultraviolet purification, ozone water disinfection, solar water disinfection, solar distillation, homemade water filters. arsenic removal filters (arf) are dedicated technologies typically installed to remove arsenic. many of these technologies require a capital investment and long-term maintenance. filters in bangladesh are usually abandoned by the users due to their high cost and complicated maintenance, which is also quite expensive. groundwater remediation groundwater pollution is much more difficult to abate than surface pollution because groundwater can move great distances through unseen aquifers. non-porous aquifers such as clays partially purify water of bacteria by simple filtration (adsorption and absorption), dilution, and, in some cases, chemical reactions and biological activity; however, in some cases, the pollutants merely transform to soil contaminants. groundwater that moves through open fractures and caverns is not filtered and can be transported as easily as surface water. in fact, this can be aggravated by the human tendency to use natural sinkholes as dumps in areas of karst topography. pollutants and contaminants can be removed from ground water by applying various techniques thereby making it safe for use. ground water treatment (or remediation) techniques span biological, chemical, and physical treatment technologies. most ground water treatment techniques utilize a combination of technologies. some of the biological treatment techniques include bioaugmentation, bioventing, biosparging, bioslurping, and phytoremediation. some chemical treatment techniques include ozone and oxygen gas injection, chemical precipitation, membrane