to test samples from every single groundwater resource. this tool also allows the user to produce probability risk mapping for both arsenic and fluoride. high concentrations of parameters like salinity, iron, manganese, uranium, radon and chromium, in groundwater, may also be of geogenic origin. this contaminants can be important locally but they are not as widespread as arsenic and fluoride. on-site sanitation systems a traditional housing compound near herat, afghanistan, where a shallow water supply well (foreground) is in close proximity to the pit latrine (behind the white greenhouse) leading to contamination of the groundwater groundwater pollution with pathogens and nitrate can also occur from the liquids infiltrating into the ground from on-site sanitation systems such as pit latrines and septic tanks, depending on the population density and the hydrogeological conditions. factors controlling the fate and transport of pathogens are quite complex and the interaction among them is not well understood. if the local hydrogeological conditions (which can vary within a space of a few square kilometers) are ignored, simple on-site sanitation infrastructures such as pit latrines can cause significant public health risks via contaminated groundwater. liquids leach from the pit and pass the unsaturated soil zone (which is not completely filled with water). subsequently, these liquids from the pit enter the groundwater where they may lead to groundwater pollution. this is a problem if a nearby water well is used to supply groundwater for drinking water purposes. during the passage in the soil, pathogens can die off or be adsorbed significantly, mostly depending on the travel time between the pit and the well. most, but not all pathogens die within 50 days of travel through the subsurface. the degree of pathogen removal strongly varies with soil type, aquifer type, distance and other environmental factors. for example, the unsaturated zone becomes "washed" during extended periods of heavy rain, providing hydraulic pathway for the quick pass of pathogens. it is difficult to estimate the safe distance between a pit latrine or a septic tank and a water source. in any case, such recommendations about the safe distance are mostly ignored by those building pit latrines. in addition, household plots are of a limited size and therefore pit latrines are often built much closer to groundwater wells than what can be regarded as safe. this results in groundwater pollution and household members falling sick when using this groundwater as a source of drinking water. sewage and sewage sludge groundwater pollution can be caused by untreated waste discharge leading to diseases like skin lesions, bloody diarrhea and dermatitis. this is more common in locations having limited wastewater treatment infrastructure, or where there are systematic failures of the on-site sewage disposal system. along with pathogens and nutrients, untreated sewage can also have an important load of heavy metals that may seep into the groundwater system. the treated effluent from sewage treatment plants may also reach the aquifer if the effluent is infiltrated or discharged to local surface water bodies. therefore, those substances that are not removed in conventional sewage treatment plants may reach the groundwater as well. for example, detected concentrations of pharmaceutical residues in groundwater were in the order of 50 mg/l in several locations in germany. this is because in conventional sewage treatment plants, micro-pollutants such as hormones, pharmaceutical residues and other micro-pollutants contained in urine and feces are only partially removed and the remainder is discharged into surface water, from where it may also reach the groundwater. groundwater pollution can also occur from leaking sewers which has been observed for example in germany. this can also lead to potential cross-contamination of drinking-water supplies. spreading wastewater or sewage sludge in agriculture may also be included as sources of fecal contamination in groundwater. fertilizers and pesticides nitrate can also enter the groundwater via excessive use of fertilizers, including manure spreading. this is because only a fraction of the nitrogen-based fertilizers is converted to produce and other plant matter. the remainder accumulates in the soil or lost as run-off. high application rates of nitrogen-containing fertilizers combined with the high water-solubility of nitrate leads to increased runoff into surface water as well as leaching into groundwater, thereby causing groundwater pollution. the excessive use of nitrogen-containing fertilizers (be they synthetic or natural) is particularly damaging, as much of the nitrogen that is not taken up by plants is transformed into nitrate which is easily leached. poor management practices in manure spreading can introduce both pathogens and nutrients (nitrate) in the groundwater system. the nutrients, especially nitrates, in fertilizers can cause problems for natural habitats and for human health if they are washed off soil into watercourses or leached through soil into groundwater. the heavy use of nitrogenous fertilizers in cropping systems is the largest contributor to anthropogenic nitrogen in groundwater worldwide. feedlots/animal corrals can also lead to the potential leaching of nitrogen and metals to groundwater. over application of animal manure may also result in groundwater pollution with pharmaceutical residues derived from veterinary drugs. the us environmental protection agency (epa) and the european commission are seriously dealing with the nitrate problem related to agricultural development, as a major water supply problem that requires appropriate management and governance. runoff of pesticides may leach into groundwater causing human health problems from contaminated water wells. pesticide concentrations found in groundwater are typically low, and often the regulatory human health-based limits exceeded are also very low. the organophosphorus insecticide monocrotophos (mcp) appears to be one of a few hazardous, persistent, soluble and mobile (it does not bind with minerals in soils) pesticides able to reach a drinking-water source. in general, more pesticide compounds are being detected as groundwater quality monitoring programs have become more extensive; however, much less monitoring has been conducted in developing countries due to the high analysis costs. commercial and industrial leaks a wide variety of both inorganic and organic pollutants have been found in aquifers underlying commercial and industrial activities. ore mining and metal processing facilities are the primary responsible of the presence of metals in groundwater of anthropogenic origin, including arsenic. the low ph associated with acid mine drainage (amd) contributes to the solubility of potential toxic metals that can eventually enter the groundwater system. oil spills associated with underground pipelines and tanks can release benzene and other soluble petroleum hydrocarbons that rapidly percolate down into the aquifer. there is an increasing concern over the groundwater pollution by gasoline leaked from petroleum underground storage tanks (usts) of gas stations. btex compounds are the most common additives of the gasoline. btex compounds, including benzene, have densities lower than water (1 g/ml). similar to the oil spills on the sea, the non-miscible phase, referred to as light non-aqueous phase liquid (lnapl), will "float" upon the water table in the aquifer. chlorinated solvents are used in nearly any industrial practice where degreasing removers are required. pce is a highly utilized solvent in the dry cleaning industry because of its cleaning effectiveness and relatively low cost. it has also been used for metal-degreasing operations. because it is highly volatile, it is more frequently found in groundwater than in surface water. tce has historically been used as a metal cleaning. the military facility anniston army depot (anad) in the united states was placed on the epa superfund national priorities list (npl) because of groundwater contamination with as much as 27 million pounds of tce. both pce and tce may degrade to vinyl chloride (vc), the most toxic chlorinated hydrocarbon. many types of solvents may have also been disposed illegally, leaking over time to the groundwater system. chlorinated solvents such as pce and tce have densities higher than water and the non-miscible phase is referred to as dense non-aqueous phase liquids (dnapl). once they reach the aquifer, they will "sink" and eventually accumulate on the top of low-permeability layers. historically, wood-treating facilities have also release insecticides such as pentachlorophenol (pcp) and creosote into the environment, impacting the groundwater resources. pcp is a highly soluble and toxic obsolete pesticide recently listed in the stockholm convention on persistent organic pollutants. pahs and other semi-vocs are the common contaminants associated with creosote. although non-miscible, both lnapls and dnapls still have the potential to slowly dissolve into the aqueous (miscible) phase to create a plume and thus become a long-term source of contamination. dnapls (chlorinated solvents, heavy pahs, creosote, pcbs) tend to be difficult to manage as they can reside very deep in the groundwater system. hydraulic fracturing the recent growth of hydraulic fracturing ("fracking") wells in the united states has raised concerns regarding its potential risks of contaminating groundwater resources. epa, along with many other researchers, has been delegated to study the relationship between hydraulic fracturing and drinking water resources. while it is possible to perform hydraulic fracturing without having a relevant impact on groundwater resources if stringent controls and quality management measures are in place, there are a number of cases where groundwater pollution due to improper handling or technical failures was observed. while the epa has not found significant evidence of a widespread, systematic impact on drinking water by hydraulic fracturing, this may be due to insufficient systematic pre- and post- hydraulic fracturing data on drinking water quality, and the presence of other agents of contamination that preclude the link between tight oil and shale gas extraction and its impact. despite the epa's lack of profound widespread evidence, other researchers have made significant observations of rising groundwater contamination in close proximity to major shale oil/gas drilling sites located in marcellus (british columbia, canada). within one kilometer of these specific sites, a subset of shallow drinking water consistently showed higher concentration levels of methane, ethane, and propane concentrations than normal. an evaluation of higher helium and other noble gas concentration along with the rise of hydrocarbon levels supports the distinction between hydraulic fracturing fugitive gas and naturally occurring "background" hydrocarbon content. this contamination is speculated to be the result of leaky, failing, or improperly installed gas well casings. furthermore, it is theorized that contamination could also result from the capillary migration of deep residual hyper-saline water and hydraulic fracturing fluid, slowly flowing through faults and fractures until finally making contact with groundwater resources; however, many researchers argue that the permeability of rocks overlying shale formations are too low to allow this to ever happen sufficiently. to ultimately prove this theory, there would have to be traces of toxic trihalomethanes (thm) since they are often associated with the presence of stray gas contamination, and typically co-occur with high halogen concentrations in hyper-saline waters. besides, highly saline waters are a common natural feature in deep groundwater systems. while conclusions regarding groundwater pollution as the result to hydraulic fracturing fluid flow is restricted in both space and time, researchers have hypothesized that the potential for systematic stray gas contamination depends mainly on the integrity of the shale oil/gas well structure, along with its relative geological location to local fracture systems that could potentially provide flow paths for fugitive gas migration. though widespread, systematic contamination by hydraulic fracturing has been heavily disputed, one major source of contamination that has the most consensus among researchers of being the most problematic is site-specific accidental spillage of hydraulic fracturing fluid and produced water. so far, a significant majority of groundwater contamination events are derived from surface-level anthropogenic routes rather than the subsurface flow from underlying shale formations. while the damage can be obvious, and much more effort is being done to prevent these accidents from occurring so frequently, the lack of data from fracking oil spills continue to leave researchers in the dark. in many of these events, the data acquired from the leakage or spillage is often very vague, and thus would lead researchers to lacking conclusions. researchers from the federal institute for geosciences and natural resources (bgr) conducted a model study for a deep shale-gas formation in the north german basin. they concluded that the probability is small that the rise of fracking fluids through the geological underground to the surface will impact shallow groundwater. landfill leachate leachate from sanitary landfills can lead to groundwater pollution. chemicals can reach into ground water through precipitation and runoff. new landfills are required to be lined with clay or another synthetic material, along with leachate to protect surrounding ground water. however, older landfills do not have these measures and