Groundwater faces many threats from the effects of agricultural intensification, urbanisation, population growth and climate change. The following section provides an overview of key groups of anthropogenic contaminants, and groundwater contamination that is exacerbated by anthropogenic activities, with a global footprint.
Elevated groundwater salinity can result from a range of processes, including natural water-rock interactions and recharge in areas dominated by evaporation. However, many groundwater salinization processes are exacerbated by anthropogenic activities; these include salinization from irrigated agriculture, over-pumping mobilising geologically old saline water, seawater intrusion into coastal aquifers and hydrocarbon production. Groundwater salinization can be exacerbated by excessive irrigation and shallow groundwater levels due to salt accumulation which is subsequently leached to groundwater. In certain cases, leaching of agricultural drainage water to groundwater increases concentrations of specific ions such as sodium and magnesium with deleterious effects to crops irrigated with sodium- and magnesium-rich groundwater. This issue is intensified in arid and semi-arid regions where there is inadequate flushing of ions due to limited rainfall recharge.
Groundwater pumping may enhance the subsurface inflow of seawater, referred to as ‘coastal intrusion’ or ‘seawater intrusion’, due to over-pumping of fresh groundwater in the coastal zone. With time, this can lead to increasing salinity levels in the abstracted groundwater and, can render the groundwater unsuitable for public supply and crop irrigation. There are many examples of this process in coastal regions globally. In some settings, pumping may enhance mobilisation, typically upward, of underlying paleo-groundwater with a higher salinity, referred to as ‘upcoming’, which can also lead to increasing salinity in the abstracted groundwater.
Groundwater salinization is also linked to climate change and rising sea levels. In low rainfall areas, salt moves up from shallow groundwater to the soil and root zone. For instance, salinization may link to changes in the intensity of tidal surges and coastal flooding in low lying regions, e.g. in polders of Bangladesh, where soils and shallow groundwater may become rapidly contaminated by episodic seawater flooding. Many of the world’s most densely populated regions are coastal, and groundwater beneath these regions will continue to be impacted by coastal salinity issues. By 2060 it is projected that 1.8 billion people will live in coastal regions, with over half of these in in Asia.
Worldwide, aquifers are experiencing an increasing threat of nitrate pollution from agricultural activities, urbanization and industrial development. Nitrate (NO3-) is the most ubiquitous nonpoint source (NPS) contaminant of groundwater resources worldwide. This well documented problem is largely driven by intensive agriculture and growing global demand for food production. After fertilizer applications, surplus nitrogen (N) can rapidly move in to groundwater systems. Nitrate is highly mobile in groundwater and there is only limited potential for denitrification. Nitrate pollution is responsible for the majority of water quality exceedances in Europe and other regions where it is routinely monitored.
Because groundwater flow is usually slow there is often a significant time lag (years-decades) for pollution to become apparent in aquifer systems. As a result, the impact of N pollution in groundwater sources and rivers sustained by baseflow may be delayed for many decades relative to the time of N inputs and last for a long time. Elevated nitrate concentrations in rivers and wetlands, due to baseflow contributions from groundwater, may lead to excessive algal growth, which results in oxygen deficiency causing fish kills, toxic algal blooms and a decrease in biodiversity.
Nitrate is a common groundwater contaminant in drinking water sources and at high concentrations can cause health problems in infants and animals. This is particularly important in peri-urban areas where untreated wastewater is used for irrigation and where groundwater is pumped for drinking purposes.
- Microbiological contamination
Globally, two billion people consume water contaminated with faeces. Groundwater is often assumed free from microbiological contamination which is not necessarily the case; indeed in the USA, up to half of all groundwater supplies have shown some evidence of faecal contamination likely resulting in many cases of waterborne transmission and illness.
Bacteria, viruses and protozoa (e.g. cryptosporidium spp.) are widely detected in groundwater systems. Faecal bacteria contamination is largely assessed through the use of faecal indicator organisms, thermotolerant (faecal) coliforms (TTC), or specifically Escherichia coli. A recent review from 2017 identified that five pathogens were responsible for most outbreaks linked to groundwater use: Norovirus, Campylobacter, Shigella, Hepatitis A and Giardia. It was estimated that between 35.2 and 59.4 million cases of acute gastrointestinal illness per year globally could be attributed to the consumption of groundwater. Pollution by microbes is especially common in private household wells, since these are often shallow, poorly located and constructed, and they generally lack water treatment. Access to ‘improved’ drinking water sources, such as deeper boreholes, may provide some protection, but does not guarantee water free from faecal contamination.
A range of pathogenic microbes are found in groundwater, particularly in vulnerable shallow groundwater supplies where high detection rates are possible. On site sanitation (pit latrines) and open defecation are major sources of faecal contamination in groundwater, but there is limited evidence to suggest pit latrine density alone is a good predictor of faecal contamination in shallow groundwater supplies. In areas where there is a low sanitation coverage, other factors such as rainfall have been shown to correlate with groundwater contamination, and significant seasonal trends are evident across a range of groundwater sources.
Contamination is often driven by poorly constructed or un-maintained groundwater sources which are then vulnerable to surface ingress of enteric bacteria and viruses. There have been a number of Cholera outbreaks in recent years, and untreated vulnerable groundwater has been shown to be a potentially important risk factor in some of these (e.g., confirming earlier anecdotal links to contaminated groundwater. In contrast, deeper well-constructed sources, such as boreholes, and other improved sources provide drinking water with significantly less contamination. Recent evidence suggests that more attention needs to be paid to reducing contamination around the immediate vicinity of the well head. Bacterial contamination in groundwater may be a greater barrier to achieving targets set for improved drinking water quality under the SDG 6 than other contaminants.
The issue of anti-microbial resistance (AMR) in vulnerable groundwater systems, driven by a range of chemical and environmental stresses, is an important emerging challenge. This issue is intimately linked to other anthropogenic contaminant challenges that can lead to a cocktail of contaminants, which both facilitate microbial activity (i.e. nutrients) and stress microbes (pharmaceuticals, pesticides, etc.) leading to AMR in polluted groundwater systems.
- Manufactured organic contaminants
Numerous manufactured organic contaminants are detected in groundwater, although on average at much lower concentrations than in surface water. Some of these are more commonly monitored and regulated in groundwater, e.g. pesticides and non-aqueous phase liquids, others such as pharmaceuticals are contaminants of emerging concern for which we have little information at present. These are emitted from a wide range of point and diffuse sources and are often very challenging to detect and treat. Concentrations can be very high in groundwater near point sources, such as fuel stations or legacy industrial sites, airfields and landfills. Industrial use of fluids (e.g. fuels and solvents) can locally cause very concentrated contamination levels through spills that form non-aqueous phase zones in groundwater. These zones may persist as a source of dissolved organic groundwater contaminants for many decades.
Pesticides are a diverse and ubiquitous group of organic contaminants (including herbicides, fungicides and insecticides) has been extensively studied in groundwater. Pesticide contamination arises from both diffuse sources such as agricultural uses and point source applications in urban settings and on transport networks (e.g. herbicides used on roads, paths and railway lines). While the concentration of individual pesticide metabolites is usually low (typically <0.1 microgram per litre [μg/L]), their diversity in a sample can be large. Legacy contamination in groundwater is widely reported where more persistent pesticides, such as atrazine and its degradation products, remain at detectable concentrations in groundwater for several decades. Pesticides can degrade in the soil and groundwater, however the degradation products can still be harmful and persist in groundwater and metabolites are often detected in groundwater at higher concentrations than parent compounds. Despite regulations to control their use, which differ significantly globally, pesticides remain a persistent issue for global groundwater resources.
- Non-aqueous phase liquids
Non-aqueous phase liquids (NAPLs) are hazardous and widely occurring point source contaminants in groundwater that can be classified as either light (L) and dense (D), according to their density relative to water. For example, benzene, toluene, ethylbenzene, and xylene (BTEX) are prominent examples of LNAPLs, while chlorinated solvents and heavy crude oil are examples of DNAPLs. The leached dissolved phase, as well as vapour phase processes, are important for transport and attenuation of NAPLs in the unsaturated zone. Monitoring and treatment of soil and groundwater contaminated by NAPLS has been hugely costly to undertake, amounting to billions of dollars globally.
- Organic contaminants of emerging concern
Organic contaminants of emerging concern (CECs) are not unknown substances, but rather groundwater pollutants about which relatively little information is currently available regarding their distribution and concentrations. Their emergence is related to the advent of suitably advanced analytical methods and sampling protocols. Associated with a wide range of anthropogenic sources of contamination, this large and diverse group of contaminants (e.g. pharmaceuticals, personal care products, perfluorinated compounds, wastewater treatment products, as well as nanoparticles and plastics) remains largely unmonitored and unregulated in groundwater. These compounds are typically detected at sub g/L concentrations in groundwater. The sources and pathways of emerging contaminants in the groundwater are as various as their chemical make-up.
Microplastics have been primarily considered a surface water pollutant, although pathways to groundwater do exist, e.g. in a recent study microplastics were detected in karst groundwater. This finding is of importance because it is estimated that 25% of the world’s population rely on karst aquifers for their drinking water supply.