Groundwater quality may be impacted by climate change, which needs to be taken into account in groundwater assessments. A well-known mechanism is through rise in sea levels and its impacts on coastal groundwater resources through coastal flooding and/or accelerated seawater intrusion. This may be exacerbated through increased pumping in coastal areas and by concomitant land subsidence. The combination of higher sea levels and more intense weather systems under future climate makes lower lying coastal regions more susceptible to episodic flooding/inundation, storm surges, tsunamis, and salinization. Certain regions, such as deltaic settings and smaller low-lying islands with naturally thin freshwater lenses underground, are particularly vulnerable.
Other impacts may be due to changes in land use that are brought about, in part, as a response to changes in climate. Examples include the intensification or expansion of agriculture, and the associated increased use of fertilizers and plant protection products (e.g. pesticides). Both of these changes can be driven by changes in climate, bringing new and different pests as well as putting more pressure on existing agricultural land. One of the drivers for urban migration is climate change, and the increases in population may lead to increased urban groundwater contamination in some regions.
Intensification of seasonal rainfall, resulting in increased flooding risk, is projected for many regions globally. This has the potential to impact groundwater quality in several ways. Firstly, directly through increased surface ingress of faecal and other surface-derived contaminants to shallow, more vulnerable groundwater sources such as springs and shallow hand-dug wells. Increased surface flooding may cause highly vulnerable groundwater sources to become unsafe for human consumption. Secondly, long-term changes in hydrology due to changes in rainfall intensities may render sites which are today only rarely affected by surface flooding unsuitable for water supply in the future. Thirdly, rapid recharge processes, for example via focussed recharge from ephemeral surface water bodies, through fissure flow in some basement and karstic terrains, may be intensified, and with that there is risk of increased contaminant loading to groundwater. Intensified and prolonged droughts, likewise projected under climate change, may increase the use of non-sewered sanitation in less developed or serviced areas, which can indirectly enhance the contamination load to groundwater.
Changes in global temperatures may impact on groundwater quality, e.g., changing survival times for groundwater microbes, changing physical and biochemical reactions in the subsurface linked to carbon breakdown, dissolution processes, denitrification and trace element mobility. Higher concentrations of algae and other microbial populations in surface water due to higher temperatures may likewise provide recharge water of relatively poorer quality. The character and mix of contaminants may also change with climate change, due to new requirements for materials, substances, pharmaceuticals, and personal care products. Through the processes described above the groundwater contaminant and treatment challenges of today may change and potentially intensify under projected climate change.
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