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The largest global source of freshwater is groundwater. The main factor that the drinkability of groundwater depends on is its concentration of dissolved organic carbon (DOC). The chemistry of water and its microbial presence is hugely dependent on the level of DOC.
In the United States (US), approximately 100,000 cancer cases are directly or indirectly associated with the presence of contaminants in drinking water.
The two main elements that are strongly linked to DOC are disinfection by-products (DBPs) and arsenic. Water treatments, such as chlorination and ozonation results in the production of harmful by-products such as 3-chloro-4-dichloromethyl-5-hydroxy-2(5 H)-furanone, brominated acetic acid, trihalomethanes (THMs), formaldehyde, and halogenated acetic acids. These by-products may be carcinogenic or genotoxic and may result in the formation of tumors.
Climatic Change and Urbanization
An increase in global temperatures can bring about a change in the rate of precipitation and evaporation, which alters biomass and surface water quantity, thereby increasing the domestic and agricultural dependence on groundwater resources.
Urbanization and rapid growth in the global population add to the reliance on groundwater. Furthermore, urbanization involves the release of DOC to groundwater through animal waste, leaking sewage, landfill leaching, fertilizer run-off, and industrial and residential waste.
Variability in Groundwater DOC
Spatial variance in the groundwater DOC concentrations and dissimilarities in DOC concentrations between countries are likely to be related to aquifer types. With reference to the US dataset, the World-wide Hydrological Mapping and Assessment Programme (WHYMAP) indicated that the concentration of DOC is dependent on the groundwater age and depth of aquifer (geological formation).This is because in deeper and older groundwaters various physical and biological processes such as oxidation processes, biodegradation and adsorption to soil and aquifer mineral surfaces result in the reduction level DOC levels. It has been observed that groundwater in younger aquifers of Cenozoic sediments contained 41% higher DOC concentrations than older Mesozoic and Paleozoic Era aquifers.
Additionally, other studies have suggested that in the case of biogeochemical hotspots, such as streams, lakes and surface waters, there are short water resting times where DOCs are rapidly produced.
Evaluating the current dataset from the Intergovernmental Panel on Climate Change (IPCC) climate projections, it was noted that changes in the concentration of DOC in groundwater are associated with temperature fluctuations along with changes in the rate and pattern of precipitation.
Furthermore, IPCC has also reported that on a normal business day of the southeastern US states, DOC concentration in hotspots of elevated groundwater increases by up to 45%, mostly due to increased temperatures in the wettest season of the year.
The rise in temperatures incites phenoloxidase activity, which brings about an increase in the surface water DOC by 5.4% which may stay all along a flow path and eventually mix with groundwater systems. This increased carbon accumulation in the groundwaters system is capable of modifying the terminal electron acceptor availability for microbial use, following which a potential enhancement in the survival rates of microbial communities is observed.
Global Impact on Underground Water
At present, 54% of the global population lives in urban areas. Considering the rapid increase in population, it has been predicted that by 2050 the world’s urban population will escalate to 66% which means that by 2030, the urban population area will reach 1.2 million km2.
This subsequently projects the very important issue of a lack of sufficient sanitation services that may significantly contaminate the groundwater systems. Countries to foresee a major urban expansion and population growth are China, India, and parts of Africa.
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In many cases groundwater DOC concentrations are reduced in the two following steps: (a) complex water treatment of groundwater and (b) adsorption process by granular activated carbon. However, these treatments are expensive. It has been recorded that the application of granular activated carbon filtration methods for a 6.6 mega gallon per day facility would increase water costs for a family of four by US$134 per year.
This article focuses on the impact of rapid changes in climate and urbanization on groundwater DOC concentrations. These impacts on groundwater DOC will not be evenly distributed. The changes in the level of DOC in groundwater stimulates microbial activity.
Furthermore, climatic changes and urbanization will not only affect the quantity of the groundwater resource but can also decrease groundwater quality and increase water treatment costs.
Researchers have primarily focused on how climate change and urbanization would bring about a change in groundwater quantities and understand the impact of climate change and urbanization on the quality of freshwater resources.
References and Further Reading
McDonough, L.K., Santos, I.R., Andersen, M.S., et al. (2020). Changes in global groundwater organic carbon driven by climate change and urbanization. Nature Communication 11, 1279.
IPCC Global Climate Change - Projections