or volcanic rocks. The distribution of these hotspots is consistent with reported sites of high fresh groundwater discharge found in North America, Europe and East Asia. However, at many hotspots, such as Iceland and parts of South America, Africa and South Asia, and many tropical islands, coastal groundwater discharge requires further exploration. In summary, Luijendijk et al. (2020) concluded that fresh groundwater discharge is insignificant for the world’s oceans, but important for coastal ecosystems. For further discussion of groundwater discharge to the oceans, see Section 2.16.
1.5.4 Global groundwater material and elemental fluxes
As an agent of material transport to the oceans of products of weathering processes, groundwater probably represents only a small fraction of the total transport (see Table 1.2). Rivers (89% of total transport) represent an important pathway while groundwater accounts for a poorly constrained estimate of 2% of total transport in the form of dissolved materials (Garrels et al. 1975). Estimates by Zektser and Loaiciga (1993) indicated that globally the transport of salts via direct groundwater discharge is approximately 1.3 × 109 t a−1, roughly equal to half of the quantity contributed by rivers to the oceans. Given a volumetric rate of direct groundwater discharge to the oceans of 2220 km3 a−1, the average dissolved solids concentration is about 585 mg L−1. This calculation illustrates the long residence time of groundwater in the Earth's crust, where its mineral content is concentrated by dissolution.
In an analysis of comprehensive datasets of the chemistry of groundwater and produced water (groundwater pumped during oil and gas extraction) compiled by the US Geological Survey (DeSimone et al. 2014; Blondes et al. 2017), together with estimates of global groundwater usage, Stahl (2019) estimated elemental fluxes from global pumping and found that groundwater fluxes contribute appreciably to the overall cycles of a number of important elements and may provide a significant portion (more than 10%) of crop requirements of key nutrients (e.g. potassium and nitrogen) where groundwater is used for irrigation. Comparing the dissolved solute flux from groundwater pumping to the dissolved solute flux of approximately 4–5 × 109 t a−1 delivered annually to the ocean by rivers (Sen and Peucker‐Ehrenbrink 2012), Stahl (2019) calculated that total pumping with and without produced waters gives fluxes of total dissolved solids (TDS) of 881 × 106 and 513 × 106 t a−1, respectively, which represent 20 and 7% of the global dissolved solute flux carried by rivers. The fact that groundwater and produced water pumping are exclusively anthropogenic fluxes of water highlights the significance of groundwater pumping in global elemental cycles.
Table 1.2 Material transport and subsurface dissolved salts discharge from groundwater to the world's oceans (Garrels et al. 1975 and Zektser and Loaiciga 1993).
(Sources: Garrels, R.M., Mackenzie, F.T. and Hunt, C. (1975) Chemical Cycles and the Global Environment: Assessing Human Influences. Kaufman, Los Altos, California; Zektser, I.S. and Loaiciga, H.A. (1993) Groundwater fluxes in the global hydrologic cycle: past, present and future. Journal of Hydrology 144, 405–427.)
| Agent or Ocean | % of total material transport (Remarks) | % of total dissolved salts transport | Subsurface dissolved salts discharge (106 t a−1) |
|---|---|---|---|
| Surface runoff | 89 (Dissolved load 19%, suspended load 81%) | 66 | – |
| Glacier ice, coastal erosion, volcanic and wind‐blown dust | ∼9 (Ice‐ground rock debris, cliff erosion sediments, volcanic and desert‐source dust) | – | – |
| Groundwater | 2 (Dissolved salts similar to river water composition) | 34 | – |
| Pacific | – | 520.5 | |
| Atlantic | – | 427.8 | |
| Mediterranean Sea | – | 42.5 | |
| Indian | – | 295.5 | |
| Arctic | – | 7.2 | |
| All oceans | 1293.5 |
1.5.5 Human influence on the water cycle
Although rarely depicted in diagrams of the water cycle, human activity alters the water cycle in three distinct, but inter‐related ways (Abbott et al. 2019). First, humans appropriate water through: (1) livestock, crop and forestry use of soil moisture that eventually flows back to the atmosphere as evapotranspiration (so‐called ‘green water’ use with an estimated flux of 15–22 × 103 km3 a−1); (2) as surface water and groundwater abstraction (‘blue water’ use with an estimated flux of 3.8–6.0 × 103 km3 a−1); and (3) as water required to assimilate pollution (‘grey water’ use with an estimated flux of 1.0–2.0 × 103 km3 a−1) (Abbott et al. 2019; Schyns et al. 2019). Second, humans have disturbed approximately three‐quarters of the Earth's ice‐free land surface through activities that include agriculture, deforestation and wetland destruction (Ellis et al. 2010). These disturbances alter evapotranspiration, groundwater recharge, river discharge and precipitation at continental scales (Falkenmark et al. 2019). Third, climate change is disrupting patterns of water flow and storage at local to global scales (Haddeland et al. 2014). These human interferences with the water cycle have created problems for billions of individuals and many ecosystems worldwide. The estimates of human green, blue and grey water use (about 24 × 103 km3 a−1) indicates that human freshwater appropriation redistributes the equivalent of half of global river discharge or double global groundwater recharge each year (Abbott et al. 2019).
1.6 Global groundwater resources
Groundwater is an important natural resource. Worldwide, more than two billion people depend on groundwater for their daily supply (Kemper 2004). Total global fresh water use is estimated at about 4000 km3 a−1 (Margat and Andréassian 2008) with 99% of the irrigation, domestic, industrial and energy use met by abstractions from renewable sources, either surface water or groundwater. Less than 1% (currently estimated at 30 km3 a−1) is obtained from non‐renewable (fossil groundwater) sources mainly in three countries: Algeria, Libya and Saudi Arabia.
The increase in global groundwater exploitation has been stimulated by the development of low‐cost, power‐driven pumps and by individual investment for irrigation (Plate 1.5) and urban uses. Currently, aquifers supply approximately 20% of total water used globally, with this
