Hydrogeology. Kevin M. Hiscock

Fig. 1.10 Conceptual diagram showing hypothetical age distributions in the Earth's critical zone. The envelopes shown indicate the mixing of water with different ages at the interfaces between hydrological compartments (Sprenger et al. 2019).

      (Source: Adapted from Sprenger, M., Stumpp, C., Weiler, Met al. (2019) The demographics of water: a review of water ages in the critical zone. Reviews of Geophysics 57, DOI: 10.1029/2018RG000633.)

      1.5.3 Groundwater discharge to the oceans

The approximate breakdown of direct groundwater discharge from continents to adjacent oceans and seas was estimated by Zektser and Loaiciga (1993) as follows: Australia 24 km³ a⁻¹; Europe 153 km³ a⁻¹; Africa 236 km³ a⁻¹; Asia 328 km³ a⁻¹; the Americas 729 km³ a⁻¹; and major islands 914 km³ a⁻¹. The low contribution from the Australian continent of direct groundwater discharge, despite its relatively large territory, is attributed to the widespread occurrence of low‐permeability surface rocks that cover the continent. At the other extreme, the overall proximity of recharge areas to discharge areas is the reason why major islands of the world contribute over one‐third of the world’s direct groundwater discharge to the oceans. The largest direct groundwater flows to oceans are found in mountainous areas of tropical and humid zones and can reach 10–15 × 10⁻³ m³ s⁻¹ km⁻². The smallest direct groundwater discharge values of 0.2–0.5 × 10⁻³ m³ s⁻¹ km⁻² occur in arid and arctic regions that have unfavourable recharge and permeability conditions (Zektser and Loaiciga 1993).

      In a later study presented by Luijendijk et al. (2020), the application of a spatially resolved, density‐driven global model of coastal groundwater discharge showed that the contribution of fresh groundwater to the world’s oceans is equal to 224 (range 1.4–500) km³ a⁻¹, and accounts for approximately 0.6% (range 0.004–1.3%) of the total freshwater input and approximately 2% (range 0.003–7.7%) of the solute input of carbon, nitrogen, silica and strontium. The uncertainty ranges reported are mostly caused by the high uncertainty of the values of permeability that were used, which is on average two orders of magnitude. Additional sources of uncertainty are the representative topographic gradient of coastal watersheds, groundwater recharge, and the size of the area that contributes to coastal groundwater discharge.

      The coastal discharge of freshwater showed a high spatial variability. For an estimated 26% (0.4–39%) of the world's estuaries, 17% (0.3–31%) of salt marshes and 14% (0.1–26%) of coral reefs, the flux of terrestrial groundwater exceeds 25% of the river flux and poses a risk for pollution and eutrophication. Catchments with hotspots of coastal groundwater discharge, where coastal groundwater discharge exceeds 100 m² a⁻¹ and 25% of the river discharge, were located predominantly in areas with a steep coastal topography due to glacio‐isostatic rebound, active tectonics or volcanic activity, and in areas consisting of permeable unconsolidated sediments, carbonates 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.)