Earth:Offshore freshened groundwater

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Short description: Water located in the sub-seafloor
Fig. 1 Global map of water stress and distribution of OFG system, thickness and minimum salinity values. Square symbols represent area where OFG thickness is unknown

Offshore freshened groundwater (OFG) is water that contains a Total Dissolved Solid (TDS) concentration lower than sea water, and which is hosted in porous sediments and rocks located in the sub-seafloor. OFG systems have been documented all over around the world and have an estimated global volume of around 1 × 106 km3.[1] Their study is important because they may represent an unconventional source of potable water for human populations living near the coast, especially in areas where groundwater resources are scarce or facing stress[2]

Elements and processes

OFG usually presents salinity values < 33 Practical Salinity Units (PSU). They are located at water depth < 100 m and within 55 km of the coast in both siliciclastic and carbonatic aquifers along active and passive margins. OFG systems are usually composed by multiple OFG bodies which are altogether < 2 km thick (Fig.1)

The principal emplacement mechanisms for OFG systems are (from the most common to the least common):

  • Meteoric recharge by rainfall which can be either a paleo-meteoric event during sea level low stands or an active-meteoric recharge via permeable connections between offshore and onshore aquifers (Fig. 2).[1][3][4][5][6][7]
  • Diagenesis due to post‐sedimentary alteration processes leading the release of freshwater and accumulation in deeply buried marine sediments in high Pressure and Temperature conditions .[8][9][10]
  • Sub‐glacial and pro‐glacial injection such as sub-glacial melting, sub-glacial drainage systems, reversal of groundwater flow direction with respect to modern flow patterns.[11][12][13][14]
  • Decomposition of gas hydrates as a result of changing in temperatures or pressures which lead to the release of low salinity pore water.[15][16][17][18]

The geological settings have a major control on OFG development: the majority are hosted in coarser siliciclastic materials, with porosity values around 30% to 60%, constraint by a permeability contrast (predominantly sand to clay).[19] Topographic gradients have a major impact on OFG emplacement[1] as topography-driven flow is one of the most important mechanisms controlling discharge of freshwater offshore.

Fig. 2 Schematic figure showing how freshened groundwater was deposited offshore when the seafloor was exposed at lower sea-levels. Credit: MARCAN project

Investigation

Different methods can be used to characterize and assess OFG occurrences:

  • Drilling, coring and wireline logging methods lead to characterized both sediments (e.g. granulometry and hydraulic properties) and pore water via geochemical analysis (e.g. salinity and chloride concentrations). Resistivity, porosity, density, sonic velocities, gamma ray content, temperature, and flow meter measurements can be then determined via in-situ measurements.
  • Reflection seismic methods provide indirect constraints on heterogeneities controlling OFG distribution.[20] Electromagnetic (EM) surveying, usually collected using controlled source electromagnetic (CSEM) systems, is used to discriminate between saturated regions with saline water (less resistive) from those containing fresh groundwater (more resistive)[21] (Fig. 3).
  • Numerical modelling approaches can lead to quantifying OFG emplacement in continental shelf environments over geologic time scales[22][23][24][25]

Applications and potential of OFG

OFG systems are receiving increasing attention as they may be used as an unconventional source of potable water in coastal areas, where groundwater resources are being rapidly depleted or contaminated.[2] 60% of the global population lives in areas of water stress[26] defined as the ratio of total water withdrawals to available renewable surface and groundwater supplies (Fig.1). Climate change, rapid population growth, and urbanization have a negative impact on water stress especially in coastal communities.[27] Therefore, OFG has been proposed as an alternative source of freshwater to mitigate water scarcity and groundwater depletion in areas of water stress[26]

Fig. 3 CSEM systems in different configurations which can be used to map OFG

References

  1. ↑ 1.0 1.1 1.2 Micallef, Aaron; Person, Mark; Berndt, Christian; Bertoni, Claudia; Cohen, Denis; Dugan, Brandon; Evans, Rob; Haroon, Amir et al. (2021). "Offshore Freshened Groundwater in Continental Margins" (in en). Reviews of Geophysics 59 (1). doi:10.1029/2020RG000706. ISSN 8755-1209. Bibcode: 2021RvGeo..5900706M. https://onlinelibrary.wiley.com/doi/10.1029/2020RG000706. 
  2. ↑ 2.0 2.1 Bakken, Tor Haakon; Ruden, Fridtjof; Mangset, Lars Erik (2012-03-01). "Submarine Groundwater: A New Concept for the Supply of Drinking Water" (in en). Water Resources Management 26 (4): 1015–1026. doi:10.1007/s11269-011-9806-1. ISSN 1573-1650. https://doi.org/10.1007/s11269-011-9806-1. 
  3. ↑ Person, Mark; Dugan, Brandon; Swenson, John B.; Urbano, Lensyl; Stott, Catherine; Taylor, James; Willett, Mark (2003). "Pleistocene hydrogeology of the Atlantic continental shelf, New England". Geological Society of America Bulletin 115 (11): 1324. doi:10.1130/b25285.1. ISSN 0016-7606. Bibcode: 2003GSAB..115.1324P. https://doi.org/10.1130/B25285.1. 
  4. ↑ Kooi, H; Groen, J (2001-06-01). "Offshore continuation of coastal groundwater systems; predictions using sharp-interface approximations and variable-density flow modelling" (in en). Journal of Hydrology 246 (1): 19–35. doi:10.1016/S0022-1694(01)00354-7. ISSN 0022-1694. Bibcode: 2001JHyd..246...19K. https://www.sciencedirect.com/science/article/pii/S0022169401003547. 
  5. ↑ Michael, Holly A.; Scott, Kaileigh C.; Koneshloo, Mohammad; Yu, Xuan; Khan, Mahfuzur R.; Li, Katie (2016-10-28). "Geologic influence on groundwater salinity drives large seawater circulation through the continental shelf: Geology Drives Seawater Circulation" (in en). Geophysical Research Letters 43 (20): 10,782–10,791. doi:10.1002/2016GL070863. http://doi.wiley.com/10.1002/2016GL070863. 
  6. ↑ Post, Vincent E. A.; Vandenbohede, Alexander; Werner, Adrian D.; Maimun; Teubner, Michael D. (2013-07-01). "Groundwater ages in coastal aquifers" (in en). Advances in Water Resources 57: 1–11. doi:10.1016/j.advwatres.2013.03.011. ISSN 0309-1708. Bibcode: 2013AdWR...57....1P. https://www.sciencedirect.com/science/article/pii/S0309170813000511. 
  7. ↑ Cohen, Denis; Person, Mark; Wang, Peng; Gable, Carl W.; Hutchinson, Deborah; Marksamer, Andee; Dugan, Brandon; Kooi, Henk et al. (2010). "Origin and Extent of Fresh Paleowaters on the Atlantic Continental Shelf, USA" (in en). Ground Water 48 (1): 143–158. doi:10.1111/j.1745-6584.2009.00627.x. PMID 19754848. Bibcode: 2010GrWat..48..143C. https://onlinelibrary.wiley.com/doi/10.1111/j.1745-6584.2009.00627.x. 
  8. ↑ Ijiri, Akira; Tomioka, Naotaka; Wakaki, Shigeyuki; Masuda, Harue; Shozugawa, Katsumi; Kim, Sunghan; Khim, Boo-Keun; Murayama, Masafumi et al. (2018). "Low-Temperature Clay Mineral Dehydration Contributes to Porewater Dilution in Bering Sea Slope Subseafloor". Frontiers in Earth Science 6: 36. doi:10.3389/feart.2018.00036. ISSN 2296-6463. Bibcode: 2018FrEaS...6...36I. 
  9. ↑ Hensen, C.; Wallmann, K.; Schmidt, M.; Ranero, C.R.; Suess, E. (2004). "Fluid expulsion related to mud extrusion off Costa Rica—A window to the subducting slab". Geology 32 (3): 201. doi:10.1130/g20119.1. ISSN 0091-7613. Bibcode: 2004Geo....32..201H. https://doi.org/10.1130/G20119.1. 
  10. ↑ Bekins, Barbara; McCaffrey, Anne M.; Dreiss, Shirley J. (1994-09-10). "Influence of kinetics on the smectite to illite transition in the Barbados accretionary prism" (in en). Journal of Geophysical Research: Solid Earth 99 (B9): 18147–18158. doi:10.1029/94JB01187. Bibcode: 1994JGR....9918147B. http://doi.wiley.com/10.1029/94JB01187. 
  11. ↑ Person, Mark; McIntosh, Jennifer; Bense, Victor; Remenda, V. H. (2007). "Pleistocene hydrology of North America: The role of ice sheets in reorganizing groundwater flow systems: PLEISTOCENE HYDROGEOLOGY OF NORTH AMERICA" (in en). Reviews of Geophysics 45 (3). doi:10.1029/2006RG000206. http://doi.wiley.com/10.1029/2006RG000206. 
  12. ↑ Mulligan, Ann; Uchupi, Elazar (2003-05-13). "New interpretation of glacial history of Cape Cod may have important implications for groundwater contaminant transport" (in en). Eos, Transactions American Geophysical Union 84 (19): 177–183. doi:10.1029/2003EO190001. Bibcode: 2003EOSTr..84..177M. http://doi.wiley.com/10.1029/2003EO190001. 
  13. ↑ Lemieux, J.-M.; Sudicky, E. A.; Peltier, W. R.; Tarasov, L. (2008-02-15). "Dynamics of groundwater recharge and seepage over the Canadian landscape during the Wisconsinian glaciation" (in en). Journal of Geophysical Research 113 (F1): F01011. doi:10.1029/2007JF000838. ISSN 0148-0227. Bibcode: 2008JGRF..113.1011L. http://doi.wiley.com/10.1029/2007JF000838. 
  14. ↑ Uchupi, E.; Driscoll, N.; Ballard, R. D.; Bolmer, S. T. (2001-01-15). "Drainage of late Wisconsin glacial lakes and the morphology and late quaternary stratigraphy of the New Jersey–southern New England continental shelf and slope" (in en). Marine Geology 172 (1): 117–145. doi:10.1016/S0025-3227(00)00106-7. ISSN 0025-3227. Bibcode: 2001MGeol.172..117U. https://www.sciencedirect.com/science/article/pii/S0025322700001067. 
  15. ↑ Hesse, Reinhard (2003-04-01). "Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface: What have we learned in the past decade?" (in en). Earth-Science Reviews 61 (1): 149–179. doi:10.1016/S0012-8252(02)00117-4. ISSN 0012-8252. Bibcode: 2003ESRv...61..149H. https://www.sciencedirect.com/science/article/pii/S0012825202001174. 
  16. ↑ Lin, In-Tian; Wang, Chung-Ho; You, Chen-Feng; Lin, Saulwood; Huang, Kuo-Fang; Chen, Yue-Gau (2010-10-01). "Deep submarine groundwater discharge indicated by tracers of oxygen, strontium isotopes and barium content in the Pingtung coastal zone, southern Taiwan" (in en). Marine Chemistry 122 (1): 51–58. doi:10.1016/j.marchem.2010.08.007. ISSN 0304-4203. Bibcode: 2010MarCh.122...51L. https://www.sciencedirect.com/science/article/pii/S0304420310000976. 
  17. ↑ Malinverno, A.; Kastner, M.; Torres, M. E.; Wortmann, U. G. (2008). "Gas hydrate occurrence from pore water chlorinity and downhole logs in a transect across the northern Cascadia margin (Integrated Ocean Drilling Program Expedition 311): GAS HYDRATE ACROSS THE CASCADIA MARGIN" (in en). Journal of Geophysical Research: Solid Earth 113 (B8). doi:10.1029/2008JB005702. http://doi.wiley.com/10.1029/2008JB005702. 
  18. ↑ Mora, Germán (2005-08-30). "Isotope-tracking of pore water freshening in the fore-arc basin of the Japan Trench" (in en). Marine Geology 219 (2): 71–79. doi:10.1016/j.margeo.2005.06.020. ISSN 0025-3227. Bibcode: 2005MGeol.219...71M. https://www.sciencedirect.com/science/article/pii/S0025322705002082. 
  19. ↑ Micallef, Aaron (2020-11-05), "Offshore freshened groundwater", Global database of offshore freshened groundwater records, doi:10.5281/zenodo.4247833, https://zenodo.org/record/4247833, retrieved 2022-10-10 
  20. ↑ Bertoni, Claudia; Lofi, Johanna; Micallef, Aaron; Moe, Henning (2020). "Seismic Reflection Methods in Offshore Groundwater Research" (in en). Geosciences 10 (8): 299. doi:10.3390/geosciences10080299. ISSN 2076-3263. Bibcode: 2020Geosc..10..299B. 
  21. ↑ Micallef, Aaron; Person, Mark; Haroon, Amir; Weymer, Bradley A.; Jegen, Marion; Schwalenberg, Katrin; Faghih, Zahra; Duan, Shuangmin et al. (2020-03-13). "3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight" (in en). Nature Communications 11 (1): 1372. doi:10.1038/s41467-020-14770-7. ISSN 2041-1723. PMID 32170097. Bibcode: 2020NatCo..11.1372M. 
  22. ↑ Thomas, A. T.; Reiche, S.; Riedel, M.; Clauser, C. (2019-11-01). "The fate of submarine fresh groundwater reservoirs at the New Jersey shelf, USA" (in en). Hydrogeology Journal 27 (7): 2673–2694. doi:10.1007/s10040-019-01997-y. ISSN 1435-0157. Bibcode: 2019HydJ...27.2673T. https://doi.org/10.1007/s10040-019-01997-y. 
  23. ↑ Zamrsky, Daniel; Karssenberg, Maria E.; Cohen, Kim M.; Bierkens, Marc F. P.; Oude Essink, Gualbert H. P. (2020). "Geological Heterogeneity of Coastal Unconsolidated Groundwater Systems Worldwide and Its Influence on Offshore Fresh Groundwater Occurrence". Frontiers in Earth Science 7. doi:10.3389/feart.2019.00339. ISSN 2296-6463. 
  24. ↑ Varma, Sunil; Michael, Karsten (2012-02-01). "Impact of multi-purpose aquifer utilisation on a variable-density groundwater flow system in the Gippsland Basin, Australia" (in en). Hydrogeology Journal 20 (1): 119–134. doi:10.1007/s10040-011-0800-8. ISSN 1435-0157. Bibcode: 2012HydJ...20..119V. https://doi.org/10.1007/s10040-011-0800-8. 
  25. ↑ Siegel, Jacob; Person, Mark; Dugan, Brandon; Cohen, Denis; Lizarralde, Daniel; Gable, Carl (2014). "Influence of late Pleistocene glaciations on the hydrogeology of the continental shelf offshore Massachusetts, USA" (in en). Geochemistry, Geophysics, Geosystems 15 (12): 4651–4670. doi:10.1002/2014GC005569. Bibcode: 2014GGG....15.4651S. http://doi.wiley.com/10.1002/2014GC005569. 
  26. ↑ 26.0 26.1 Damania et al., (2017). Uncharted waters: The new economics of water scarcity and variability. Washington, D.C.: World Bank. https://openknowledge.worldbank.org/bitstream/handle/10986/28096/211179v2.pdf?sequence
  27. ↑ Hofste, R. W., Kuzma, S., Walker, S., Sutanudjaja, E. H., Bierkens, M. F., Kuijper, M. J., ... & Reig, P. (2019). Aqueduct 3.0: Updated decision-relevant global water risk indicators. World Resources Institute, 1-53.



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