The onshore influence of offshore fresh groundwater

Knight, Andrew C.; Werner, Adrian D.; Morgan, Leanne K.

doi:10.1016/j.jhydrol.2018.03.028

Cited by 34 publications

(40 citation statements)

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“…Preferential flow paths can enhance connectivity relative to homogeneous aquifers in systems with karst conduits (Evans & Lizarralde, ; Xu et al, ), palaeochannels (Mulligan et al, ), natural faults (Varma & Michael, ), and a range of other heterogeneous geologic settings (Houben et al, ; X. Li et al, ; Michael et al, ). Knight et al () analyzed 27 coastal sites globally and found that widespread losses of offshore freshwater are associated with onshore pumping due to connections between offshore and onshore fresh groundwater reserves. One example is Gippsland Australia, where offshore pumping has caused onshore resource conflicts (Figure b) and drawdowns have reached up to 50 m in 40 years (Varma & Michael, ).…”

Section: Introductionmentioning

confidence: 99%

Offshore Pumping Impacts Onshore Groundwater Resources and Land Subsidence

Michael

2019

Geophysical Research Letters

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Freshened groundwater exists offshore along many coastlines worldwide. Pumping of these offshore resources has been proposed for more efficient oil production as well as drinking and agriculture. Although pumping can occur tens to hundreds of kilometers offshore, these activities may threaten connected onshore aquifer systems. We conducted numerical simulations of variable‐density groundwater flow and salt transport in coastal aquifers with different geologic structure subject to offshore pumping to assess potential impacts. Results show that offshore pumping can diminish both onshore groundwater availability and submarine groundwater discharge and can cause widespread land subsidence. Heterogeneity increases water resource vulnerability relative to equivalent homogeneous and layered systems and exacerbates the magnitude of land subsidence. More continuous geologic structure causes propagation of maximum subsidence farther landward. This work suggests that coastal aquifers may be vulnerable to offshore pumping activities and that these effects should be considered in feasibility assessments for offshore drilling.

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Section: Introductionmentioning

confidence: 99%

Offshore Pumping Impacts Onshore Groundwater Resources and Land Subsidence

Michael

2019

Geophysical Research Letters

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“…ast offshore bodies of fresh and moderately brackish groundwater (concentration of total dissolved solids of <10 g l −1 ) have been documented up to 100 km from modern shorelines and down to 4.5 km below the seafloor (bsf) 1 . The majority of offshore freshened groundwater (OFG) is hosted in shallow (<300 m), poorly consolidated, clastic sediments in seawater depths less than 50 m [1][2][3] . Nearly all discoveries of OFG have been made along passive continental margins, mostly in the Atlantic US and European margins 4,5 .…”

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confidence: 99%

“…These include active groundwater migration across topographic gradients via present day, permeable connections between offshore and onshore aquifers 6 , recharge during Pleistocene sealevel lowstands 7,8 , sub-glacial and pro-glacial injection 9 , entrapment of connate water in subsiding basins 10 , and gas hydrate dissociation 11 . Global volumetric estimates of OFG were derived in passive margins and are on the order of 10 5 km 3 . This is two orders of magnitude greater than the volume of groundwater that has been extracted globally from continental aquifers since 1900 (refs.…”

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confidence: 99%

“…Groundwater resources are declining in terms of quantity and quality as a result of climate change, pollution and overexploitation caused by population growth and urbanisation, particularly in coastal regions and island nations 13,14 . OFG may provide a buffer to increased demand during periods of intense drought and, in some coastal areas, is already being inadvertently exploited by onshore pumping 3,15 . Industrial sectors involving seafloor engineering, carbon dioxide sequestration, and ore deposit and petroleum exploration have a direct interest in the evolution of OFG systems because these can place better constraints on past fluid migration histories 16 .…”

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3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight

et al. 2020

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Although offshore freshened groundwater (OFG) systems have been documented in numerous continental margins worldwide, their geometry, controls and emplacement dynamics remain poorly constrained. Here we integrate controlled-source electromagnetic, seismic reflection and borehole data with hydrological modelling to quantitatively characterise a previously unknown OFG system near Canterbury, New Zealand. The OFG system consists of one main, and two smaller, low salinity groundwater bodies. The main body extends up to 60 km from the coast and a seawater depth of 110 m. We attribute along-shelf variability in salinity to permeability heterogeneity due to permeable conduits and normal faults, and to recharge from rivers during sea level lowstands. A meteoric origin of the OFG and active groundwater migration from onshore are inferred. However, modelling results suggest that the majority of the OFG was emplaced via topographically-driven flow during sea level lowstands in the last 300 ka. Global volumetric estimates of OFG will be significantly revised if active margins, with steep coastal topographies like the Canterbury margin, are considered.

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“…Recent analytical and SEAWAT (Langevin et al, 2008) modeling studies underline the importance of heterogenic geological conditions (mainly presence and extension of aquitards) on potential OFGV occurrence (Van Engelen et al, 2018;Knight et al, 2018;Morgan et al, 2018;Solórzano-Rivas and Werner, 2018), turning away from earlier simulations embedding homogeneous geological conditions (Ranjan et al, 2009;Michael et al, 2013;Werner et al, 2013;Ketabchi et al, 2016) and moving toward more complex heterogeneous subsurface representation (e.g., Michael et al, 2016;Zamrsky et al, 2018;Thomas et al, 2019).…”

Section: Introductionmentioning

confidence: 99%

Geological Heterogeneity of Coastal Unconsolidated Groundwater Systems Worldwide and Its Influence on Offshore Fresh Groundwater Occurrence

et al. 2020

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Numerous coastal areas worldwide already experience fresh water shortages due to overexploitation and salt water intrusion. Future climate change and population growth will further intensify this threat in more areas in coming decades. Therefore, it is necessary to explore any potential fresh water source, such as offshore fresh groundwater, that could alleviate this fresh water shortage and provide valuable time for adaptation measures implementation and changes in water management strategies. Recent evidence suggests that a disproportionally large portion of human population living in coastal areas relies on groundwater resources stored in underlying unconsolidated groundwater systems. These systems are often very heterogeneous, combining numerous high permeability aquifers interlaid with low permeability aquitards with varying total thickness. This heterogeneity is a major control on the fresh groundwater volume and groundwater salinity distribution within such systems. Thus, the quantification of geological heterogeneity is often the limiting factor when estimating fresh groundwater volumes, both inland and offshore, along the global coastline. To overcome this obstacle, we combine conceptual geological models with available state-of-the-art global datasets to derive a set of geological heterogeneity parameter distributions quantifying geological heterogeneity of coastal unconsolidated groundwater systems (CUGSs) as formed over last 1 Ma. These are then used in an algorithm designed to build synthetic heterogenic parameterizations of CUGSs along the global coastline. These, in turn, provide key input for modeling variabledensity groundwater flow and coupled salt transport to analyze changes in groundwater salinities and offshore fresh groundwater volume (OFGV). Such an analysis is performed over one full glacial-interglacial cycle (the last 0.13 Ma) to account for oscillating sealevel conditions and shifts in coast-line positions and salinity incursions. Our simulation results show a close match between the modeling scenarios and values presented by literature sources demonstrating the potential of the hereby presented methodology to be applied in similar future studies.

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The onshore influence of offshore fresh groundwater

Abstract: Thus, offshore freshwater should be assessed in coastal water balances presuming that it serves as an existing freshwater input, rather than as a new potential freshwater resource.

Cited by 34 publications

References 58 publications

Offshore Pumping Impacts Onshore Groundwater Resources and Land Subsidence

Offshore Pumping Impacts Onshore Groundwater Resources and Land Subsidence

3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight

Geological Heterogeneity of Coastal Unconsolidated Groundwater Systems Worldwide and Its Influence on Offshore Fresh Groundwater Occurrence

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