Abstract
AbstractFirst reported in the 1960s, offshore freshened groundwater (OFG) has now been documented in most continental margins around the world. In this review we compile a database documenting OFG occurrences and analyze it to establish the general characteristics and controlling factors. We also assess methods used to map and characterize OFG, identify major knowledge gaps, and propose strategies to address them. OFG has a global volume of 1 × 106 km3; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during Pleistocene sea level lowstands. Key factors influencing OFG distribution are topography‐driven flow, salinization via haline convection, permeability contrasts, and the continuity/connectivity of permeable and confining strata. Geochemical and stable isotope measurements of pore waters from boreholes have provided insights into OFG emplacement mechanisms, while recent advances in seismic reflection profiling, electromagnetic surveying, and numerical models have improved our understanding of OFG geometry and controls. Key knowledge gaps, such as the extent and function of OFG, and the timing of their emplacement, can be addressed by the application of isotopic age tracers, joint inversion of electromagnetic and seismic reflection data, and development of three‐dimensional hydrological models. We show that such advances, combined with site‐specific modeling, are necessary to assess the potential use of OFG as an unconventional source of water and its role in sub‐seafloor geomicrobiology.
Highlights
Offshore freshened groundwater (OFG) is water stored in pores of sediments and fractures of rocks in the sub‐seafloor and with a total dissolved solid (TDS) concentration below that of seawater
Key knowledge gaps, such as the extent and function of offshore freshened groundwater (OFG), and the timing of their emplacement, can be addressed by the application of isotopic age tracers, joint inversion of electromagnetic and seismic reflection data, and development of three‐dimensional hydrological models. We show that such advances, combined with site‐specific modeling, are necessary to assess the potential use of OFG as an unconventional source of water and its role in sub‐seafloor geomicrobiology
We update the global database of OFG records in Post, Groen, et al (2013) and analyze this in the context of recent and past studies to (i) establish the current understanding of the general characteristics, emplacement mechanisms, and controlling factors of OFG in continental margins; (ii) present the geophysical, geochemical, and modeling approaches that are currently used to detect and characterize OFG; (iii) identify the major knowledge gaps and uncertainties that exist with respect to the above and make recommendations on how these should be addressed; and (iv) explore two frontier applications of OFG research that will benefit from an improved understanding of OFG
Summary
Offshore freshened groundwater (OFG) is water stored in pores of sediments and fractures of rocks in the sub‐seafloor and with a total dissolved solid (TDS) concentration below that of seawater. We update the global database of OFG records in Post, Groen, et al (2013) and analyze this in the context of recent and past studies to (i) establish the current understanding of the general characteristics, emplacement mechanisms, and controlling factors of OFG in continental margins; (ii) present the geophysical, geochemical, and modeling approaches that are currently used to detect and characterize OFG; (iii) identify the major knowledge gaps and uncertainties that exist with respect to the above and make recommendations on how these should be addressed; and (iv) explore two frontier applications of OFG research (exploitation as a water resource and geomicrobiology) that will benefit from an improved understanding of OFG.
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