Research ArticleGEOPHYSICS

Marine electrical imaging reveals novel freshwater transport mechanism in Hawai‘i

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Science Advances  25 Nov 2020:
Vol. 6, no. 48, eabd4866
DOI: 10.1126/sciadv.abd4866
  • Fig. 1 Study area and survey layout.

    Map of the study area parallel to the Hualalai terrestrial aquifer at Kona, offshore west of Hawai‘i. The black lines denote the survey towlines (10 inlines and 2 crosslines). White lines denote depth contours of 200 m, and gray lines denote the depth contours of 1000 m. Inset map: The island of Hawai‘i, with black rectangle indicating the main map area. Areas with no bathymetry data are shown in white. Bathymetry data: Courtesy of Hawai‘i Mapping Research Group.

  • Fig. 2 Multilayer electrical resistivity formation offshore the island of Hawai‘i.

    Fence diagram showing 2D isotropic CSEM inversion models of 20 discretized survey lines parallel to the Kona coastline and two crosslines. The color scale shows log10[ρ(ohm·m)], with blue and red colors corresponding to resistive and conductive features, respectively. Blue shaded areas starting at ∼100-m depth denote horizontal layers of resistive anomalies that represent freshened water-saturated basalts, confined by low-permeability horizons of ash/soil (black dashed lines). White lines denote the deeper boundary of these freshened horizontal layers. The spatially extensive and highly resistive area (∼1000 ohm·m) offshore Kailua-Kona represents a large-scale freshwater reservoir that extends from ∼250- to ∼500-m depth. The models derived from the data acquired by three/four surface-towed CSEM receivers at 3, 7, and 13 Hz (10 models), and 3 and 7 Hz (12 models). The models’ vertical exaggeration is approximately 16. Table S1 presents the parameterization and properties of the inversion models.

  • Fig. 3 Line 3 north inversion model.

    Model showing the electrical resistivity structure of line 3 north (see line location in Figs. 1 and 2). The color scale shows log10[ρ(ohm·m)]. This multilayer inversion model is composed of four lateral formations: two conductive seawater-saturated basalt layers intermitted by two resistive freshened water-saturated basalts. Low-permeability thin horizons of ash/soil (dashed lines) separate the conductive and resistive layers. The model was derived from the data acquired by three surface-towed CSEM receivers at 3 and 7 Hz. Inversion error floors include amplitude, 7%, and phase, 4%.

  • Fig. 4 Line 2 south inversion model.

    Model showing the electrical resistivity structure of line 2 south (see line location in Figs. 1 and 2). The color scale shows log10[ρ(ohm·m)]. The deep and laterally continuous resistive body (∼1000 ohm·m) represents a large-scale freshwater reservoir (area bounded by a white line). A low-permeability thin layer (black dashed line) separates between the upper seawater-saturated (resistivity of ∼1 to 3 ohm·m) basalts and the freshened water layer (∼5 to 50 ohm·m) situated between ∼190- to 320-m depths. A moderately resistive body (∼5 to 10 ohm·m) exists between ∼60- to 100-m depths, interpreted as an intermediate layer of freshened water. The conductive layer beneath the seafloor (∼30- to 50-m depth) indicates seawater-saturated sediment, weathered ash, and basalts. The model was derived from the data acquired by four surface-towed CSEM receivers at 3 and 7 Hz. Inversion error floors include amplitude, 7%, and phase, 4%. This inversion converged to an RMS misfit of 0.99 after eight iterations. The model-to-data fits and normalized residuals are shown in fig. S4.

  • Fig. 5 Fresh groundwater onshore-to-offshore transport mechanism.

    Illustration showing a multilayer conceptual model of the transport mechanism of fresh groundwater from onshore to offshore in Hawai‘i. Fresh groundwater recharge from rainfall infiltrates the subsurface basalts and migrates toward the coastline. Low-permeability ash/soil layers intercept and perch the downslope migration (hydrostatic head driven) of freshwater where the freshwaters are above the water table. Below the water table, the low-permeability ash/soil layers act as confining formations. The freshwaters trapped below the confining formations flow thorough permeable fractured basalts and mix with seawater to form freshened groundwater while displacing gravitationally denser seawater. At the shelf edge, the freshened groundwater flows are released to the ocean as springs. Above and below the freshened water-saturated basaltic formations, seawater-saturated basalts exist as a result of seawater intrusions from the ocean toward the land.

Supplementary Materials

  • Supplementary Materials

    Marine electrical imaging reveals novel freshwater transport mechanism in Hawai‘i

    Eric Attias, Donald Thomas, Dallas Sherman, Khaira Ismail, Steven Constable

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    • Figs. S1 to S5
    • Table S1

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