Self-Potential as a Predictor of Seawater Intrusion in Coastal Groundwater Boreholes

M. T. Graham; D. J.  MacAllister; J. Vinogradov; M. D. Jackson; A. P.  Butler

doi:10.1029/2018WR022972

Self-Potential as a Predictor of Seawater Intrusion in Coastal Groundwater Boreholes

M. T. Graham (Corresponding Author), D. J. MacAllister, J. Vinogradov, M. D. Jackson, A. P. Butler

Engineering

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Abstract

Monitoring of self-potentials (SP) in the Chalk of England has shown that a consistent electrical potential gradient exists within a coastal groundwater borehole previously affected by seawater intrusion (SI) and that this gradient is absent in boreholes further inland. Furthermore, a small but characteristic reduction in this gradient was observed several days prior to SI occurring. We present results from a combined hydrodynamic and electrodynamic model, which matches the observed phenomena for the first time and sheds light on the source mechanisms for the spatial and temporal distribution of SP. The model predictions are highly sensitive to the relative contribution of electrochemical exclusion and diffusion potentials, the ‘exclusion efficiency’, in different rock strata. Geoelectric heterogeneity, largely due to marls and hardgrounds with a relatively high exclusion efficiency, was the key factor in controlling the magnitude of the modelled SP gradient ahead of the saline front and its evolution prior to breakthrough. The model results suggest that, where sufficient geoelectric heterogeneity exists, borehole SP may be used as an early warning mechanism for SI.

Original language	English
Pages (from-to)	6055-6071
Journal	Water Resources Research
Volume	54
Issue number	9
Early online date	6 Sept 2018
DOIs	https://doi.org/10.1029/2018WR022972
Publication status	Published - Sept 2018

Bibliographical note

This work was supported by the Natural Environment Research Council in the UK, as part of the Science and Solutions for a Changing Planet Doctor Training Partnership, run by the Grantham Institute for Climate Change at Imperial College London. We thank Southern Water for access to the boreholes at Saltdean and Balsdean. We thank Southern Water and Atkins Global for funding the installation of the equipment. We also thank Dr Amadi Ijioma for providing a prototype of the electrodynamic modelling code in MATLAB, which has since been adapted for use in a coastal chalk aquifer. Three anonymous reviewers are thanked for their comments, which greatly helped to improve the manuscript. The data used in this paper are in the tables, figures and cited information. The authors have no conflicts of interest to declare.

Keywords

self-potentials
seawater intrusion
coastal aquifiers
modeling

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10.1029/2018WR022972Licence: CC BY

Self‐Potential as a Predictor of Seawater Intrusion in Coastal Groundwater Boreholes
©2018. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. https://creativecommons.org/licenses/by/4.0/
Final published version, 3.87 MBLicence: CC BY
Early Online: Self‐Potential as a Predictor of Seawater Intrusion in Coastal Groundwater Boreholes
©2018. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. https://creativecommons.org/licenses/by/4.0/
Final published version, 3.28 MBLicence: CC BY

Cite this

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title = "Self-Potential as a Predictor of Seawater Intrusion in Coastal Groundwater Boreholes",

abstract = "Monitoring of self-potentials (SP) in the Chalk of England has shown that a consistent electrical potential gradient exists within a coastal groundwater borehole previously affected by seawater intrusion (SI) and that this gradient is absent in boreholes further inland. Furthermore, a small but characteristic reduction in this gradient was observed several days prior to SI occurring. We present results from a combined hydrodynamic and electrodynamic model, which matches the observed phenomena for the first time and sheds light on the source mechanisms for the spatial and temporal distribution of SP. The model predictions are highly sensitive to the relative contribution of electrochemical exclusion and diffusion potentials, the {\textquoteleft}exclusion efficiency{\textquoteright}, in different rock strata. Geoelectric heterogeneity, largely due to marls and hardgrounds with a relatively high exclusion efficiency, was the key factor in controlling the magnitude of the modelled SP gradient ahead of the saline front and its evolution prior to breakthrough. The model results suggest that, where sufficient geoelectric heterogeneity exists, borehole SP may be used as an early warning mechanism for SI.",

keywords = "self-potentials, seawater intrusion, coastal aquifiers, modeling",

author = "Graham, {M. T.} and MacAllister, {D. J.} and J. Vinogradov and Jackson, {M. D.} and Butler, {A. P.}",

note = "This work was supported by the Natural Environment Research Council in the UK, as part of the Science and Solutions for a Changing Planet Doctor Training Partnership, run by the Grantham Institute for Climate Change at Imperial College London. We thank Southern Water for access to the boreholes at Saltdean and Balsdean. We thank Southern Water and Atkins Global for funding the installation of the equipment. We also thank Dr Amadi Ijioma for providing a prototype of the electrodynamic modelling code in MATLAB, which has since been adapted for use in a coastal chalk aquifer. Three anonymous reviewers are thanked for their comments, which greatly helped to improve the manuscript. The data used in this paper are in the tables, figures and cited information. The authors have no conflicts of interest to declare.",

year = "2018",

month = sep,

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AU - Jackson, M. D.

AU - Butler, A. P.

N1 - This work was supported by the Natural Environment Research Council in the UK, as part of the Science and Solutions for a Changing Planet Doctor Training Partnership, run by the Grantham Institute for Climate Change at Imperial College London. We thank Southern Water for access to the boreholes at Saltdean and Balsdean. We thank Southern Water and Atkins Global for funding the installation of the equipment. We also thank Dr Amadi Ijioma for providing a prototype of the electrodynamic modelling code in MATLAB, which has since been adapted for use in a coastal chalk aquifer. Three anonymous reviewers are thanked for their comments, which greatly helped to improve the manuscript. The data used in this paper are in the tables, figures and cited information. The authors have no conflicts of interest to declare.

PY - 2018/9

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N2 - Monitoring of self-potentials (SP) in the Chalk of England has shown that a consistent electrical potential gradient exists within a coastal groundwater borehole previously affected by seawater intrusion (SI) and that this gradient is absent in boreholes further inland. Furthermore, a small but characteristic reduction in this gradient was observed several days prior to SI occurring. We present results from a combined hydrodynamic and electrodynamic model, which matches the observed phenomena for the first time and sheds light on the source mechanisms for the spatial and temporal distribution of SP. The model predictions are highly sensitive to the relative contribution of electrochemical exclusion and diffusion potentials, the ‘exclusion efficiency’, in different rock strata. Geoelectric heterogeneity, largely due to marls and hardgrounds with a relatively high exclusion efficiency, was the key factor in controlling the magnitude of the modelled SP gradient ahead of the saline front and its evolution prior to breakthrough. The model results suggest that, where sufficient geoelectric heterogeneity exists, borehole SP may be used as an early warning mechanism for SI.

AB - Monitoring of self-potentials (SP) in the Chalk of England has shown that a consistent electrical potential gradient exists within a coastal groundwater borehole previously affected by seawater intrusion (SI) and that this gradient is absent in boreholes further inland. Furthermore, a small but characteristic reduction in this gradient was observed several days prior to SI occurring. We present results from a combined hydrodynamic and electrodynamic model, which matches the observed phenomena for the first time and sheds light on the source mechanisms for the spatial and temporal distribution of SP. The model predictions are highly sensitive to the relative contribution of electrochemical exclusion and diffusion potentials, the ‘exclusion efficiency’, in different rock strata. Geoelectric heterogeneity, largely due to marls and hardgrounds with a relatively high exclusion efficiency, was the key factor in controlling the magnitude of the modelled SP gradient ahead of the saline front and its evolution prior to breakthrough. The model results suggest that, where sufficient geoelectric heterogeneity exists, borehole SP may be used as an early warning mechanism for SI.

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DO - 10.1029/2018WR022972

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JO - Water Resources Research

JF - Water Resources Research

IS - 9

ER -

Self-Potential as a Predictor of Seawater Intrusion in Coastal Groundwater Boreholes

Abstract

Bibliographical note

Keywords

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