A geophysical investigation of saline intrusion and geological structure beneath areas of tidal coastal wetland at Langstone Harbour, Hampshire, UK

Abstract

Experiments are described that test how tide and geological structure affect saline intrusion beneath an area of coastal wetland in Hampshire, southern England. Resistivity tomography and time-dependent ground conductivity surveys were carried out at two closely located survey sites. Resultant maps and sections show clear geophysical anomalies that can be attributed to tidal saline intrusion and to the position of geological structures. Results suggest that migration of saline groundwater is, in places, out of phase with the tidal cycle, and is concentrated along sub-surface tidal channels, gravel aquifers and depressions in Chalk bedrock topography. We conclude that the seawater-fresh-water boundary in the studied wetland environment is complicated by shallow-level saline flow along geologically definable routes within the Quaternary deposits above the less permeable Chalk bedrock.

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

“…Monitoring of borehole water levels and hydrochemistry often fails to predict the timing of SI, particularly in heterogeneous aquifers, where seawater may be transported along a small number of preferential flow paths. Time‐lapse resistivity and electromagnetic surveys have been used in various studies to investigate SI (e.g., Comte & Banton, ; Fitterman, ; McDonald et al, ), although these typically require a large footprint for the long‐term installation of monitoring apparatus or repeated surveys during the predicted period of SI risk.…”

“…Monitoring of borehole water levels and hydrochemistry often fails to predict the timing of SI, particularly in heterogeneous aquifers, where seawater may be transported along a small number of preferential flow paths. Time-lapse resistivity and electromagnetic surveys have been used in various studies to investigate SI (e.g., Comte & Banton, 2007;Fitterman, 2014;McDonald et al, 1998), although these typically require a large footprint for the long-term installation of monitoring apparatus or repeated surveys during the predicted period of SI risk. This paper explores whether borehole measurements of self-potential (SP) may represent an alternative means of identifying a nearby saline front, as concentration gradients are known to generate SP, through the development of an electrochemical exclusion-diffusion potential V EED (e.g., Jackson, 2015;Jouniaux et al, 2009;Lanteri et al, 2009;Leinov & Jackson, 2014;Martínez-Pagán et al, 2010;Revil, 1999;Westermann-Clark & Christoforou, 1986).…”

Palaeogeographic Reconstruction in the Transition Zone: The Role of Geophysical Forward Modelling in Ground Investigation Surveys

“…Typical ranges of geophysical properties such as electrical resistivity for unconsolidated and consolidated materials together with their varying pore water types are given in a number of standard texts [see Table after Telford et al () and Reynolds ()]. Of particular importance to working in the transition zone, as we have already noted, is the large influence that saline saturation has on the relative resistivity of different materials (McDonald et al , ) where up to four‐orders of magnitude difference can be seen between salt‐saturated sand and dry sand. The empirical relationship presented by Archie () demonstrates that bulk resistivity (or its inverse bulk conductivity) is related to pore fluid conductivity, porosity and degree of saturation where a material contains little or no clay.…”

“…In the two studies reported here, it is the relative contrasts in conductivity (or resistivity) that are shown to be important for discrimination of subsurface geology and thus palaeogeographic reconstruction. McDonald et al () clearly demonstrated the influence of geology, and in particular the effect of porosity and permeability, on saltwater intrusion in a study of coastal wetland at Langstone Harbour, UK. Here a buried channel complex consisting of muds, sand, peats and gravels on an undulating chalk bedrock was investigated with ERT and FDEM.…”

A study of saline incursion across an inter‐tidal zone on Anglesey, Wales using airborne conductivity data

“…Additional control is also required to distinguish between fluid conductivity contributions and those due to clay component litholologies (e.g., Gabriel et al 2003). Onshore ground electrical and electromagnetic geophysical measurements related to saline conditions at coastal and wetland locations in the UK were reported by Barker (1982Barker ( , 1990 and McDonald et al (1998). Ground geophysical investigations of the coastal zone of the Welsh mainland in the vicinity of Anglesey were reported by Obikoya and Bennell (2008).…”