Abstract
AbstractMonitoring of self‐potentials (SPs) 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 modeled 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.
Highlights
Groundwater provides the main source of water for human consumption and is critically important for agriculture in many countries (WWAP, 2014)
The elevated electrical conductivity of clay minerals in the marl bands does not greatly affect the magnitude of the SP gradient, it does appear to be responsible for the deviation in observed SP adjacent to the Shoreham Marl, just below À20 mAOD
Data collected from a groundwater observation borehole in a coastal aquifer near the south coast of England show a strong and consistent SP gradient ahead of a saline front
Summary
Groundwater provides the main source of water for human consumption and is critically important for agriculture in many countries (WWAP, 2014). Groundwater demand is high in coastal areas, where population density is more than three times the global average (Small & Nicholls, 2003). Traditional approaches for characterizing SI fall into three main categories (Werner et al, 2013): borehole hydrochemistry, monitoring of groundwater levels, and geophysical investigations. Monitoring of borehole water levels and hydrochemistry often fails to predict the timing of SI, 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), these typically require a large footprint for the long-term installation of monitoring apparatus or repeated surveys during the predicted period of SI risk
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