Use of electrical resistivity tomography to reveal the shallow freshwater–saline interface in The Fens coastal groundwater, eastern England (UK)

The Fens is a region that contributes 11% of the agri-food economy from just 4% of the agricultural land covering England (UK). This region is vulnerable to soil salinisation from sea-level rise with estimated 100-year flood events projected to be observed up to every 2 years by 2100. Seawater intrusion and upwelling of saline groundwater can provide an additional pathway; however, the area’s groundwater has not been assessed and the risk is unknown. This study used data from the British Geological Survey’s stratigraphic core archive to produce the first stratigraphic map of the loosely consolidated Holocene deposits in the South Holland–Holbeach Marsh region. There is a sandy unconfined aquifer towards the coast, a semiconfined central region with a silty cap and a clay confining cap in the north region. Electrical resistivity tomography data indicate water level depths of 0.58 ± 0.37 m above mean sea level (msl) in February 2021 and 0.01 ± 0.72 m msl in August 2021. The saline–freshwater boundary was at 1.70 ± 0.82 m msl in February 2021, deepening to 2.00 ± 1.02 m msl in August 2021, but the only evidence of seasonal fluctuation was within 10 km of the coast. A potential, but unverified, freshwater lens up to 3.25 m thick may exist beneath the surface. These results suggest that freshwater–saline interface fluctuations may primarily be driven by surface hydrology and would be vulnerable to climate-change-induced future variations in factors that affect surface water