Satellite data interpretation of causes and controls on groundwater-seawater flow directions, Merseyside, UK: implications for assessing saline intrusions

Abstract. Groundwater in the Triassic Sherwood Sandstone aquifer, Liverpool, UK, has locally elevated chloride concentrations (~4000 mg/l) in parts of the coastal region although there is freshwater right up to the coast line in other areas. The aquifer is cut my numerous faults with vertical displacements of as much 300 m. SPOT satellite data have been used for the Merseyside area of Liverpool. The satellite data revealed and confirmed the location of some of the main faults since the fault zones of the aquifer have low permeability (due to grain crushing, cataclasis, and clay smearing). Where fault zones outcrop at the surface, below the well-developed regolith, there is locally elevated soil water and thus anomalous vegetation patterns in comparison to unfaulted and highly porous aquifer. The ability to identify fault zones by this satellite-based method strongly suggests that they are at least partially sealing, sub-vertical features in the aquifer. Digitally enhanced and processed satellite data were used to define the relative proportions of sand and clay in the near-coastal (inter-tidal) part of the Mersey estuary. Sand-dominated sediment has higher pixel values in comparison with clay deposits in the near infrared spectral region (NIR). Where open and weathered fault rocks crop out at the surface near the intertidal zone, water movement in these potential surface water conduits is limited where the intertidal zone is clay-dominated since clay will plug the conduit. Where these weathered and open fault-rocks crop out against sand-dominated parts of the coastline, fresh water outflux into the seawater has been imaged using the satellite data. Furthermore, the high and low chloride concentration parts of the aquifer are separated by major, sub-vertical fault zones and have allowed a very steep water table gradient to remain in the aquifer.