Impact of River Water Seepage on Reduction of Saltwater Volume in Offshore Coastal Aquifers

Investigation on available freshwater below the sea in offshore aquifers (i.e. part of the coastal aquifer is stretched beneath the sea) can provide a resource for ecological demand in the future. In this study, to understand the response of intruded seawater to freshwater recharge that is leaked from the riverbed, various scenarios with different aquitard length and permeability or river conductance have been introduced. SEAWAT dispersive code is used for seawater intrusion simulation and saltwater wedge toe position, mixing zone thickness and saltwater volume were estimated. The results showed that the river conductance influences the volume of freshwater in offshore aquifers significantly. Thereby, small conductance value (i.e. 0.0025 m2/d in this study) leads to a negligible impact on saltwater characteristics but as this coefficient grows up to a specific amount, more freshwater is released from the riverbed which could significantly affect saltwater characteristics (60% reduction in brackish water volume). Nonetheless, extra-large conductance (i.e. 375 m2/d in this study) cannot sharply affect seawater recession due to high-velocity magnitude of recharged freshwater and limited contact time with the saltwater wedge. Hydraulic conductivity and length of aquitard also affect the results; so that the extension of aquitard length to the dry land boundary will cause reduction in the recharged freshwater and consequently reduce the intruded saltwater especially in the vicinity of aquifer bed.