Tipping points for seawater intrusion in coastal aquifers under rising sea level

This study considers different projections of climate-driven sea-level rise and uses a recently developed, generalized analytical model to investigate the responses of sea intrusion in unconfined sloping coastal aquifers to climate-driven sea-level rise. The results show high nonlinearity in these responses, implying important thresholds, or tipping points, beyond which the responses of seawater intrusion to sea-level rise shift abruptly from a stable state of mild change responses to a new stable state of large responses to small changes that can rapidly lead to full seawater intrusion into a coastal aquifer. The identified tipping points are of three types: (a) spatial, for the particular aquifers (sections) along a coastline with depths that imply critical risk of full sea intrusion in response to even small sea-level rise; (b) temporal, for the critical sea-level rise and its timing, beyond which the change responses and the risk of complete sea intrusion in an aquifer shift abruptly from low to very high; and (c) managerial, for the critical minimum values of groundwater discharge and hydraulic head that inland water management must maintain in an aquifer in order to avoid rapid loss of control and complete sea intrusion in response to even small sea-level rise. The existence of a tipping point depends on highly variable aquifer properties and groundwater conditions, in combination with more homogeneous sea conditions. The generalized analytical model used in this study facilitates parsimonious quantification and screening of sea-intrusion risks and tipping points under such spatio-temporally different condition combinations along extended coastlines.