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

Abstract

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.

Intensively exploited Mediterranean aquifers: resilience to seawater intrusion and proximity to critical thresholds

“…These aquifers have also been studied previously by several scientists using variable-density models such as 2D-FED: NDA (Sherif and Singh, 1999), FEFLOW: NDA (Sherif et al, 2012), ICA (Yechieli et al, 2010), SUTRA: CAA (Prieto, 2005;Prieto et al, 2006). However, the present study is the first that addresses their vulnerability in direct relation to the high non-linearity of the seawater intrusion process with associated tipping points for complete aquifer intrusion (Mazi et al, 2013), in conjunction with consideration of critical seawater intrusion to different possible groundwater pumping locations (Koussis et al, 2010a, b), and comparatively so across the different aquifers.…”

“…They are also useful for screening the impact of various groundwater pumping alternatives in management scenarios (Koussis et al, 2012;Mantoglou, 2003). After developing a sound conceptual model, properly schematizing aquifer geometry, and carefully estimating dominant hydraulic parameters and forcing quantities, analytical solutions can be used for systematic analyses, elucidating the way in which various physical and management parameters impact the seawater intrusion behavior of coastal aquifers, and determining likely intrusion responses to various forcing conditions (Werner and Simmons, 2009;Ferguson and Gleason, 2012;Mazi et al, 2013).…”

“…To compare our results to those obtained with variable-density models, we assigned to the sharp interface the 50 % salinity line (17 500 ppm) of the transition zone, as suggested by Reilly and Goodman (1987) and generally adopted. With the use of this generalized model and approach, Koussis et al (2012) have identified important criteria useful in the management of an aquifer, and Mazi et al (2013) have shown the aquifer's highly non-linear response to seawater intrusion and associated tipping points that should not be crossed in order to avoid rapid loss of control and complete seawater intrusion.…”

“…However, aquifers in general and especially coastal aquifers are often exploited so intensively that their natural hydrological regime is strongly disturbed and may be thrown out of balance (Custodio, 2010). In particular, changes of seawater intrusion are highly non-linear and exhibit important thresholds, or tipping points, beyond which full seawater intrusion into a coastal aquifer may occur in response to even small sea level and/or groundwater management changes (Mazi et al, 2013). The possibility of such acceleration into full seawater intrusion is a major concern for the resilience and sustainability of coastal populations that depend on groundwater for their water supply.…”

“…In this paper, we study three coastal aquifers in the southeastern Mediterranean region that are already suffering seawater intrusion, with the aim to determine how close these aquifers are to reaching critical seawater intrusion extents in relation to different possible groundwater pumping locations (Koussis et al, 2010a, b) and/or to tipping points for complete seawater intrusion in the aquifer (Mazi et al, 2013). The studied aquifers ( Fig.…”