Impact of Sea‐Level Rise on Sea Water Intrusion in Coastal Aquifers

TLDR

Previous studies on sea‑level rise effects on coastal aquifers have been site‑specific, and a generalized systematic analysis of sea‑water intrusion response has not been reported. This study uses a simple conceptual framework to provide a first‑order assessment of sea‑water intrusion changes in coastal unconfined aquifers due to sea‑level rise. The authors test two conceptual models—flux‑controlled systems with persistent groundwater discharge to the sea, and head‑controlled systems where abstractions or surface features maintain aquifer head—under steady‑state, sharp interface, homogeneous isotropic conditions with constant recharge. For constant flux conditions, sea‑water intrusion increases by no more than 50 m for a 1.5 m sea‑level rise, whereas constant head conditions can cause hundreds of meters to several kilometers of salt‑water toe migration, highlighting the critical role of inland boundary conditions and identifying hydrogeologic parameter combinations that dictate the magnitude of migration.

Abstract

Despite its purported importance, previous studies of the influence of sea-level rise on coastal aquifers have focused on specific sites, and a generalized systematic analysis of the general case of the sea water intrusion response to sea-level rise has not been reported. In this study, a simple conceptual framework is used to provide a first-order assessment of sea water intrusion changes in coastal unconfined aquifers in response to sea-level rise. Two conceptual models are tested: (1) flux-controlled systems, in which ground water discharge to the sea is persistent despite changes in sea level, and (2) head-controlled systems, whereby ground water abstractions or surface features maintain the head condition in the aquifer despite sea-level changes. The conceptualization assumes steady-state conditions, a sharp interface sea water-fresh water transition zone, homogeneous and isotropic aquifer properties, and constant recharge. In the case of constant flux conditions, the upper limit for sea water intrusion due to sea-level rise (up to 1.5 m is tested) is no greater than 50 m for typical values of recharge, hydraulic conductivity, and aquifer depth. This is in striking contrast to the constant head cases, in which the magnitude of salt water toe migration is on the order of hundreds of meters to several kilometers for the same sea-level rise. This study has highlighted the importance of inland boundary conditions on the sea-level rise impact. It identifies combinations of hydrogeologic parameters that control whether large or small salt water toe migration will occur for any given change in a hydrogeologic variable.