The Effects of Late Pleistocene Sea‐Level Fluctuations and Sediment Transport Processes on the Sequestration of Fresh and Brackish Water in Continental Shelf Environments

Abstract

Offshore fresh groundwater (OFG) is regarded as a significant, albeit typically non‐renewable, unconventional water resource stored within continental shelves. Many models of offshore freshwater emplacement use present‐day stratigraphy while applying Pleistocene sea‐level fluctuations. However, these static‐grid models do not account for sediment deposition or erosion, processes that could affect the volume and distribution of OFG. Here, we couple a sediment transport model (Sequence) with a variable‐density groundwater flow and solute transport model to reconstruct OFG sequestration over the last 500 kyr BP in response to late Pleistocene sea‐level changes. Our simulations incorporate the dynamic evolution of continental margin stratigraphy and associated hydrogeological processes. We examine six scenarios, including varying sea‐level fluctuations, deposition/erosion dynamics, subsidence rate, sediment flux, and offshore sediment diffusivity. Our findings demonstrate that overpressure generation due to relatively rapid sedimentation inhibits offshore freshwater emplacement. We also found that static‐grid models can significantly overestimate the volume of OFG. A second key observation is that the largest volumes of fresh groundwater are emplaced within the shallowest confined aquifer. Progressive burial of this unit by overlying confining units promotes salinization of the fresh groundwater through isolation from direct recharge and variable‐density flow effects. Our simulated OFG volumes range from 0.07 to 6.18 per km length of coastline, consistent with field‐based estimates from continental shelf margins around the world. These findings showcase the importance of incorporating sediment dynamics into coastal hydrogeologic modeling to accurately predict the OFG distribution and support sustainable water management practices.