Detailed monitoring and simulation of groundwater salinity in response to extractions in coastal aquifers

High-resolution three-dimensional variable-density groundwater flow and coupled salt transport models (3D-VD-FT models) are useful instruments to support coastal groundwater management strategies and to project impacts of climate change. However, the ability of 3D-VD-FT models to provide accurate groundwater salinity predictions depends on computational capabilities, availability of sufficient and adequate high-resolution temporal and spatial data and knowledge of groundwater salinity processes in the subsurface. Current understanding in saltwater intrusion research is mainly based on theoretical, experimental and numerical studies, with intensive monitoring field studies being uncommon. In this paper, we describe a methodology that combines a high-resolution 3D-VD-FT model with an intensively monitored pilot in a coastal area in the Netherlands to improve our understanding of fresh-saline groundwater dynamics in response to multi-level groundwater extractions. We assess the applicability of 3D-VD-FT models to reproduce observed groundwater salinity changes in response to extractions. Moreover, we evaluate multiple extraction regimes of fresh and brackish groundwater. Subsequently, critical pumping rates are determined to secure fresh groundwater supply and the preventive effect of brackish groundwater extractions on saltwater intrusion is evaluated. Preliminary results show improvement of predictions on the scale of individual wells compared to previous studies conducted on similar scales. The 3D-VD-FT model captures the observed salinity trends such as downconing fresh groundwater and upconing saline groundwater, both of which occurred in response to withdrawals. However, the model’s absolute accuracy of downconing and upconing groundwater salinity still requires improvement.