Groundwater age distribution in a highly dynamic coastal aquifer

Abstract

Besides other environmental tracers, the use of the 3H-3He method enables the estimation of modern groundwater ages. Widely used in fresh groundwater studies, the combination of tritium (3H) and its stable daughter product tritiogenic helium-3 (3Hetrit), however, has so far only been applied in a few studies to date groundwater in coastal aquifers. The coastal aquifer presented in this study is located at the very young eastern part of the North Sea barrier island Spiekeroog, the so-called ‘Ostplate’, that is characterized by a currently developing freshwater lens. Previously field and numerical modeling studies showed that the groundwater salinity distribution within this coastal aquifer is largely determined by seawater infiltration during storm tides. The main aim of this study was to characterize the groundwater age distribution in a highly dynamic coastal aquifer using the 3H-3He age dating method. In addition, a transient 2-D density-dependent flow and transport model that was calibrated to groundwater heads and salinity was utilized to assess whether measured 3H-3He ages can be reliably simulated despite the mixing of distinctly different and time-variable 3H endmembers, i.e., freshwater and seawater. The numerical calculation of the 3H-3He age distribution, thereby, was based on the simulation of the fictive tracer ‘stable tritium’ (sum of 3H and 3Hetrit) and 3H. Even though the freshwater lens is very young, both field observations and simulation results indicate notably lower ages within the freshwater lens as compared to the age of the lens itself. Simulation results show that frequent seawater infiltration, most importantly during winter storm tides, and discharging water from the freshwater lens lead to a disturbance of the vertical age stratification below the beach. A comparison with direct ages shows that the chosen 2-tracer approach improves system understanding, especially with regard to dispersion effects.