Field-scale modelling reveals dynamic groundwater flow and transport patterns in a high-energy subterranean estuary

Abstract. Subterranean estuaries (STEs) are biogeochemical reactors modifying the chemistry of salt- and freshwater as they flow through the subsurface sediments. Boundary conditions such as tides, waves, beach morphology, seasonal meteoric groundwater recharge and storm events control endmember mixing and residence time distributions within STEs. These in turn affect biogeochemical reactions and thus elemental fluxes discharging to the ocean via submarine groundwater discharge (SGD). Especially at high-energy beaches exposed to high tidal ranges and high wave energy, boundary conditions are very dynamic and likely imprint on groundwater flow and reactive transport within the STEs. A quantitative understanding of mixing processes and residence time distributions is necessary in order to adequately describe biogeochemical processes and can be achieved with the help of numerical modelling. Yet, transient field-scale modelling approaches calibrated to comprehensive observational data sets are still lacking, in particular for real-world high-energy STEs. In the present study, for the first time a density-dependent groundwater flow and transport model was developed and calibrated for a high-energy beach. The north beach of the barrier island Spiekeroog, northern Germany, thereby served as an example field site exposed to high-energy characteristic boundary conditions. The model was calibrated to a 1.5-year extensive dataset of groundwater heads, salinities, temperatures and 3H / He groundwater ages at various shore-perpendicular locations along the beach at depths down to 24 m below ground surface. The calibrated model is able to replicate the principal behaviour of the highly transient system and enabled the identification of hot spots of high temporal variability in the investigated state-variables. The dynamics in salinity are most intense at the in- and exfiltration locations of the tide-induced recirculating seawater. The groundwater age variability was largest seawards of the low tide mark as well as below the deep recirculating seawater cell at around 20–30 m depth near the dunes, where very old freshwater from the islands' freshwater lens mixes with young brackish water from the upper beach. Temperature variations were seasonal and confined to the upper 5–10 m below the beach. Computed saline SGD water fluxes varied considerable on daily and spring-neap time scales, as well as on the longer term, i.e., monthly to yearly time scales. The rather gradual, longer-term changes in flux appear to be mainly controlled by changes in spatial variability of the beach slope. The simulated groundwater age of the fresh SGD component varied between 4 and 25 years, and predominantly depended on the magnitude of saline SGD flux. Overall, the model provided important insights into the dynamics of the flow and transport processes.