A conceptual model for groundwater discharge to a coastal brackish lagoon based on seepage measurements (Ringkøbing Fjord, Denmark)

Abstract

Groundwater discharge to a brackish lagoon (Ringkøbing Fjord) was quantified with seepage meters along four transects perpendicular to the shore during four seasons in 1 year. The objectives were to develop a conceptual model of the spatiotemporal variability of seepage and how landscape characteristics and saltwater intrusion affect seepage fluxes. The electrical conductivity of the groundwater was measured in vertical sediment profiles up to 3 m below the lagoon bed to assess the effects of the freshwater-saltwater interface location on flux distribution. The main differences compared with existing theoretical models are a lower discharge near the shore line (0-5 m) and two discharge peaks more offshore (15-20 and >25 m), which allowed the development of a conceptual model that is different from the classical concepts introduced for marine or lake environments and which can be representative of other similar areas. These differences are explained by the vegetation and organic material deposition in the near-shore environment reducing discharge, the presence of the saline wedge leading to upward flow, and the increase of recirculation of brackish water due to wave pumping in areas offshore most exposed to the wind. The seasonal variability in discharge is smaller than the spatial variability between and within transects along the shoreline. Based on the conceptual model, fresh water discharge in a 20-m-wide fringe was calculated to be between 66 and 388 L·day −1 ·m −1 shore line, corresponding to a difference of a factor of six due to the variations in coast morphology and local characteristics of the area. The seasonal changes were responsible for changes of only 8% to 75% depending on the location. KEYWORDS groundwater discharge to lagoon, seepage meter, spatial-temporal variability, variable salinity

Application of Stable Isotopes of Water to Study Coupled Submarine Groundwater Discharge and Nutrient Delivery

“…However, the groundwater flow with the associated nutrient input to the lagoon has not been researched. Previous groundwater-related research focused exclusively on the physical processes that determine the flow of groundwater to the lagoon [22,37], and the development of methodologies to help quantify water fluxes [38,45].…”

“…The data collection aimed to characterize the hydrogeological properties, fluxes, and hydrochemistry, including: freshwater aquifer inland (Inland wells A and B), shallow part of the aquifer underneath the lagoon (sampling points 1-30), and groundwater-lagoon water fluxes. The area was investigated in previous studies [22,38], allowing the optimization of the placement of sampling points 1-30 and the requirements to complete field work. Sampling points 1-30 extended along a transect from the shoreline up to 40 m offshore, with small differences in the water depth of the lagoon (water depth of 30 to 50 cm).…”

“…Saltwater fluxes are associated with density-driven flow [1,21]. The benthic fluxes, referred to here as the exchange between groundwater and surface water across the sea or lagoon bed [22], can be generated by different mechanisms including, among others, tides and waves [23]. Also, the mixing of water with different properties and ionic strengths can trigger multiple chemical transformations [24,25], making an accurate estimation of mass loads difficult.…”

Groundwater-Surface Water Interactions in “La Charca de Suárez” Wetlands, Spain

“…The explanation could be due to the shallow depth of the wells and the lagoons and therefore, the regional discharge of the aquifer to the coast did not impact the water level measurements neither the groundwater-surface water interactions. The low-permeability sediments deposited on the lagoon-aquifer interface could have greatly influenced seepage rates [40], as well as the presence of organic sediments observed in other locations [54,55]. These could be the causes of the low percentage of the net groundwater exchange obtained in the water balance.…”

Evaluation of Temperature Profiling and Seepage Meter Methods for Quantifying Submarine Groundwater Discharge to Coastal Lagoons: Impacts of Saltwater Intrusion and the Associated Thermal Regime

“…The current study is interested in evaluating the methods for quantifying SGD and its inherent spatial heterogeneity. Duque et al [7] recently showed that spatial heterogeneity in exchange fluxes was more important than temporal heterogeneity and highlighted the impact of a correct characterization of the spatial heterogeneity on the discharge estimations, so this study focuses on capturing SGD as a snapshot in time.…”