Vadose zone oxygen dynamics control all subsurface redox reactions and play a decisive role in maintaining groundwater quality. Although drying and wetting events are common in artificial recharge, their effects on subsurface oxygen distribution are poorly documented. We monitored oxygen concentration in the unsaturated zone in a mid-scale (1 m high) laboratory soil lysimeter, which was subjected to short wetting and drying cycles that simulated a highly permeable shallow aquifer recharged by river water. Ten cycles of varying duration were performed for a period of 85 days. Measurements of oxygen in the liquid and the gas phases were recorded every 20 s using non-invasive optical fibers (PreSens). The results provided high-resolution (in time) oxygen concentration maps. The infiltration rate revealed a decreasing trend during wetting cycles associated with biological clogging. Such a decrease with time was accompanied by a depletion of O2 concentration, occurring within the first few hours of the infiltration. During drying, O2 concentrations recovered rapidly at all depths owing to air flushing, resulting in a stratified vertical profile consistent with the biological consumption of O2 along the air infiltration path. Furthermore, drying periods caused a potential recovery of the infiltration capacity while preserving the soil biological activity. Scraping also led to the recovery of the infiltration capacity of the soil but was less effective than drying. Our experiment suggests that the small-scale heterogeneity played a key role in accurately mapping pore-scale O2 concentrations and should be considered in modeling O2 fluxes of unsaturated soils under natural or managed recharge conditions.Peer ReviewedPostprint (author's final draft

Surface water-groundwater interactions were studied in a coastal lagoon performing 180 seepage meter measurements and using heat as a tracer in 30 locations along a lagoon inlet. The direct seepage meter measurements were compared with the results from analytical solutions for the 1D heat transport equation in three different scenarios: (1) Homogeneous bulk thermal conductivity (K_e); (2) horizontal heterogeneity in K_e; and (3) horizontal and vertical heterogeneity in K_e. The proportion of fresh groundwater and saline recirculated lagoon water collected from the seepage experiment was used to infer the location of the saline wedge and its effect on both the seepage meter results and the thermal regime in the lagoon bed, conditioning the use of the thermal methods. The different scenarios provided the basis for a better understanding of the underlying processes in a coastal groundwater-discharging area, a key factor to apply the best-suited method to characterize such processes. The thermal methods were more reliable in areas with high fresh groundwater discharge than in areas with high recirculation of saline lagoon water. The seepage meter experiments highlighted the importance of geochemical water sampling to estimate the origin of the exchanged water through the lagoon bed.

Submarine groundwater discharge (SGD)—including terrestrial freshwater, density-driven flow at the saltwater–freshwater interface, and benthic exchange—can deliver nutrients to coastal areas, generating a negative effect in the quality of marine water bodies. It is recognized that water stable isotopes (¹⁸O and ²H) can be helpful tracers to identify different flow paths and origins of water. Here, we show that they can be also applied when assessing sources of nutrients to coastal areas. A field site near a lagoon (Ringkøbing Fjord, Denmark) has been monitored at a metric scale to test if stable isotopes of water can be used to achieve a better understanding of the hydrochemical processes taking place in coastal aquifers, where there is a transition from freshwater to saltwater. Results show that ¹⁸O and ²H differentiate the coastal aquifer into three zones: Freshwater, shallow, and deep saline zones, which corresponded well with zones having distinct concentrations of inorganic phosphorous. The explanation is associated with three mechanisms: (1) Differences in sediment composition, (2) chemical reactions triggered by mixing of different type of fluxes, and (3) biochemical and diffusive processes in the lagoon bed. The different behaviors of nutrients in Ringkøbing Fjord need to be considered in water quality management. PO₄ underneath the lagoon exceeds the groundwater concentration inland, thus demonstrating an intra-lagoon origin, while NO₃, higher inland due to anthropogenic activity, is denitrified in the study area before reaching the lagoon.

Wetlands are extremely dynamical systems and their behavior depends on the characteristics of the surroundings (topography, geology and vegetation, among others) as well as on meteorological and hydrological processes. Wetlands receive groundwater through diffuse upwelling and through springs. Studying upwelling is of great importance to e.g. evaluate the overall ecology or capacity to remove nitrate of the wetland system. One problem is that diffuse upwelling is difficult locate and measure. We analyze the temporal dynamics of a groundwater-fed wetland in central Jutland (Denmark) by the use of a range of thermal methods across a lowland stream valley. A monitoring system consisting of Distributed Temperature Sensing (DTS), wells with temperature depth profiles and thermal infrared (TIR) imaging on an unmanned aerial vehicle, in conjunction with hydrological and atmospheric data, provide a quasi 3D time-lapse characterization of the thermal behavior of the system, both on the ground and in the subsurface, over a period of two years. We infer potential locations of groundwater upwelling to the land surface by studying the temperature in both the wetland surface and the groundwater. Each thermal method provides different, partially overlapping estimates of the upwelling location and magnitude, highlighting the need to incorporate classic hydrological metrics to constrain the results obtained using heat as a tracer. The integration of these data indicates that temperature measurements can be used to study groundwater upwelling in stream valleys.