Presentation: Groundwater – surface water interactions in a lowland stream valley: thermal characterization of groundwater upwelling in a wetland

Event: AGU Fall Meeting 2019, San Francisco (USA)
Presentation by Joel Tirado-Conde, Majken Looms, Peter Engesgaard

Abstract

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 are wet partly because they receive groundwater (or drain water) 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 various 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 a UAV, 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 around two years.

By analyzing the temporal evolution of the temperature in both the wetland surface and the groundwater, we can infer potential locations of groundwater upwelling to the land surface and subsequent overland flow. This is relevant as previous studies have shown that it is a generally overlooked flow component that may have a big impact relative to base flow. Moreover, it serves as a test for the feasibility of using heat as a tracer to study groundwater – surface water exchanges in wetlands.


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

in Water 11:9 (September 2019)
by Carlos Duque, Soren Jessen, Joel Tirado-Conde, Sachin Karan and Peter Engesgaard
https://doi.org/10.3390/w11091842

Abstract

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.

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Publication: 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

in Water 11:8 (August 2019)
by Joel Tirado-Conde, Peter Engesgaard, Sachin Karan, Sascha Müller and Carlos Duque
https://doi.org/10.3390/w11081648

Abstract

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 (Ke); (2) horizontal heterogeneity in Ke; and (3) horizontal and vertical heterogeneity in Ke.

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.

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Presentation: Benchmarking the use of heat as a tracer by the use of integrated surface and subsurface hydrologic models

Event: Computational Methods in Water Resources XXII, Saint-Malo (France) June 2018
Presentation by Joel Tirado-Conde, Majken Looms and Peter Engesgaard

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Abstract

The use of integrated surface and subsurface hydrologic models (ISSHM) to understand the flow exchange processes occurring in stream-wetland areas may give important insights to researchers and water resource managers in terms of understanding where groundwater preferentially discharge to stream valleys. How to best monitor and quantify the surface water – groundwater interaction is a non-resolved issue. Several techniques have been proposed based on the use of heat as a tracer and they aim to provide robust and reliable results. An ISSHM of a stream-wetland area in central Jutland (Denmark) is developed in order to create a benchmark model that could better constrain the applicability of such techniques.


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