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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Poster: Temperature profiles to measures groundwater discharge to Ringkøbing Fjord

Event: 33rd Nordic Geological Winter Meeting 2018
Poster by Joel Tirado-Conde, Carlos Duque, Peter Engesgaard and Sachin Karan


Abstract

Groundwater discharge or upwelling plays an important role in the ecological and hydrological dynamics in coastal areas, bringing fresh water inputs to saline water systems. However, locating it both in space and time as well as quantifying how much groundwater flows upward to coastal areas requires a big effort since these are very heterogeneous systems. Seasonal changes in rainfall, temperature and water level lead to temporal variability, while variations in the hydraulic properties and hydrological processes can generate spatial heterogeneity, making the process of measuring those fluxes complicated and requiring multiple measurements to obtain accurate results.

Furthermore, the non-steady position of the fresh water-salt water interface increases the uncertainty surrounding these processes. Using temperature as a tracer, the groundwater inputs to a surface water body can be calculated by means of solving analytically the conduction-convection equation, shortening considerably the amount of field work needed to obtain groundwater upwelling fluxes.

In this work, groundwater upwelling in the Ringkøbing Fjord coastal area was obtained using two methods: direct upward flow measurements and indirect flow calculations with shallow fjord bed temperature profiles. These two different sets of data are compared in order to assess their feasibility to map and quantify upwelling. Addressing the strengths and weaknesses of each method, we aim to better constrain the reliability of them in order to improve the quality of the data collection process.


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