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Application of advanced technologies and stochastic modeling in the study of submarine groundwater discharge

Final Report Summary - SUBGRODIS (Application of advanced technologies and stochastic modeling in the study of submarine groundwater discharge)

The sea is the final destination of the contamination that takes place inland. One of the main problems that menaces estuaries and seashores is the delivery of nutrients that alters the coastal ecosystems. While surface water discharge e.g. from rivers can easily be measured and detected, measurement of groundwater discharging to the sea remains a challenging task as it is invisible and can transport high concentrations of harmful components. Submarine Groundwater Discharge (SGD) is the flow of groundwater to marine environments and it can be classified into Fresh Submarine Groundwater Discharge (FSGD) and Saline or Recirculated Submarine Groundwater Discharge (RSGD) depending on the origin and the processes that drive the flow. Both have important implications for the transport of nutrients to coastal areas, the chemical reactions triggered by the mixing of water with different properties, the ecological status of the coastal ecosystems, and even the chemical composition of the sea.

Despite of the importance of SGD, there are persistent uncertainties in studying it because of (1) the difficulties in measuring it directly (it takes place under the sea), (2) the challenge in understanding the associated chemical processes due to the interaction of multiple factors and origins of water, and (3) the problems in quantifying impacts due to the spatial variability of fluxes and chemical reactions. This project addressed all these questions by combining field methods and numerical approaches to propose new ways of quantifying SGD. The individual methods and approaches are already known, but here they have been used in new, different configurations and combinations that allow determination of patterns and trends of FSGD and RSGD. This gave new understanding of SGD and will be a helpful tool for the conceptualization of the physical, chemical and reactive processes that coalesce in the study of SGD.

SGD is controlled by the geological architecture of the subsurface. Geological heterogeneity is a fundamental issue in hydrogeology because it controls the way in which groundwater flows. The natural heterogeneity of hydraulic properties of the subsurface is often complex and therefore difficult to define and quantify accurately. A methodology using a systematic distribution of measuring points was developed to determine both trends and spatial variability of FSDG and RSGD fluxes, and use of the methodology was demonstrated at field scale. While FSDG usually is larger than RSGD, the spatial extension of each of the zones can be highly variable among study areas, affecting the bulk impact of SGD in the coastal surface waters differently. The spatial variability of fluxes must be considered for the development of numerical models using probabilistic approaches to predict SGD and its ecological implications at large scale. Probabilistic modelling is the most appropriate approach for managers to do cost-benefit analysis in connection with coastal protection decision making.

Due to the aforementioned spatial variability, a strategy for quantifying fluxes discharging into coastal areas is to use tracer measurements because they integrate processes taking place at local scale to provide estimates at larger scale. This is one of the main reasons for using radioactive tracers (222Rn,223Ra, 224Ra, 226Ra, and 228Ra) with different properties such as half-life and affinity to salt-fresh waters. These radioactive tracers are generated naturally in the sediments and therefore groundwater can be enriched in them, contrary to surface and sea water. The tracers have been applied preciously to quantify SGD, but there are uncertainties about the accuracy of these estimates and also about the circumstances for which they should be applied. In this project we demonstrated at a field site that a spatially dense network of radioactive tracer measurements allows determination of significant FSDG variability at metric scale. This shows the importance of the hydrogeological flow paths and has implications for the design of sampling campaigns at field sites to be investigated in the future. The use of the ratio of short life isotopes (223Ra and 224Ra) to long life isotopes (226Ra, 228Ra) has thus proven to be a powerful tool for understanding the hydrogeological system in coastal areas.

The discharge of subsurface continental water to estuaries and the sea has become increasingly in focus during recent years due to both its environmental and societal implications. The discharge of groundwater with high contaminant load to coastal areas induces eutrophication processes with negative environmental and economic ramifications. This project has shown by detailed hydrogeological characterization of the study area, that while nitrate coming from far inland is located in the freshwater discharge zone, ammonium, connected to the remineralization of organic matter in the bay sediments, is present in the saltwater zone. This indicates freshwater originating inland carries contaminants from human activities (farming or contaminant spills) to the sea, while the presence of ammonium indicates recirculation of seawater that infiltrates groundwater for shorter periods of time. Therefore, there are two types of water mixing near the freshwater-saltwater interface.

Our detection of large spatial variability at meter-scale indicates a high risk of failure whenever large-scale assessment studies are based on just a few punctual measurements of flux, nutrients and radioactive tracers. The contaminant load to a marine environment cannot be assessed from a concentration alone, because areas with lower concentration can actually be the major contributors of contaminant load if they are associated with high fluxes. The magnitude of fluxes can also have an impact on the extent of reactive processes since high/low fluxes can favor or hinder chemical reactions degrading or generating nutrients. This amplifies the importance of studying and understanding the details of the hydrogeological system in coastal areas.

The main conclusions of this study are that: 1) SGD should be studied keeping the hydrogeological conditions of the coastal areas in mind; 2) the distribution of salinities and fluxes can have a high impact on the applicability of radioactive tracers to estimate groundwater discharge; 3) the variability of fluxes can impact both the selection of areas with high study relevance and the chemical processes causing decomposition of harmful nutrients before they reach coastal ecosystems. In brief, spatial hydrogeological variability must be considered to properly explain and quantify SGD and its implications. Therefore use of a combination of detailed field measurements is recommended for future field studies of submarine groundwater discharge within the European Union.