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Co-ordinated Research Programme on Nuclear and Isotopic Techniques for the Characterization of Submarine Groundwater Discharge (SGD) in Coastal Zones

carried out together with IAEA's Isotope Hydrology Section, and in collaboration with Intergovernmental Oceanographic Commission of UNESCO, International Hydrological Programme of UNESCO and 9 Member States laboratories

SGD has been recognised as an important pathway for material transport to the marine environment. It is important for the marine geochemical cycles of elements and can lead to environmental deterioration of coastal zones. While the major rivers inputs to the ocean are gauged and well analysed, thus allowing relatively precise estimates of fresh water and contaminants inputs to the ocean, assessing groundwater fluxes and their impacts on the near-shore marine environment is much more difficult, as there is no simple means to gauge these fluxes (and contaminants) to the sea. SGD to the world ocean may be as high as 30-50% of total river runoff in temperate and tropical forested regions, as well as in karstic regions. SGD represents therefore an important part of the continental water balance requiring more detailed investigations (Fig. 1).

The direct discharge of groundwater into the near-shore marine environment may also have significant environmental consequences because groundwater in many areas have become contaminated with a variety of substances like heavy metals, radionuclides and organic compounds. As almost all coastal zones are subject to flow of groundwater either as submarine springs or disseminated seepage, coastal areas are likely to experience environmental degradation. Transport of nutrients to coastal waters may trigger algae blooms, including harmful algae blooms, having negative impacts on the economy of coastal zones.

Figure 1 SGD in coastal zones.

Groundwater seepage is usually patchy, diffuse, temporally variable, and difficult to quantify. Specific methods have been developed for simulating seawater-freshwater interactions and seawater intrusion using salinity/temperature time variations, tide pumping, wind and wave modelling. For the estimation of SGD flux to the sea, the most frequently used method is based on seepage rates measurements, although because of seawater circulation in the coastal areas it need not give a realistic value for fresh water input into the sea via SGD.

Isotopic studies of SGD using stable (2H, 3He, 4He, 13C, 15N, 18O, 87/86Sr, etc.) as well as radioactive (3H, 14C, Ra isotopes, radon, U isotopes, etc.) isotopes have been developed recently, and studies of SGD have been carried out in several coastal regions (e.g. Atlantic coast of USA, Florida, Australia, Sicily, Brazil, etc.). New technologies have enabled to carry out stationary and spatial monitoring of radon and radon decay products. Stable and radioactive isotopes together with salinity and seepage measurements can thus provide a complex approach to SGD investigations, enabling separate estimation of fresh water as well as saline re-circulated groundwater fluxes to the sea.

The IAEA has a long tradition and wide experience in the use of isotope techniques in freshwater and seawater studies. Past SGD activities have included:

The objectives of the CRP can be summarized as:

(i) To improve the capability of the Member States for water resources and environmental management of coastal areas. (ii) To identify and integrate the application of isotope methods appropriate for detection of SGD in coastal areas. (iii) To explore application of recently developed nuclear and isotopic techniques suitable for quantitative estimation of various components of SGD. (iv) To develop a better understanding of the influence of SGD on coastal oceanographic processes and on groundwater regime for better management of groundwater resources in coastal areas. (v) To develop numerical models of SGD in coastal areas.

The CRP has been benefiting from the cooperation established between IAEA-UNESCO (both the IOC and IHP) and MS laboratories participating in the CRP:

Figure 2 Salinity vs. counting rate under the 214Bi peak (222Rn decay product) measured by the underwater gamma-ray spectrometer.

A complex approach in characterisation of SGD carried out as a pilot SGD investigation in the framework of the CRP offshore of south-eastern Sicily in 2001 and 2002, comprising of applications of radioactive and non-radioactive tracers, direct seepage measurements of SGD, a geophysical survey and a numerical modelling, have shown (i) successful applications of radioactive and stable isotopes as tracers for characterisation of groundwater discharge to the sea; (ii) an existence of two main processes affecting the hydrogeochemistry of the studied area-seawater mixing and the water-rock interaction, also influenced by anthropogenic effects (nutrients, heavy metals); (iii) SGD fluxes in the Donnalucata port basin ranging from 290 to 1000 m3/day (based on direct seepage measurements), which are comparable with the total SGD flux in the basin of the order of 1500 m3/day obtained from radon measurements; (iv) coastal SGD fluxes of the order of 5x106 m3/km day (based on Ra isotope measurements); (v) possibilities of temporal and spatial investigations of SGD by monitoring radon and its decay products in seawater (Fig. 2); (vi) possibilities of numerical modelling of SGD in coastal region as a complimentary method for studying the SGD phenomena in coastal regions.

CRP results will be published in a comprehensive forms as special issues of scientific journals. The Sicily issue is under a review process and the Brazil issue is under in preparation.

Further activities will include a sampling expedition to Mauritius planned for April 2005 and a final RCM to be held in Vienna in December 2005.