Submarine groundwater discharge and its influence on the coastal environment
Submarine groundwater discharge (SGD) may have an influence on the coastal environment. This article examines the forces behind this process and the importance of coastal zone management. In order to assess the magnitude of SGD , this process can be investigated from both the land and the sea using varies methods. Indirect indicators of SGD are also considered.
Phenomenological description and drivers
Submarine groundwater discharge (SGD) is defined as the discharging flow out of the aquifer across the sea floor . It is a process complementary to saltwater intrusion, which is defined as the invasion of seawater at the lower part of the aquifer due to the density difference between fresh groundwater and seawater. Both processes are shown schematically in Figures 1 (a & b).
If mixing between fresh groundwater and seawater wedge is weak, it can be assumed that the interface between the two areas is sharp (Figure 1a). In such cases, during the winter period, SGD consists of fresh groundwater, which flows to the sea above the saltwater wedge, and the interface is located more landward than in summer. This is due to the relatively low groundwater levels occurring in fall. In summer, the interface moves seaward due to the rise of the groundwater level occurring in spring, while, simultaneously to fresh groundwater discharge, seawater outflow takes place from the area of saltwater wedge. The seawater outflow can be greater than the fresh groundwater discharge .
In most cases, the interface between the freshwater and the saltwater is not sharp. Due to dispersion and molecular diffusion a freshwater-seawater mixing zone is built between the two areas. The mixing causes the salt water in the deeper part of the aquifer to become lighter, rise up and discharge back to the sea (Figure 1b). In this case, SGD consists of a mixture of fresh groundwater and re-circulated seawater. The re-circulation of seawater, in the near shore part of the aquifer, is enhanced by further mechanisms acting from the marine side such as tidal pumping and wave set-up. The variation of SGD in time exhibits components of small time scale (minutes, hours, days), which are due to the wave action and tides, as well as components of large time scale, which are due to the seasonal moving of the mixing zone and large-scale sea level variations.
Concerning the contribution of the fresh groundwater flow and the seawater re-circulation to the total SGD, it is known that:
- the terrestrially derived SGD ranges from 6 to 10% of the surface waters discharging into the ocean .
- in cases that re-circulation is only due to density effects and mixing, recirculated seawater can be up to 70% of the SGD  ;
- in cases that re-circulation is due to wave setup and tide, the recirculated sea water may constitute up to 96% of the SGD .
SGD can take place not only near the shore but also at great distances. The latter occurs when confined aquifers, underneath the continental shelf, have their outcrop on the sea bottom and a large distance from the shore. SGD is usually distributed over large areas. Concentrated outflows at the sea bottom occur in cases where the outcrops of alluvial aquifers are of limited area. Such outflow locations on the sea bottom sometimes form pockmarks in the fine-grained sediments of the sea floor. Particularly strong, concentrated, submarine groundwater discharges occur in case of karstic aquifers discharging into the sea (karstic submarine springs).
Importance for coastal zone management
SGD has been recognized in the last decades as a mechanism for transporting land derived pollutants to the sea . Nutrients, organics, metals and pathogens, which are dissolved in terrestrial groundwater, can induce chemical and biological effects in the near shore sea area, where groundwater discharges. These effects can be enhanced by the re-circulated seawater as it reacts with the aquifer sediment and can be enriched by the pollutants. The pollutant concentration in the near shore water and its impact on the chemistry and biology of this area depends not only on the pollutant fluxes but on the intensity of mixing and the exchange with the open ocean as well. Thus, coastal zone managers, in order to determine whether SGD in an area of interest is of actual or probable importance, need to estimate:
- the magnitude of SGD and
- the intensity of mixing in the near shore sea area and the exchange between the near shore sea and the open ocean.
Estimation of the magnitude of SGD
In addition to a large number of individual studies addressing the problem of SGD estimation, an initiative was launched by the International Atomic Energy Agency (IAEA) and UNESCO in 2000 to assess the importance of SGD and the effectiveness of prediction methodologies for coastal zone management. The results of the projects undertaken in the framework of this initiative have been presented in SCOR-LOICZ (2004) as well as in Burnett et al. (2006 ). A further comprehensive report concerning current knowledge about SGD has been presented by Gallardo and Marui (2006 ).
The methods to estimate SGD can be distinguished in:
- (a) investigations from the land side and
- (b) investigations from the sea side.
Investigations from the land side
This group invovlves the hydrogeological methods, with the most important being the water balance approach and numerical models.
In water balance approach, the estimation of SGD is based on the water balance equation at the basin scale. For extended periods (i.e. years), over which the change of water storage is negligible, precipitation, evapotranspiration and surface discharge must be accurately known in order to reliably estimate SGD. The water balance approach only provides the fresh groundwater component of SGD as the total volume of groundwater discharging to the sea during the selected time period. Thus, the results of the water balance approach are not comparable with locally measured values or with results of numerical models, which provide the distribution of SGD over the discharge area. An example of the water balance approach is the work of Sekulič and Vertačnik (1996 ), who estimated SGD in the Adriatic Sea.
Numerical models used in SGD studies are of different complexity. Groundwater models neglecting density effects, as for instance MODFLOW , have been widely used. They provide only the freshwater component of SGD and can appropriately simulate the flow in the part of the coastal aquifer upstream of the saltwater intrusion area. SGD in such simulations is the groundwater flux at the seaward model boundary, provided that there is no groundwater extraction in the area downstream of this boundary up to the coast. The collection of the data needed for the model calibration requires a considerable effort and it is time consuming particularly for the parameters varying in time such as water levels and fluxes. However, in areas, in which such data are available, numerical models represent an efficient tool for the estimation of the freshwater part of SGD.
In the last decades numerical models considering density effects (an extensive review of such models has been presented in Bear et al. 1999 ) have been applied in SGD studies. These models include the near shore part of the aquifer as well as parts of it lying below the sea bottom, where density effects are relevant, provide both the fresh groundwater discharge as well as the discharge of the seawater re-circulated in the aquifer. Their application is reasonable, only if data is available for the near shore part of the aquifer. However, particularly for the part of the aquifer below the sea floor, the collection of such data is difficult. Therefore applications of such models, as those presented by Smith and Turner (2001 ), Kaleris (2002 ), Langevin (2003 ) and Destouni and Prieto (2003 ), are limited.
Investigations from the sea side
The most important methods for investigations of SGD from the seaside are seepage meter measuring techniques and the use of geochemical tracers such as radium isotopes and radon. Both tracers are enriched in groundwater relative to seawater.
An overview of different types of seepage meters used to measure SGD at the bottom of the sea, as well as conclusions concerning their applicability in the field, have been presented by Taniguchi et al. (2003 ). Seepage meters provide local SGD values because of the variability of the hydraulic properties of the sea bottom material and the relatively small area (usually <<1m2), over which SGD is measured. Taking into consideration the variability of SGD in time, it can be concluded that in order to obtain representative values of SGD for a shore, which can be a few kilometres long, a large number of measurements at different locations and in different times must be performed. The results of seepage meter applications and comparisons with other methods have been presented by Burnett et al. (2006 ).
Radon is produced by the decay of radioactive isotopes in the sediments. Therefore, it is strongly enriched both in the freshwater part as well as in the re-circulated part of SGD relative to coastal marine waters . Thus, these two parts of SGD can hardly be distinguished if its value is estimated from radon measurements. To what extent a near shore water sample, used to measure radon activity, provides representative values for a larger part of the near shore water body, depends on the degree of mixing of SGD with the surface water.
Radium is absorbed onto the surface of the geologic materials and is immobile in fresh groundwater. When saltwater intrudes the aquifer and replaces freshwater in the pores, radium is desorbed from sediment and becomes mobile . Thus, radium is transferred to the near shore seawater primarily by the re-circulated seawater actively involved in saltwater intrusion . Since this water discharges into the sea mixed with the fresh groundwater, it is not possible, as in case of radon, to distinguish the portions of fresh groundwater and seawater in SGD.
An overview of the technologies developed in the last years for the measurement of radon and radium activities, advantages and disadvantages of the methods as well as applications for the estimation of SGD, have been presented by Burnett et al. (2006 ).
Indirect indicators of SGD
The application of the methods described above, involves considerable financial, labor and/or computational effort. Thus, before such methods are applied in coastal zone management studies, it is important to determine if the magnitude of SGD is relevant. Results of investigations presented in the literature can hardly help in such assessments as they vary widely. High values (above 100 cm/day) as well as low values (below 5 cm/day) have been observed . A way to determine the importance of SGD in an area, is to use indirect indicators , for example:
- the groundwater escaping from the sea bottom might be coloured by tiny gas bubbles;
- surrounding sediment might be stained red by the oxidation of iron;
- there are cold-water anomalies in the open water during the summer and warm-water anomalies in winter as well as salinity anomalies;
- the levels of radium, radon, methane, hydrogen sulphide or carbon dioxide might be elevated.
Mixing in the near shore sea
The concentration of pollutants, transferred in the near shore sea area by SGD, depends on the intensity of mixing in this area, which is due to hydrodynamic circulation, transport and dispersion. The estimation of the pollutant concentration in the near shore sea using numerical models, which simulate the forementioned processes, is computationally intensive and requires data, which in most cases is not available. A more convenient approach is to use cell models (well mixed reservoir models).
An application of a cell model is given by Uchiyama et al. (2000 ), who investigated the nutrient concentration in the near shore sea due to SGD. They found that the long-term pollutant concentration depends on the relationship between the pollutant fluxes due to SGD and the exchange flux between the near shore sea and the open ocean. For the conditions of the investigated case, these authors found that the volumetric exchange flux between the near shore sea and the open ocean is three orders of magnitude larger than the SGD flux. Thus, the long term pollutant concentrations, resulting under the assumption that the pollution of the open ocean water is negligible, were found to be of minor importance for the near shore sea environment.
For estimations of the exchange of water between the near shore sea (surf zone) and the offshore zone, a relatively simple model presented by Inman et al. (1971 ) can be used. The data required are the wave and beach characteristics (breaker height, breaker angle, beach slope in the surf zone, beach slope offshore). For open beaches, typical wave and beach characteristics, , it is probable that the water exchange between the surf zone and the offshore strongly dilutes pollutants transported in the surf zone by SGD, provided that SGD is of the order of magnitude determined in the existing studies, which are reported in Burnett et al. (2006 ) and Gallardo & Marui (2006 ). However, in cases of closed bays or near shore seas with weak wave action and circulation, it can be shown using cell models, that the long term pollutant concentration in the near shore environment, which is due to the impact of SGD, is not negligible. In such cases managerial measures, which could reduce the pollutant load of groundwater, are required.
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