It consists of benthic chambers attached to a plastic bag partially filled with a known volume of seawater. The volume changes in the bag represent the flux of water out of or into the sediment. The main problem with seepage meters is that: the flow can be extremely variable in both time and space; as the seepage meter cover only a small area, many measurements might be needed to get reliable averages.
Chemical tracers natural and artificial tracers have been applied to evaluate groundwater discharge rates into the ocean in many ways. The most popular is the use of enriched geochemical tracers in the groundwater relative to the seawater. Therefore, small temporal and spatial variabilities tend to be smoothed out over time and space. Geophysical tracers such as conductivity and temperature can also be used to estimate groundwater discharge rates.
The electrical bulk ground conductivity or its inverse, resistivity is a measure of how much salt is present in marine sediments, which is a function of sediment porosity, pore water salinity and temperature. Permeable sediments containing seawater have high conductivity, which decreases with salinity. Thus, bulk ground conductivity profiles are very useful in mapping subsurface zones of brackish or freshwater, and can be employed to investigate the flow paths of fresh, terrestrially-derived groundwater.
Taking into account the length of the South American coastline about 30, km , specific SGD rate estimates have been performed on a few areas Table 1.
In order to compile existing SGD data in South America, we present all available studies that estimated or indicated the occurrence of SGD in the studied area Figure 2. In this study, we considered indistinctly all fresh, recirculated seawater and salt water components of SGD see section 2. As all over the world, the earlier studies of seawater-groundwater exchange in South America were largely motivated by groundwater resources issues. Estimative based on hydrogeological model has been conducted for the entire South America coast by Zektser et al.
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This approach assumes that specific hydrogeological provinces have the groundwater input to rivers similar to groundwater discharge to the ocean. Based on published data, the groundwater input to the rivers was estimated, and the expected discharge per kilometer of river with the shoreline length of each province was scaled. Then, the groundwater fluxes from each province to the ocean were provided. According to the authors, this approach may miss fluxes from zones deeper than the upper zones draining into rivers.
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Considering the heterogeneity of SGD, models are useful for investigating SGD, but they probably are not yet a substitute for direct measurements. In the coastal region of Lima Peru , the hydrogeological map given in Karakouzian et al. The SGD rate was estimated, through water balances, between 2. In , Dold 32 addressed the remediation strategy of a costal tailing deposit in Bahia de Ite Peru.
Through geochemical modeling and isotopic signatures, these authors described the element cycling in the subterranean estuary of this man-impacted area. Additionally, they mentioned that alkaline fresh groundwater from the high Andes flows through the coastal aquifer and tailings deposit toward the sea, which was further discussed by Dold et al.
Submarine Groundwater Discharge: An Overlooked Water Resource
However, a quantitative approach has not been reported. A large amount of relatively fresh groundwater was found further offshore of Suriname coast. The potential biogeochemical impact of SGD-related input was first addressed in the north coast of Bahia. The authors also showed the impact of population growth on the groundwater quality, and the eutrophication of coral reefs due to the high nutrient load through SGD.
Studies in Bahia State were expanded to its southern region, where coral reef demise has been related to the high SGD-input of nutrient. Considering the entire bay, it was found a total groundwater flux up to m 3 s -1 , which represents three times the average river discharge into the bay. Preliminary data from the coastal water of Pernambuco, characterized by the presence of coral reefs, showed activities of Rn from 1. Although further investigations are needed, this might indicate SGD toward the coral reefs, as it has been shown for the coast of Bahia.
These authors reported an advective rate between 1. Smoak et al. The normalized SGD rate reported is approximately 5. In , Godoy et al. These authors found, through multiple tracers of SGD, an advective rate of 6. In , Windom et al. Using the advective rate 3. Another study, based on numerical modeling, demonstrated that geochemical tracer techniques such as radium quartet characterize only the component of benthic discharge flux that transports the tracer.
Well along, Niencheski et al.
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Also reported by Moore, 65 the results suggest that the chemistry of the shelf waters of this region is substantially affected by SGD. Indeed, studying the hydrochemistry of the distribution of dissolved Ra, and trace elements on the entire shelf adjacent to Rio Grande do Sul State , Niencheski et al.
From a dataset of groundwater and surface water analysis, Niencheski and Windom 41 demonstrated that the Patos Lagoon Barrier Aquifer acts as a reactive reservoir that alters the land-sea flux. For example, it enhances the flux of Fe and Mn and diminishes the flux of Cu, significantly influencing adjacent coastal water chemistry. Further south in Rio Grande do Sul State In , Schmidt et al. These authors suggested instead that the rainwater infiltrating the aquifer is the main source of groundwater discharging into the ocean. Later, Attisano et al. In the same state, Dias et al. However, no quantitative estimation was provided.
In , a study related to marine shore tailing deposits and their remediation described the discharge of highly saline groundwater from the Salar de Pedernales salinity up to 56 into the Bay of Chanaral, Atacama Region. In another study of surf clam ecology, Riascos et al. According to the authors, the features of low salinity groundwater flow at Hornitos most likely correspond to SGD.
Fanjul et al. In , Marchetti and Carrillo-Rivera 33 investigated the groundwater flow to a floodplain in Argentina, and suggested that groundwater occurs in the sea, based on the topographic altitude. The main goal of the research is to understand the role of SGD inputs on the marine biogeochemistry of the region. Preliminary data of Rn and radium isotopes suggest that SGD can be an important process in particular areas of Patagonia's coastal region.
In this region, the hydraulic gradient is likely the main driving force. The compilation of South America SGD studies shows that, from the 21 investigated sites, a quantitative approach either the rate or the total flux has been conducted to only nine of them. Thus, a continent-wide estimative of SGD must be much greater. We were unable to find any indications of groundwater discharge reported for Guyana, French Guiana, Venezuela, Colombia, Ecuador, and Uruguay.
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This indicates where future effort should be concentrated to advance the current state of the art concerning the distribution and magnitude of SGD in South America. Although significant advances and insights have occurred in the preceding two decades, key questions remain:. Efforts approaching representative types of coastal aquifers, climate conditions and environmental dynamics are necessary to more completely describe SGD on regional and global scales.
We believe that international cooperative research programs may promote and improve our understanding of this important exchange process between land and sea in South America, which is recognized as a major component of the hydrological cycle. The authors would like to thank the referees for their valuable comments which helped to improve the manuscript. 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.
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