Heavy metal distributions in the Gulf of Cadiz
[Raul Perianez is the lead author of this article]
Heavy metal distributions in the Gulf of Cadiz are generated chiefly by coastal riverine discharge, and thus concentrations of heavy metals are highest at sediments in the river mouths. Soluble heavy metals are also found farther from the coastal zone, and the dissolved heavy metal plumes are evident as distant as the Strait of Gibraltar. Heavy metal discharges have been documented to have occurred here since early Roman times.
Physical description of the Gulf of Cadiz
The Gulf of Cadiz (GoC) is the sub-basin of the Atlantic Ocean which is nearest to the Strait Of Gibraltar, connecting the Atlantic Ocean and the Mediterranean Sea. Its northern, southern and eastern boundaries are, respectively, the Atlantic coasts of Spain, Morocco and the Strait of Gibraltar. The western boundary is usually defined by the 9º meridian.
The continental shelf extends to water depths of some 100 to 130 metres, (m) where the shelf edge is located. Shelf width is some five kilometres (km) in the Portuguese coast, increasing towards the east to more than 30 km off the Gualdalquivir River.
The amplitude of the main semidiurnal tide is about one m over all the GoC, decreasing near the Strait of Gibraltar. Associated currents are weak, with amplitudes below 0.10 m/s over most of the GoC. The tidal current amplitude increases as approaching the Strait entrance, where currents about 0.8 m/s are produced.
The mean surface circulation in the northern GoC is characterized by a current directed to the southeast along the Spanish coast. This circulation is a rather constant pattern during most of the year. Part of the flow enters the Strait of Gibraltar and part is deflected to the south. Below the surface, the Mediterranean waters flow into the Atlantic and mainly direct to the NW.
Main rivers discharging in the GoC drain the southern Iberian Peninsula. They are the Guadiana, Odiel-Tinto and Gualdalquivir rivers. Suspended matter released from them is transported to the southeast along the Spanish coast, which is the mean current direction in this area of the GoC. As widely discussed in current literature, there is a dominant eastward transport throughout the entire northern GoC. Some authors have postulated the existence of some westward transport of sediments released by the Guadalquivir River in the inner shelf, although little indication of it has been found in other works.
Particle concentrations of the order of 10 grams/cubic meter are measured near the river mouths. Concentrations of the order of 10-1 grams/cubic meter are obtained in most part of the northern GoC and much smaller values are apparent to the south. In the deepest water layer concentrations are slightly higher than at the surface over most of the northern GoC. In some areas as the Strait of Gibraltar and the Spanish continental slope, concentrations are more significantly enhanced in the deep water. This is probably due to erosion produced by stronger tidal currents (in the area of the Strait) and also produced by the Mediterranean Water deep current.
Heavy metal sources
The freshwater inputs of rivers discharging in the GoC are relatively small. However, the Guadalquivir, Guadiana and Odiel-Tinto rivers present strongly enhanced heavy metal concentrations since they drain the Iberian Pyrite Belt, one of the most important mining areas in the south of Europe. Mineral resources have been extracted in the last 5000 years during two main periods: the Roman age and the last two centuries. During the last period, intensive exploitation has led to a relevant environmental impact, with vast surfaces covered with mining residues and subjected to erosion. On April 25, 1998, the retaining dam of a tailing reservoir at the Los Frailes mine collapsed and resulted in a very high input of heavy metals into the Guadiamar River, which discharges into the Guadalquivir River. However, six months after the incident, no evidence of the spill could be detected in the Guadalquiver River plume in the GoC.
The GoC is responsible for five to ten percent of fish and shell-fish catches of Spain and Portugal, holding important living resources of commercial and ecological interest. Consequently, it is relevant to understand the geochemistry and dispersion patterns of heavy metals in the GoC system, since it helps assessing the potential influence of metals on ecosystem functioning.
The concentrations of several metals have been measured in the fine sediment fraction (<63 μm) along the Spanish coast from the Guadiana to the Guadalquivir mouths and also at some points closer to the Strait of Gibraltar. Samples were collected at an approximate distance of 500 m from the shoreline. An example of results for three metals is presented in the following figure.
This figure illustrates the two characteristic metal distributions found in these coastal sediments. For some metals as zinc (Zn), cadmium (Cd), copper (Cu), lead (Pb), Cd, arsenic (As) and iron (Fe), maximum concentrations are obtained in the area of the Odiel-Tinto mouth. Other metals (chromium (Cr), nickel (Ni), aluminum (Al) and manganese (Mn)) behave differently and there is not a significant increase in concentrations in the Odiel-Tinto mouth area.
Metal concentrations are very low westward from the Guadiana River. There is an increase in concentrations here since, as has already been mentioned, the three rivers drain the Iberian Pyrite Belt. Maximum concentrations exist in the mouth of the Odiel-Tinto rivers for some metals. Although river flows are much smaller than those of the Guadiana and Guadalquivir, Odiel-Tinto rivers are considerably more contaminated and, indeed, they have been recognized as the main source of metals along the coast.
There is not an appreciable metal enhancement in the area of the Guadalquivir mouth (except in the case of Ni, Cr and Mn). Metal concentrations at the east of the Odiel-Tinto rivers decrease much slower than at the west of the Guadiana River, which is due to the mean currents in the shelf, which flow towards the Strait of Gibraltar. Indeed, it has already been found that coastal waters transport dissolved metals (which contaminate bed sediments as metals travel over them) from the Odiel-Tinto rivers to a distance of more than 200 km. Moreover, it has been found that these rivers constitute a source of natural radionuclides into the Mediterranean through the Strait of Gibraltar. Natural radionuclides have been introduced into the Odiel-Tinto estuary due to the releases of phosphogypsum from a fertilizer processing complex.
The analysis of metal distributions among different sediment phases indicated that Zn is the most mobile of all studied metals, showing the highest percentage in the acid-soluble fraction. On the other hand, the highest percentages of Fe, Cr, Pb and Ni are found in the residual fraction, which implies that these metals are strongly bound to sediments. For other metals as Cd, their distribution varies widely with position. Cu is the element showing the highest proportion in the oxidisable fraction, highlighting the relevant role played by organic matter.
Concentrations of dissolved metals have been measured in the northern GoC in the frame of the TOROS project in summer 1997. Dissolved metal concentrations decrease quickly with distance from the Spanish coast. The highest concentrations are obtained in the mouth of the Odiel-Tinto rivers, obviously as in the case of bed sediments. The plume of dissolved metals reaches the Strait of Gibraltar, as has been
commented above. A contour plot obtained from empirical data is presented in the figure, as an example, for the case of Zn. It may be clearly seen that, effectively, the impacts from river outflow are restricted to a narrow band along the shore. Samples were not collected in the coastal area from Cadiz to the Strait, thus the impact is apparently restricted in the experimental contours to the zone located to the north of Cadiz.
Dissolved metal concentrations deeper in the water column (not shown) are rather uniform (for instance in the range 3-5 nM for the case of Zn) since deeper waters (depth >50 m) are not affected by river discharges. Indeed, it has been found that the main core of metal enriched water extends to about the 50 m isobath. Along Spain, this isobath is typically some 20 km from the shore.
Models have been widely applied to simulate contaminant dispersion since they may provide insights on the main environmental processes governing such dispersion and, consequently, may help to describe and characterize the environment. In particular, models have been widely applied to heavy metal and radioactive element dispersion in coastal waters.
Although some interesting modeling works describing water circulation off Iberia and Morocco coasts have been published, these models have a relatively low resolution, not providing detailed information about the GoC basin circulation features. Other modeling works are specifically devoted to the study of Mediterranean water spreading.
Published models describing trace metal dispersion in the GoC consider metals as conservative tracers, no interacting with sediments and without any other sources and sinks. Analysis of model results showed that sources/sinks of metals due to interactions with sediments (adsorption/desorption reactions as well as erosion and deposition processes) were apparent.
More recently, the dispersion of heavy metals in the GoC has been studied by means of numerical modeling including, for the first time, uptake-release reactions between the dissolved phase and suspended matter and bed sediments. Computed metal distributions in water and coastal sediments could be compared with measurements, both set of data being in generally good agreement. As an example, a comparison of modeled and measured Cu concentrations in surface water may be seen in the figure.
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