Impact of the Prestige oil spill on marsh soils: Relationship between heavy metal, sulfide and total petroleum hydrocarbon contents at the Villarrube and Lires marshes (Galicia, Spain)

Impacto de la marea negra del Prestige en suelos de marisma: relación entre los contenidos de metales pesados, sulfuros e hidrocarburos en las marismas de Villarrube y Lires (Galicia, España)

L. Andrade^1*, P. Marcet¹, L. Fernández-Feal², C. Fernández-Feal², E.F. Covelo¹ and F.A. Vega¹

¹ Department of Vegetable Biology and Soil Science Apartado 874 36200 Vigo, Spain. * E-mail: mandrade@uvigo.es

Recibido en junio de 2003;
aceptado en marzo de 2004.

Abstract

The objectives of this study were to determine the effect of the Prestige oil spill on the total petroleum hydrocarbons and heavy metal contents of soils in two marshes (Lires and Villarrube, Galicia, Spain) and the relationship between their oxidation-reduction potential and the solubility of heavy metals with sulfide and sulfate contents. Soil samples were taken from polluted and unpolluted areas and their petroleum hydrocarbon contents, heavy metal contents and other chemical characteristics were measured. The soils affected by the oil spill show remarkable contents of Cr, Cu, Ni, Pb and V. The Lires marsh soils are more affected by fuel oil than Villarrube marsh. The effects of the contaminating agents on the soils reach distances of up to 500 m from the coastline. In the first 400 m, there are important spatial variations because the fuel oil penetrated into the soils through tidal action and not directly. The Cr, Cu, Ni, Pb and V contents of polluted soils were between 50 and 200 times higher than those of their unpolluted counterparts and the background concentrations in Galician coastal sediments. In the case of Cr, Cu, Ni, Pb and V, their originthrough the fuel oil was corroborated by the high correlation (r > 0.90) between the concentrations of these metals and the total petroleum hydrocarbon content of the polluted soils, which shows the combined addition of these metals through the fuel oil.

Key words: heavy metals, hydrocarbons, oil spill, marsh soils.

Resumen

Palabras clave: metales pesados, hidrocarburos, marea negra, marismas, suelos.

Introduction

Marshes are an important part of fluvial, estuarial and coastal ecosystems. The short-term effects of fuel oil on marshes depends primarily on the vegetative responses. The adverse effects of oil on plants range from reductions in transpiration and carbon fixation to plant mortality (Pezeshki et al., 2000), and the long-term effects depend more on the response of the soil microbial community that controls the remineralization of nutrients, soil redox potential and fuel oil degradation (Nyman and Patrick, 1995). The response of the soil microbial community may be influenced by the content of heavy metals in the marsh soils.

The Villarrube (4°23'40" W, 43°38'10" N) and Lires (9°15'06" W, 43°00'08" N) marshes are located in northwestern Galicia (Spain), in the estuaries of the rivers Ferrerías (Cedeira Ría) and Castro (Lires Ría), respectively. Both marshes were considered ecosystems that had not been negatively affected by human action (Fernández-Feal, 1999; Marcet et al., 2000).

Low bulk densities, high organic matter content and high sulfide content often characterize tidal marsh soils (Griffin and Rabenhorst, 1989; Fernández-Feal, 1999; Marcet et al., 2000; Andrade et al., 2002). The organic matter accumulated by superficial deposition of marsh grass detritus and urban effluents is very important to the fate of trace metals (Griffin and Rabenhorst, 1989). The oxidation status of soils and sediments affects the distribution of some trace metals among bound, unavailable, soluble and available forms (Gambrell et al., 1991).

The combination of anaerobic conditions and high organic matter content makes the salt marsh environment ideal for bacterial reduction of sulfates to sulfides (Pons and Van Breemen, 1982). This process plays an important role in the development of marsh sediments and in the control of heavy metal solubility.

This paper reports the effects of the Prestige oil spill on the hydrocarbon and heavy metal contents of the soils of two affected estuarine environments two weeks after the spill had occurred.

One sample of emulsified fuel oil was taken from the sea and another two from each marsh. The fuel oil characterization and analytical methods are described briefly in table 1.

The soil sampling sites extended 750 m from the coastline during low tide (fig. 1). Six soil samples (Thionic Fluvisol) were taken during low tide at each marsh (samples V and L) two weeks after the spillage and immediately after cleaning the beaches, one sample every 100 m and two controls (at 750 m, control L and control V). The sampling sites were chosen to obtain a representative measure of the pollution conditions in the marshes.

Topsoil (0-30 cm) samples were collected using an Eijkelkamp sampler and the samples were stored in polyethylene bags in darkness at 4°C. Five samples of each site were taken at each sampling point. The samples from each site were air-dried, passed through a 2-mm sieve, mixed and homogenized in the laboratory. Subsequently, five subsamples from each aggregate sample were taken.

Redox potential (Eh) was measured on site, using a platinum electrode and a calomel reference electrode. Both electrodes were introduced into the topsoil to a depth of 1520 cm. Ten measurements were made at each sampling site, and the results are the average of all of them. The Eh was measured based on a standard hydrogen reference electrode.

Sulfide was determined starting from the difference between the iron extracted with hydrochloric acid and nitric acid (Tabatabai, 1982), and the adsorbed S-SO₄^-2 was extracted with a solution containing LiCl₂ 0.1M and P (500 mg L^-1) (Tabatabai, 1982). Both were analyzed by turbidimetry with barium acetate.

The available content of Cr, Cu, Ni, Pb and V was extracted by the DTPA method developed by Lindsay and Norwell (1978). Total contents were extracted by acid digestion using a mixture of concentrated nitric, hydrochloric and hydrofluoric acids (1:3:3 v/v) in Teflon reactors placed in a microwave oven (Marcet et al., 1997).

The analysis of Cr, Cu, Ni, Pb and V was carried out by ICP-OES (Perkin-Elmer Optima 4300 DV).

The efficiency of the extraction and analysis procedures was controlled by analyzing international standard reference material of marine sediment from estuaries such as MESS-3 (10 repetitions for each element) from the Marine Analytical Chemistry Standards Program of the Canadian National Research Council (Ottawa, Canada); the procedure is described in Marcet et al. (1997). MESS-3 is estuarial sediment characterized by a low average metal concentration, making the sediments an appropriate tool for the analytical control of the samples from the Galician marshes. The confidence intervals of the values obtained were similar to those certified by the Canadian National Research Council (table 2).

The data were statistically analyzed and the least significant differences (LSD), at 5% level, used to separate means. The relationship between the different variables was evaluated by a simple correlation and regression analysis (Neter et al., 1996).

Results and discussion

The heavy metal contents in the soils of both marshes are higher that the normal levels from petrogenic origin in these zones (tables 3, 4) (Marcet et al., 1997; Fernández-Feal, 1999; Carballeira et al., 2000; Marcet et al., 2000), which seems to indicate the effect of the oil spill on these soils.

Polluted soils had significantly higher values than control soils (P < 0.05) with respect to total Cr, Cu, Ni, Pb and V contents. The heavy metal concentrations found in the polluted soils also exceeded the average concentrations in unpolluted Galician coastal sediments and soils (Barreiro et al., 1988, 1994; Marcet et al., 1997; Fernández-Feal, 1999; Carballeira et al., 2000; Marcet et al., 2000) and the background levels (table 3) (Barreiro et al., 1988, 1994; Carballeira et al., 2000). The highest levels were found at Lires marsh, yielding evidence in the sense that this marsh was more affected by the fuel than the Villarube one.

The combined addition of these metals through the fuel oil seems to be corroborated by the significant positive correlation between the total heavy metal content with TPH content (r > 0.90, P < 0.01) in all cases (fig. 2), and also significant mutual correlation (r > 0.90, P < 0.01) (table 5) among total Cr, Cu, Ni, Pb and V levels (table 5).

DPTA-extractable metal contents were likewise low in relative terms (table 4). The predominance of insoluble forms is attributable to the low Eh values of these soils, and hence partly, in the case of the polluted soils, to their TPH contents (table 4).

The differences in heavy metal content in the soils are not related to their nature and properties (grain-size, organic matter content).

So, the redox potential has a decisive effect on the amount of Cr, Cu, Ni, Pb and V in insoluble form. Table 4 shows that the highest concentrations occur under reduced conditions, gradually decreasing when they change to oxidant values. Gambrell et al. (1991) found that soluble heavy metals were hardly affected by the oxidation-reduction potential except at intermediate oxidation-reduction potentials similar to those found in the soils studied in this paper.

However, the scarce amount of S-SO4^-2 and metals in available form (table 4) shows that when oxidation is produced by the fluctuation of the tide, or desiccation, a great proportion of these metals will become easily soluble forms, which can affect waters, fauna and flora, risking their entry into the trophic chain.

The effect of the fuel oil spill on polluted marsh soils was displayed by the Eh values and the low concentrations of Cr, Cu, Ni, Pb and V in available form, against high total contents, precipitated as sulfides.

The effects of the contaminant substances on the soils begin to weaken 500 m from the coastline. In spite of the correlation established between the distance to the coastline and heavy metal and total petroleum hydrocarbon content (table 5), in the first 400 m, there are important spatial variations because the metals and hydrocarbons penetrate into the soils through tidal action and not directly.

Finally, it must be noted that at several points in both marshes, there are very high total amounts of Cr, Cu, Ni, Pb, V and hydrocarbons (table 3). This may seriously jeopardize the fauna and flora if the acidic conditions allow part of these contents to become soluble. It must be highlighted that according to data provided by Andrade et al. (2002), the heavy metal content in the contaminated soils is about 50 to 200 times greater than the content in uncontaminated soils (control). This, in the near future, will have repercussions on the available amounts of heavy metal content and consequently the environmental quality of these areas.

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