Sediment accumulation rate and radiological characterisation of the sediment of Palmones River estuary (southern of Spain)

Journal of Environmental Radioactivity 65 (2003) 267–280 www.elsevier.com/locate/jenvrad

Sediment accumulation rate and radiological characterisation of the sediment of Palmones River estuary (southern of Spain) L. Rubio a,∗, A. Linares-Rueda a, C. Duen˜as b, M.C. Ferna´ndez b, V. Clavero c,1, F.X. Niell c, J.A. Ferna´ndez a a

Department of Plant Biology, Faculty of Sciences, University of Ma´laga, Campus de Teatinos s/n, E29071 Ma´laga, Spain b Department of Physics, Faculty of Sciences, University of Ma´laga, Campus de Teatinos s/n, E-29071 Ma´laga, Spain c Department of Ecology, Faculty of Sciences, University of Ma´laga, Campus de Teatinos s/n, E-29071 Ma´laga, Spain Received 6 March 2002; received in revised form 2 August 2002; accepted 5 August 2002

Abstract Chemical analyses and radioecological methods were combined in order to estimate the sediment accumulation rate in the upper 20 cm depth of the Palmones River estuary. Organic matter, total carbon, C:N and 137Cs vertical profiles showed changes at 13 cm depth. These changes could be associated with the decrease in river input since 1987 when a dam situated in the upper part of the estuary started to store water. Using 1987 as reference to date the sediment, accumulation rate was 1.2 cm yr⫺1. As alternative method, two layer model of 210Pbxs vertical distribution showed a sedimentation rate of 0.7 cm yr⫺1 with a surface mixing layer of 7 cm thickness. The high ammonium, potassium and sodium content in pore water and the strong correlation between 137Cs activities and organic matter in dry sediment suggests that 137Cs (the only anthropogenic product detected) is mainly accumulated in the estuary associated with the particulate organic material from the catchment area.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Sedimentation rate; Organic matter; Radiological characterisation; Palmones estuary

∗

1

137

Corresponding author: Tel.: +34-95-2132007; fax: +34-95-2131944. E-mail address: [email protected] (L. Rubio). In memoriam

0265-931X/03/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0265-931X(02)00102-9

Cs,

210

Pb, sediment;

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1. Introduction Estuarine sediments act as a temporary store of inorganic and organic materials. In high productive system, the decomposition of organic matter consumes oxygen, and the sediment becomes anoxic. The reduced conditions cause chemical transformations of metals and important anions in the sediment, as phosphate, ammonium, iron, manganese and bicarbonate could be released to the water and production increases in an amplified positive feedback (Nixon et al., 1986; Hopkinson and Wetzel, 1982; Van Cappellen and Gaillard, 1996). The Palmones River estuary is located near the Strait of Gibraltar (Southern Spain), under the influence of important anthropogenic activities. Several changes have been described in species abundance, diversity and evenness due to organic sewage, decrease of river discharges and accumulation of sediment at the river mouth since 1987. In this date, a dam located in the upper part of the catchment area (Charco Redondo) started to store water from the river (Estacio et al., 1998). Furthermore, recent climatic changes, characterised by severe drought during 1996 and 1997, have decreased river discharges, enhancing the influence of tides on the water estuarine flow (Carreira et al., 1995; Clavero et al., 1997, 1999). All these climatic events and human manipulations lead to a change of sedimentation rate in the estuarine area. Thus, a refined method must be applied in order to describe the sediment accumulation in the coastal area of the basin. Artificial and natural radioisotopes provide useful information as tracers to identify time scales for various estuarine processes, being 137Cs and 210Pb the main radioisotopes used to determinate the sediment accumulation rate (Koide et al.,1972, 1973; Robbins, 1978; Goldberg and Burland, 1974; Comans et al., 1989; Warner and Harrison, 1992; Clifton et al., 1995; Appleby & Oldfield, 1992; Appleby, 1997; Kirchner and Ehlers, 1998; Carroll et al., 1999; Radakovitch et al., 1999; Smith, 2001). Cs+ is known to interact strongly with micaceous clay minerals in soils and sediments (Francis and Brinkley, 1976; Evans et al., 1983; Cremers et al., 1988; Comans and Hockley, 1992; Comans et al., 1997; Hird et al., 1995). 137Cs (half life 30.2 yr) is produced by nuclear fission and has been released to the environment as a result of nuclear weapons testing during 1950–1970, with maximum atmospheric values in 1963, and from accidents such as Chernobyl in 1986, which impact has not been important in Spain (Warner and Harrison, 1992; De Cort et al., 1998). On the other hand, 210Pb (half-life 22.3 yr) is continuously settled onto the soil and sediment surface. The total 210Pb that is present in the sediment has two components, first a minor part in equilibrium with 226Ra fixed to the sediment from 238U decay. Second, 210Pb have a major part that is associated to the particulate matter, named 210Pd in excess (210Pbxs). This 210Pb in excess is formed in the atmosphere after 222Rn decay and is deposited on the surface with the particle material, so the exponential decrease of the accumulated 210Pbxs can be used to estimate the sediment accumulation rate (Goldberg and Burland, 1974; Robbins, 1978; Clifton et al., 1995; Appleby, 1997; Gasco´ et al., 1999; Radakovitch et al., 1999). The aim of this study is to estimate the sediment accumulation rate in the upper 20 cm of the Palmones River estuary using as reference the construction of Charco

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Redondo dam in 1980s and to compare with the sedimentation rate obtained by the vertical distribution of 137Cs and the accumulation of 210Pbxs. Moreover, this work includes the radioisotopes inventory of the sediment of the Palmones River estuary up to 1998, before the accidental 137Cs release in the ACERINOX factory, placed near the estuary in Algeciras Bay (Rojas and Calvo, 1998).

2. Sampling and methods 2.1. Sampling area The Palmones River estuary is located in Southern Spain (Fig. 1) at the end of a small catchment area (97 km2), in Algeciras Bay. It is a small, shallow, well mixed estuary with tidal movements which have a maximum amplitude of 2 m and extensive areas of mud that emerge daily at low tide. The sediment is sandy and slimy, clay minerals represents about 5% of the composition. Porosity decreases in the upper 10 cm from 0.9 to 0.7 and changes seasonally

Fig. 1.

Map of the Palmones River estuary, indicating the sampling area.

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(Clavero, 1992). The main transformations in the catchment area of Palmones River which have happened since 1990s are related to the reduction of the river water flow since the construction of a dam in its upper part and the climatic changes which affected the flow of water in the river between the dam and the estuary (Clavero et al., 1997, 1999). The place chosen for the present study is situated in the middle part of the estuary, where sediment and water characteristics have been documented since 1987 (Pe´ rezLlorens and Niell, 1990; Clavero et al., 1991; Clavero, 1992; Clavero et al., 1997, 1999; Clavero et al., 2000; Herna´ ndez et al., 1997). 2.2. Sampling Sediment cores were taken in February 1998 at low tidal regimen to minimise the effects of tides in the sediment vertical structure (Clavero et al., 1997). Two PVC cores tubes (15 cm i.d., 60 cm long) were inserted by hand into the sediment, separated by 5 m from each other. Both cores were retrieved and the ends covered with silicone tops to minimise contact with the air and transported to the laboratory in an icebox at 4 °C. In the laboratory, the sediment was frozen and extruded from the PVC and sliced into 1 cm segments. Once melted, each slice was weighed and centrifuged to extract pore water (5000 rpm, 20 min, 4 °C) which was passed through Whatman GF/C filters. The sediment was dried at 60 °C for 24 h and divided into subsamples to determine dry mass, organic matter, carbon, nitrogen and radioisotopes activity. Results are the means ± SD of the two analysed cores. 2.3. Analytical methods Organic matter content was determined by weight loss after 4 h ashing at 550 °C. Total carbon and nitrogen were determined with a CHN Perking Elmer elemental analyser (Model, 2400). Pore water was analysed to determine ammonium concentrations (Slawyk and MacIsaac, 1972). Furthermore, sodium, calcium and potassium content were measured in a Corning flame photometer, model 410C (Golterman et al., 1978). The radioisotope activities were measured in dry sediment using a reverse type germanium detector (model GR 2520-7500; Camberra). The detector efficiencies were 10–25% for the 1.33 MeV 60Co energy and the resolution was 1.8–2 keV. Efficiency calibrations were obtained with different densities of pitchblende sources prepared in various geometric forms. The calibration energy ranged from 46 to 2.2 MeV (Liger, 1996). Activities in the sediment cores were decay corrected to the date of sample collection. Dry sediment samples (15–20 g) were loaded into a cylindrical vial (5 cm i.d.), sealed and counted over 24 h. 137Cs and 210Pb were determined using the 661.6 and 46.5 keV photopeaks respectively. The samples were counted again at least 30 days after sealing and the 226Ra content was estimated from the 214Bi (609.3 keV) and

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214

Pb (352.0 keV) photopeaks (Gilmore and Hemingway, 1995). The unsupported Pb were determined as the difference between total 210Pb and 226Ra activities (Radakovitch et al., 1999). A two-layers model with constant mixing in the upper layer (Christensen, 1982) was tentatively applied to estimate the sedimentation rate in the upper 21 cm depth assuming a constant rate of unsupported 210Pb supply and a surface mixing layer of 7 cm thick. 210

3. Results 3.1. Sediment characterisation Organic matter content of dry sediment ranged from 7 to 21% (Fig. 2a). The maximum value was situated at 13 cm depth, after that the organic matter content decreased to the bottom (21 cm depth). The lower values were found at the surface. Total carbon content ranged between 4 and 20 mmol C g⫺1 dry mass (Fig. 2b). The lower value was found at surface and the maximum was at 15 cm depth. Total nitrogen in dry sediment was round 1 mmol N g⫺1 dry mass (Fig. 2c). There was a maximum value (1.53 mmol N g⫺1 dry mass) at 9 cm depth and the lower values were measured at surface (0.5 mmol N g⫺1 dry mass). The C:N ratio ranged from 8.5 to 21.2 (Fig. 2d). Vertical profile showed two different ranks of values (1-way ANOVA was significant, a ⫽ 0.05). From sediment surface to 12 cm depth C:N values were close to 12, and in the deeper layer (13– 21 cm depth) the values were higher. In pore water, pH was basic and constant (1-way ANOVA, a ⫽ 0.05). Values were 8.3 ± 0.3 pH units (data not shown). It was observed an increase in ammonium and sodium concentrations in pore water with depth. Ammonium concentrations (Fig. 3a) ranged from 0.3 to 1.7 mM, the lower value is at surface and the maximum is at 19 cm depth. Sodium ranged from 250 to 800 mM (Fig. 3b). From 7 cm depth to the profile bottom sodium concentrations were higher than sodium concentration in seawater (450 mM, Riley and Chester, 1971). The maximum value was situated at 17 cm depth. Pore water calcium content was higher than in seawater (10 mM; Riley and Chester, 1971) ranging from 14 to 50 mM. The lower values were detected at surface and bottom, and there were two maximums at 6 and 10 cm depth, respectively (Fig. 3c). Finally, pore water potassium concentrations (Fig. 3d) were closed to potassium concentrations in seawater (10 mM; Riley and Chester, 1971) apart from 8, 14, 15 and 17 cm depth where potassium reached maximum values. Changes in the trend of organic matter, total carbon and C:N ratio vertical distributions round 13 cm depth could be explained by the construction of a dam in the upper part during the 1980s which started to store water in 1987. Using this date as reference a sediment accumulation of 1.2 cm yr⫺1 can be computed.

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Fig. 2. Vertical distributions of dry sediment (a) organic matter, (b) total carbon, (c) total nitrogen content and (d) C:N ratio (means ⫹ SD) within the upper 21 cm of sediment in the Palmones River estuary.

3.2.

137

Cs profiles

From the sediment surface to a depth of 21 cm 137Cs activities in dry sediment ranged from undetectable values (Minimum Detectable Activity to 137Cs is 6 Bq kg⫺1) to 18 Bq kg-1 (Fig. 4), lower values were situated at the surface. In the same way as has been observed in organic matter, total carbon and C:N ratio vertical distributions, changes in the 137Cs vertical profile can be observed (1-way ANOVA was significant, a ⫽ 0.05), at 13 cm depth. In the upper part, activities were close to 12 Bq kg ⫺1, while values close to 17 Bq kg⫺1 were recorded between 13 and

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Fig. 3. Vertical distribution of pore water (a) ammonium, (b) sodium, (c) calcium and (d) potassium ( means ⫹ SD) within the upper 21 cm of sediment in the Palmones River estuary.

21 cm depth. There is no maximum value that can be directly associated to the historical fallout of 137Cs in the upper 21 cm depth in the sediment of the Palmones River estuary.

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Fig. 4. 137Cs activities (means ⫹ SD) in Bq kg⫺1 versus depth (cm) in dry sediment from the Palmones River estuary. 137Cs activities were determined with a 2 s uncertainty and decay corrected. The error bars on activity determination are indicated.

3.3.

210

Pb profiles

Processes that dominate 210Pbxs profiles in sediment include accumulation and mixture, as well as the radioactive decay. Thus, in a given interval, assuming a constant input of 210Pbxs, the vertical distribution of 210Pbxs could be expressed as: 210

Pbxs(z) ⫽

210

210

Pbxs(z0)e⫺lz/w

(1) 210

where Pbxs(z) is the activity of Pbxs at the bottom of a given profile and 210 Pbxs(z0) is the activity at the end of the mixing layer, both in Bq kg⫺1. l is the decay constant of 210Pb (0.03114 y⫺1) and w, (cm y⫺1) is the sediment accumulation rate. In the sediment of the Palmones River estuary 210Pbxs activities of dry sediment ranged from 50 to 130 Bq kg⫺1 (Fig. 5). Lower values were detected in the first cm and at 21 cm depth. Maximum value was situated at 7 cm depth, after that 210Pbxs activities decreased exponentially (R2 ⫽ 0.5) to the end of the core (21 cm depth). According to the exponential model of 210Pb decay [Eq. (1)] a sedimentation rate of 0.7 cm y⫺1 can be calculated between 7 and 21 cm depth. 3.4. Radiological characterisation in the Palmones River estuary sediment In addition to 137Cs and 210Pb, different natural radioisotopes from natural decay of 238U and 232Th series (Table 1) were detected in dry sediment. 40K showed the major activities (250–600 Bq kg⫺1). Since the sediment cores were taken in February

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Fig. 5. Vertical profile of 210Pb and 210Pbxs (means ⫹ SD) in Bq kg⫺1 in dry sediment from the Palmones River estuary. 210Pbxs activities were determined by subtracting the activity of 214Bi (considered as the 226Ra in equilibrium with 210Pb) from the total 210Pb activities. Activities were measured with a 2 s uncertainty and decay corrected. The error bars on activity determination are indicated.

Table 1 Activity ranges of the radioisotopes detected in dry sediment of the Palmones River estuary from 1 to 21 cm depth Radioisotope

Activity range (Bq kg⫺1)

210

50–130 25–100 60–150 20–45 15–25 10–15 15–25 ⬎6–18 250–600

Pbxs Th 226 Ra 212 Pb 214 Pb 208 Tl 214 Bi 137 Cs 40 K 234

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1998, this is the gamma radioisotopes content in the sediment of the Palmones River estuary before the accidental 137Cs release in a steel factory situated in Algeciras Bay (May, 1998; Rojas and Calvo, 1998).

4. Discussion 4.1. Sedimentation rate Recent papers (Clavero et al., 1999, 2000) have shown that Palmones River discharges have decreased dramatically since the Charco Redondo dam started to store water. The sediment of the estuary has returned to be dominated by tidal influence and an increase of the organic materials accumulation from the high eutrophication of the estuary have been observed (Carreira et al., 1995; Niell et al., 1996; Clavero et al., 1997, 1999, 2000). From the C:N ratio in the sediment, the upper layer (1– 12 cm depth) would store marine rather than terrestrial organic matter, thus the qualitative changes in organic matter, total carbon and C:N vertical profile around 13 cm depth could be associated with a low input from the Palmones River since the dam began operating. C:N ratio (a good index of the organic matter quality) reveals significant differences between the two depth layers (1–12 cm versus 13–21 cm depth). The lower values of the C:N ratio in the upper layer suggest the presence of slow growth plankton and Ulva sp., (Niell et al., 1996). While, the deep C:N values can be related to a highly degraded allocthonus organic matter. Thus, since the early 1980s, processes in the Palmones River estuary have changed, suggesting the tides as one of the main factors in the progressive eutrophication in the estuary (Clavero et al., 1999). Several studies have shown the importance of particle mixing on the vertical 210 Pbxs distribution in sediment of coastal zones (Christensen, 1982; Radakovitch et al., 1999; Smith, 2001). In most cases, little is known about the use of an alternative method for sediment dating. Nevertheless, in this work we use the changes in vertical distribution of organic matter, total carbon and C:N vertical profiles associated to dam operating to date the sediment. 210Pbxs vertical distribution permit us to estimate a sedimentation rate of 0.7 cm yr⫺1, assuming a two layers model with constant mixing layer in the upper 7 cm depth (Christensen, 1982). The use of 210Pbxs is tentative because the application of the Constant Rate of Supply (CRS) model is not possible due to the available data only corresponding to the upper 20 cm depth, where the activities of 210Pbxs do not reach equilibrium. Both sediment accumulation rate (1.2 and 0.7 cm y⫺1 with a surface mixing layer of 7 cm thick) are high and suggest a quick filling of the estuary, according to the progressive eutrophication estimated in this area (Clavero et al., 1999, 2000). 4.2.

137

137

Cs vertical profile

Cs activities could have several origins: global direct fallout from atmospheric nuclear weapons testing, fallout from the Chernobyl accident in a minor way, but

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mainly the erosion of soils in the Palmones River catchment area that were contaminated by these historical events. In agreement with the preferential distribution of the radioisotopes released from the Chernobyl accident over the east of Europe (De Cort et al., 1998), there are not apparent maximal values of 137Cs activity associated to this accident in the Palmones River estuary. Taking into account the sedimentation rates calculated in this work, the maximum 137Cs related to the global fallout of nuclear weapons testing in 1963, should be situated round 35 cm depth. Therefore the thickness of the present profile is not sufficient to detect the 1963 peak. Thus, the presence of 137Cs in the sediment of the estuary could be mainly associated with the accumulation of materials from areas contaminated during the global fallout of nuclear weapons testing and a more contemporaneous mobilisation from the Palmones River catchment area, since the vertical distribution of 137Cs is similar to the recorded organic matter and C:N vertical profiles. The association of 137Cs with the soils is regulated by a small number of highly selective ion-exchange sites, located at the frayed edge sites of clay minerals (Francis and Brinkley, 1976; Evans et al., 1983; Cremers et al., 1988). Univalent cations with low hydration energy and an ionic radius similar to Cs+ have been described to compete for these same bound sites (Comans et al., 1989, 1990). Cations such as K+ dehydrate and form polar bonds with the structural oxygen atoms on the silicate. These bonds result in a collapse of clay minerals layers. Larger cations with a stable hydration shell, such as Ca+2, separate the interlayers and are not selectively retained (Comans and Hockley, 1992). In coastal environments, competition is expected to be mainly with Na+ and K+ (Martin et al., 1994) as well as NH4+, which reaches a high concentration in anoxic pore waters of eutrophic sediments (Comans et al., 1989). Moreover, in sediments or soils with high organic content there is a significant association between Cs+ and organic matter (Martin et al., 1994; Sansone et al., 1997). Data from this study showed a strong positive correlation among 137Cs vertical distribution in dry sediment with ammonium and sodium concentrations in pore water (r ⫽ 0.90 and r ⫽ 0.78, respectively, a ⫽ 0.05). Also, 137Cs vertical distribution is positively correlated with the vertical distribution of organic matter (r ⫽ 0.78, a ⫽ 0.05) and with the total carbon in the sediment (r ⫽ 0.78, a ⫽ 0.05). Both conclusions suggest that the major part of the 137Cs measured in the Palmones River estuary is corresponded to non interchangeable form. On the other hand, the multiple regression analysis showed that ammonium and sodium in pore water and sediment total carbon and organic matter explain 95% of the total variation of 137Cs activities. The standard partial regression coefficients were positive to ammonium, total carbon and organic matter and negative to sodium. [Eq. (2)]. 137

Cs⬘ ⫽ 0.7[NH+⬘ ] ⫹ 0.3[C⬘] ⫹ 0.1(%OM) ⫺ 0.2[Na+⬘] R2 ⫽ 0.95 4

(2)

where 137Cs⬘ is the standardised activity of 137Cs; [NH4+⬘], [C⬘], [Na+⬘] and (%OM) are the standardised values of ammonium, total carbon and sodium concentrations

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and organic matter content, respectively. The 1-way ANOVA was significant, a ⫽ 0.001. The 137Cs activities in the Palmones River estuary are bigger than the reported values for this radioisotope in the Spanish Mediterranean coast (Gasco´ et al., 1999). In sediments, the high ammonium content in pore water would be associated to low 137 Cs activities (Comans et al., 1989), nevertheless, the positive correlation observed between 137Cs activities and organic matter would explain the positive correlation with the pore water ammonium due to the high degradation of the organic matter under anoxic conditions. Recent papers (Clavero et al., 1999, 2000) have shown that the Palmones River discharges have decreased dramatically since the Charco Redondo dam started to store water. The sediment of the estuary has begun to be dominated by tidal influence and an increase in the organic materials accumulation from the high eutrophication of the estuary (Carreira et al., 1995; Niell et al., 1996; Clavero et al., 1997, 1999, 2000). According to the C:N ratio in the sediment, the upper layer (1–12 cm depth) would store marine rather than terrestrial organic matter, thus the decrease in 137Cs activities could be associated to low input of the Palmones River since the dam was built. 4.3. Radiological characterisation The radiological composition of the sediment has been determined for the first time in the Palmones River estuary. The results show the lack of artificial gamma radioisotopes, except 137Cs, which shows activity higher than those detected in Mediterranean coast (Gasco´ et al., 1999). Besides 137Cs, natural radioisotopes were also detected (Table 1). The recorded activities were lower than those reported in the estuary formed by the Odiel and Tinto rivers in southwest Spain, where the increase in natural radioisotopes was associated with the radiological impact of phosphate factories (Travesı´ et al., 1997).

Acknowledgements This work has been supported by grant AMB 19999-0782 of the Spanish Commission of Science and Technology.

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