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Geochemical evidences of the anthropogenic alteration of trace metal composition of the sediments of Chiricahueto marsh (SE Gulf of California)
Environmental Pollution 125 (2003) 423–432 www.elsevier.com/locate/envpol
Geochemical evidences of the anthropogenic alteration of trace metal composition of the sediments of Chiricahueto marsh (SE Gulf of California) M. Soto-Jime´neza, F. Pa´ez-Osunab,*, A.C. Ruiz-Ferna´ndezb b
a Programa de Posgrado en Ciencias del Mar y Limnologı´a, Unidad Acade´mica Mazatla´n, UNAM, Mexico Unidad Acade´mica Mazatla´n, Instituto de Ciencias del Mar y Limnologı´a, UNAM, Apdo. Postal 811, Mazatla´n 82000, Sinaloa, Mexico
Received 19 September 2002; accepted 21 February 2003
‘‘Capsule’’: Principal component analysis allowed separation of natural from anthropogenic origin metals. Abstract Seven sediment cores (60–80 cm) were collected at Chiricahueto marsh, a salt marsh influenced by agrochemical, domestic and industrial effluents. The concentrations of Ag, Al, Cd, Co, Cu, Fe, Li, Mn, Pb, V and Zn were studied in the solid phase at each 1cm section. The profiles of Ag, Cd, Cu, Mn, Ni, Pb and Zn showed a slight recent pollution in the site with enrichment and anthropogenic factors higher than unity; and correlation analysis indicated a direct association with organic carbon. Al, Co, Cr, Fe, Li, and V concentration profiles displayed a negative correlation with organic C and positive with mud content and no consistent enrichment at surface. Based on the principal component analysis and correlation analysis, two principal groups of metals were identified. The first group includes Al, Co, Cr, Fe and Li, that are derived predominantly from the weathering of parent materials in the local bedrock; and the second group include most of the metals, which were relatively enriched at surficial sediments, that are produced mainly by anthropogenic activities such as agriculture (Cd, Cu and Zn), sewage effluents (Ag, Cd, Cu, Ni, Pb and Zn) and in lesser extent atmospheric deposition (Cd and Pb). # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Metals; Agriculture; Sewage; Chiricahueto marsh; Gulf of California
1. Introduction The Sinaloa state lies on the west coast of Mexico beside the Pacific ocean and the Gulf of California. It is the most important national producer in economic activities such as agriculture, fishing and aquaculture. Annually more than 8 million ton of vegetables, fruits and grains are harvested and destined for internal consumption and for exports to the United States. During 1999 total exports from the state were more than 1 billion USD derived principally from the agricultural sector (INEGI, 2000). The Culiaca´n valley represents one third of the agricultural area of the state. Drastic changes in the use of land and water occurred at the middle of the last century, transforming the Culiaca´n valley * Corresponding author. Tel.: +52-669-852845; fax: +52-669-9826133. E-mail address: [email protected] (F. Pa´ez-Osuna).
from a quiet rural hamlet into an area of intensive agriculture. The region was converted in a developing center that resulted in a rapid increasing population growth, from 49,000 inhabitants in the 1950’s to close to 1 million inhabitants at the end of the century (INEGI, 1999). However, as occurred in other countries with emergent economies, the development of the region did not consider the environmental implications with respect to biodiversity and other aspects of ecological change, combined with insufficient environmental protection measures. Pollution problems have been reported in the coastal area adjacent to Culiaca´n valley related to metals, nutrients and pesticides (Pa´ez-Osuna et al., 1992, 1998; Readman et al., 1992; Green-Ruiz and Pa´ezOsuna, 2001; Ruiz-Ferna´ndez et al., 2001a, 2001b, 2002; Soto-Jime´nez et al., 2003). Chiricahueto marsh is a wetland that has been particularly impacted due to excessive conversion of forest in upland areas to cultivated and urbanized lands and
0269-7491/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0269-7491(03)00083-6
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mangrove forest in lowlands to farmlands and shrimp aquaculture facilities. Located on the natural drainage outlet of the agricultural fields (ca. 130,000 ha), Chiricahueto marsh acts as a trap of sediments and other materials, including nutrients and metals, carried through the wastes from culture lands and the polluted runoff from sub-urban and agricultural areas. Additionally, Chiricahueto marsh receives the untreated sewage from Culiaca´n city (14.7103 m3 day1; INEGI, 1999) via a combined sewer system designed to collect rainwater runoff, domestic sewage and industrial wastewater in the same pipe line. The average daily loads of suspended solids of untreated effluents discharged by the Chiricahueto drain are about 3.5 t day1. Another potential source of metals is the discharge of wastewater derived from 14,500 residential septic systems that collect the residues from 127,000 inhabitants established on the drainage basin as well as through atmospheric deposition. The present study investigated the metal pollution in sediments from this anthropogenically altered marsh, using multivariate methods, correlation and principal component analysis (PCA), to define the different origins of these metals (Szefer and Kaliszan, 1993; Callaway et al., 1995; Ruiz-Ferna´ndez et al., 2001b).
2. Material and methods 2.1. Study area Altata-Ensenada del Pabello´n (AEP) is a lagoon system located in the northwestern coast of Mexico (24 200 –24 400 N; 107 300 –107 580 W) with an area of 330 km2 and a drainage basin of 17,700 km2. Chiricahueto lagoon is a wetland marsh type that is part of the AEP (Fig. 1) with an area of ca. 23 km3, characterized by a shallow marsh (< 0.5 m depth) and limited water exchange with the adjacent Ensenada del Pabello´n lagoon. Average annual precipitation values for the region is 673 mm, mostly occurring in the summer months. Wetland water losses to the atmosphere occur from water surfaces and moist soil surfaces (evaporation) and from the interior of the emergent portions of plants (transpiration); they have been estimated as 1500 mm per year (De la Lanza-Espino and Flores-Verdugo, 1998). 2.2. Sample collection and processing Seven undisturbed cores (60–80 cm depth) were taken from an intertidal marsh flat in April–June 1999 (Fig. 1). The cores were immediately transported to the laboratory and the supernatant liquid was collected (about a 5-cm thick layer above the water–sediment interface). Two sediment cores were sectioned at 1-cm intervals, lyophilized (72 h at 40 C and 36103 mBar) and mechanically homogenized with a mortar and pestle.
One core was sectioned at 1-cm intervals and treated with H2O2 for textural analysis. The remaining cores were used for radionuclide examination (Ruiz-Ferna´ndez et al., 2001a) and porewater analysis (Soto-Jime´nez et al., 2003). 2.3. Analytical procedure Grain size was determined separating the fine silt and clay from the coarse fraction (sand) of sediments by wet sieving using a 63 mm mesh stainless steel sieve. The contents of silts and clays were evaluated by pipette analysis (Folk, 1974). Sediment porosity was estimated from the weight loss of the sediment after drying and assuming a specific weight of 2.65 kg dm3. The carbonate content was determined by treating the sediments with 1 M HCl, and the excess of acid was quantified by back-titration with 0.5 M NaOH using phenolphthalein as an indicator (Rauret et al. 1988); organic C content (OC) was analyzed by chromic acid oxidation (Loring and Rantala, 1992). Sediment samples were digested with HCl, HNO3 and HF according to the procedure reported by Loring and Rantala (1992). Samples (0.15 g) of dried, ground and homogenized sediment from each section were transferred into Teflon vessels (100 ml), digested with 1 cm3 of inverted aqua regia (HNO3:HCl, 1:3 v/v) and 6 cm3 HF in a microwave oven (CEM, MSD 2000) (two steps of 30 min at 100% power). Samples digested were transferred into the polypropylene tubes containing 5 g of boric acid and diluted to 50 cm3 with water Milli-Q. Aluminum, Cd, Cr, Co, Pb and V were analyzed by graphite furnace atomic absorption spectroscopy (Varian GTA 110 GFAAS). Metals such as Cu, Fe, Mn, Ni and Zn were determined by flame atomic absorption spectroscopy (Varian SpectrAA 220 AAS). Analytical performance was evaluated with standard reference materials: polluted marine sediment IAEA-356 (IAEA, 1994) and marine sediment BCSS-1 (Loring and Rantala, 1992). Metal recovery depending on element varied between 90 and 104%. Analytical precision of replicates (n=6) varied from 4 to 11% relative to standard deviation. 2.4. Data processing The PIRLA procedure (Binford, 1990) was applied to identify the metal background levels from the elemental profiles. This method involves the calculation of the mean (x) and standard deviation () of the apparently constant values found at the bottom of the core; if the next higher concentration was minor than the mean value plus one standard deviation (x+), then this concentration was included in the set of constant values and the mean and standard deviation were recalculated. The procedure continued until the concentration in the
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Fig. 1. Location of the study area. Chiricahueto salt marsh in the Altata-Ensenada del Pabello´n lagoon system.
next higher level was greater than x+. The asymptotic levels estimated were assumed as the geochemical background concentrations for the different elements in the marsh. Considering that the textural characteristics of sediments can have an influence on the concentration of metals, an enrichment factor (EF) was calculated by using the ratio between the metal concentration of the sample and the average metal concentration of the earth’s crust (Salomons and Fo¨rstner, 1984; Szefer et al., 1996; Soto-Jime´nez and Pa´ez-Osuna, 2001). Additionally, in order to evaluate the human-caused changes registered in the Chiricahueto marsh sediments, an anthropogenic factor (AF) was calculated (Szefer and Skwarzec, 1988; Szefer and Glasby, 1996) as the quotient of the mean metal concentration at the surface (0– 5 cm) and the geochemical background metal con-
centrations estimated with the PIRLA procedure described before. Finally, in order to identify the provenance and distribution of the metal load, a correlation analysis in combination with principal components analysis (PCA) were applied on the entire data by using Statistica for Windows version 4.5 (StatSoft Inc., USA) (Table 1).
3. Results A description of the sedimentary column and the vertical distribution patterns for metals and other sediment proprieties are shown in Figs. 2 and 3. The results obtained correspond to the average of two cores examined, only the 70–80 section was individually analyzed. A very thin brown oxidized layer was identified in the top of the sediment (< 1 cm depth), while in the down-
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Fig. 2. Description of the sedimentary column, and mud, carbonates, Fe, Al, Li, Cr, Co and V profiles in sediment cores from Chiricahueto marsh.
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Fig. 3. Organic carbon content, Ag, Cd, Cu, Mn, Ni, Pb and Zn profiles in sediment cores from Chiricahueto marsh.
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Table 1 Mean metal concentrations in surface (0–5 cm) and bottom layers ( >52 cm) of the profile, average composition of the crust and enrichment (EF) and anthropogenic (AF) factors Metala
Al Ag Cd Co Cr Cu Fe Li Mn Ni Pb V Zn
Sediment concentrations
Factors
Earth crustb
Bottom ( <52 cm)
Surface (0–5 cm)
69 0.8 0.2 13 71 32 36 42 720 49 16 97 127
36.1 5.4 0.2 0.1 0.3 0.1 8.2 0.7 44 7.2 26 5.2 35 2.7 28 4.2 477 31 21 2.3 10 1.0 82 4.2 79 17
354.9 3.90.7 1.20.3 8.91.0 453.9 602.8 292.6 232.8 1256121 423.5 182.6 452.8 13414
EFAl
AF
4.9 6.0 0.7 0.6 1.9 0.8 0.6 1.7 0.9 1.1 0.5 1.1
25.1 4.3 1.1 1.0 2.3 0.8 0.9 2.6 2.0 1.8 0.5 1.7
a Metal concentrations are in mg kg1, except Al and Fe, which are in g kg1. b Martin and Maybeck (1979).
core the prevalence of reducing conditions was evident by the dark gray to black with strong odor of H2S. Observations of the irrigation and bioturbation were neglected because no tube dwellings were observed and no animals were found in the sediment. The sediments were mainly composed of silts and clays (< 63 mm) with a mud content greater than 96% formed by the settlement of fine particulate mainly transported with the material fluvial from agricultural operations. The mud content profile is very regular and does not display any trend with depth, which underlines the good sediments column homogeneity. Low CaCO3 contents (0.3–2.2%) with an irregular downcore increase was observed in the sedimentary column. The organic C in the solid phase ranged from 0.4 to 4%, the highest concentrations were consistently observed in the upper layers showing an exponential decrement with depth. The kinetic of the decomposition and the diagenesis of the organic matter has been discussed by Soto-Jime´nez et al. (2003). It is clear that the diagenetic processes are mainly controlled by the exponential decomposition of organic matter that takes place at the sediment–water interface and in the sedimentary column, under oxic and suboxic-anoxic conditions, respectively. The vertical distribution patterns for Al, Fe, Li, Cr and V showed a slight decrease towards the surface. The profiles of Ag, Cd, Cu, Ni, Pb, and Zn presented a concentration increment from low or background values in the bottom-most layer, followed by fluctuations and surface or subsurface peak values. This pattern is more or less the same as Mn, which showed the highest values in the surface oxidized zone and decreasing concentrations with depth.
PIRLA procedure revealed relatively little variation with depth and low concentration of nutrients (SotoJime´nez et al., 2003) and of most metals in sediments deposited below 52 cm depth. The average concentrations for each element were estimated from these low and relatively constant levels and established as the regional background values (Table 1). Compared with the background levels in the earth’s crust (Martin and Maybeck, 1979), the regional baseline values estimated for metals were significantly lower, except for Cd and Fe (P < 0.05, Table 1). When the baseline levels here proposed are compared with those obtained for rocky samples collected in the drainage basin (Green-Ruiz, 2000), Ag, Cd, Co, Mn and Pb have comparable levels, while Cr, Cu, Fe, Li, Ni, V and Zn showed concentrations more elevated. Above the 52-cm section, the concentrations of most metals and organic C displayed a similar trend to increase upwards with peaks in the subsurface and surface sediments. The metal concentration averages in the surface section (0–5 cm) are also listed in Table 1. Mean enrichment factors in surface sediments using Al as normalizer element show that Fe, Ni, Pb and Zn are close to unity and Co, Cr, Li and V below unity (Table 1). Additionally, the AF values of the Co, Cr, Fe, Li and V were near unity. Both Efs and AFs for surface sediments of the study area were much greater than 1.0 for Ag, Cd, Cu and Mn, suggesting a significant enrichment for these metals in recent times. In the case of Ni, Pb and Zn the enrichment in surficial layers was evident with AFs higher than unity, although the EFs were close to unity due to differences between the background levels in continental crust and natural levels in the region. There is a good correlation between organic C and most of metals (Ag, Cd, Cu, Mn, Ni, Pb and Zn), with high values of Pearson’s correlation coefficients (P < 0.05). Obviously, there is also a very significant relationship among concentrations of these metals (Table 2). A significant negative correlation was observed between Al and Fe with the OC but positive with mud content. Cobalt only correlated directly with Pb and inversely with Ag, Cd and V. In the case of carbonate content there was no significant linear association with any element. After checking the suitability of the dataset for factor analysis, a principal component analysis with Varimax rotation was run. An eigenvalue less than unity was chosen as the break off test for the factors. Based on the loading distribution of the variables in PCA (accounting for 77.2% of total variance), two principal groups were extracted (Fig. 4). The first group includes a strong association between Al, Fe and Li and mud content, whereas Ag, Cd, Cu, Mn, Ni, Pb and Zn together with OC constitute a second group enriched in the surface layers. Co, Cr, V and CaCO3 cannot be clearly included in any of these groups.
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Table 2 Pearson correlation matrix for the metal concentrationsa
OC Ag Al Cd Co Cr Cu Fe Li Mn Ni Pb V Zn a
OC
Ag
1.00 0.95 0.71 0.74 0.34 0.36 0.75 0.70 0.32 0.69 0.81 0.53
1.00 0.69 0.76 0.37 0.40 0.84 0.78 0.32 0.74 0.82 0.55
0.50
0.65
Al
1.00 0.58 0.53 0.46 0.79 0.56 0.68 0.48 0.46
Cd
1.00 0.69
Co
Cr
Cu
Fe
1.00 0.32 0.39
1.00 0.78
Li
Mn
Ni
Pb
V
1.00 0.65 0.36
1.00
Zn
1.00
0.48 0.55 0.29 0.41 0.37
0.58 0.37 0.41
0.48 0.65
0.50
0.70 0.90 0.75 0.46 0.83
1.00 0.40 0.79 0.68 0.66 0.51 0.55
1.00 1.00 0.65 0.70 0.59 0.39
1.00 0.77 0.66
1.00
All correlations are significant at P<0.05.
4. Discussion 4.1. Metals from natural source The significant correlation coefficients found between element concentrations, being in agreement with the principal component analysis results, indicates that there are two main groups of elements in the sediments. The first formed by Al, Fe and Li (and possibly Cr and V) related to mud content, and the other constituted by the rest of metals, except Co, closely associated to organic carbon. These associations between elements were used to establish the major sources that control their profiles into the sedimentary column in Chiricahueto marsh. For example, it is clear that Al, Fe and
Li are elements derived from the weathering of parent materials in the local bedrock and reflect the granular variability in the sediments (Martin and Maybeck, 1979; Szefer, 1990; Soto-Jime´nez and Pa´ez-Osuna, 2001). For this reason, the selection of Al as a reference element for the estimation of enrichment factors was correct. Chromium and V with profiles similar to Al, Fe and Li with the EF < 1, are metals attribute commonly to lithogenic components, but the separation observed for both metals in PCA results may indicate a partial remobilization out of the solid phase controlled by organic matter degradation in reducing conditions (Soto-Jime´nez and Pa´ez-Osuna, 2001). Iron is an element that in the reducing conditions can be mobilized during diagenesis, however, for the sediments studied in this work it was not clearly evidenced as is explained in a posterior section. The levels of Co appeared to be uniformly distributed and the levels were generally low and did not indicate any significant input, thus are derived predominantly from natural source lithogenous material. 4.2. Metals from anthropogenic sources
Fig. 4. PCA results: plot of loading of the three first components obtained in the analysis.
High levels of total Ag in the surface sediments, characterized by EF > > 1, are mainly related to the discharges of untreated urban wastewater (Bryan and Langston, 1992; Hornberger et al., 1999). Any other source is clearly identified for this element. In the case of Cu, two sources can be distinguished, one is due to the application of Cu fungicide, as CupravitR (Cu2(OH)3Cl) or other similar compounds in the tomatoes cultivates (Lycopersicon esculentum Mill) that in the Culiaca´n valley is greater than 20,000 ha. Given that the dose of this product (concentration 85%) varies between 1 and 2 kg ha1 per cycle, the annual input of Cu to the agriculture soils ranges between 0.3 and 1.2 kg ha1, which are
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potentially exposed to be transported to the sediment marsh by fluvial load. Another source is the untreated sewage, as has been previously mentioned in other studies (e.g. Bothner et al., 1994; Alloway, 1995; SotoJime´nez and Pa´ez-Osuna, 2001). The influent Cu concentrations from year 2000 in the municipal effluents ranged from < 0.1 to 0.19 mg l1 (Anonymous, 2001), which give a net input into marsh of about 550–1020 kg each year considering a discharge constant over time in the last recent years. The AF and EF values of Cd for recent sediments of the study area suggest a significant anthropogenic input for this metal, which has been primarily associated to the application of high amounts of phosphate fertilizers produced from phosphate rocks containing from 3 to 150 mg Cd/kg (de Meeuˆs et al., 2002). The application rates of fertilizers as simple or triple super-phosphate calcite to the agricultural lands is a common practice in different crops in the region (INIFAP, 1997): tomatoes, sugar-cane, corn, soybeans, chili-pepper and others, and vary from 30 to 150 kg ha1. For illustrative purposes it was considered that the concentrations in phosphatefertilizers range from 30 to 100 mg Cd kg1 and an annual application rate of 100 kg P2O5, this is equivalent to an applied loading of 3–10 g Cd ha1 year1. Other sources of Cd to marsh sediments include the discharge of domestic effluents ( < 0.066 mg l1) estimated as < 350 kg year1 and the atmospheric deposition that were not evaluated in this work. Another metal derived mainly from agriculture activities is Zn, which results from the direct application of chemicals to the cultivated lands as pesticide and fertilizers (Fo¨rstner and Wittmann, 1979). Zn inputs via the application of fertilizers were estimated between 70 and 220 g ha1 year1, considering a concentration of 1.45 g Zn kg1 P2O5 (Alloway, 1995). In addition the application of the etilen-bis-dithiocarbamate, that contain zinc salts [(C4H6 Mn Zn N2S4)], is common in the region for control of plagues in different crops (INIFAP, 1997). It was estimated that the application rate of 1–2 kg ha1 of a formulation to 80% (20% w/w of Zn), generates an input rate between 160 and 320 g ha1 year1. Similarly, to Ag and Cu, other significant source of Zn is the untreated sewage from Culiaca´n city. This source was estimated as about 970 kg Zn year1 discharged directly to the Chiricahueto marsh, considering an average annual concentration of 0.18 mg Zn l1 in the wastewater (Anonymous, 2001). Lead in the surface sediments (0–1 cm) was found to be enriched relative to crustal material indicating that continental weathering does not directly control its concentrations. Particulate Pb-oxides from combusted fuel in road runoff are likely to be a major source of Pb in the region. Although Chiricahueto marsh is in a rural catchment and is not affected by high road traffic, the Pb enrichment in the sediments could be explained by
mobilization in the combined sewer systems sewage from Culiaca´n city. About 100,000 vehicles of different types circulate in Culiaca´n city and the surrounding areas, which generate road runoff that is lixiviated by fluvial precipitation, mixed together with municipal effluents and partially transported via the Chiricahueto channel toward the saltmarsh. Additionally, Pb is derived from atmospheric input deposited on agriculture lands and lixiviated from agricultural drain. Similar results were obtained by Ruiz-Ferna´ndez et al. (2001b) in a previous study carried out in sediments from Culiaca´n river estuary, in which it was concluded that the metal levels in the column sedimentary are related to the eroding soil of the watershed and anthropogenic sources. High Ag levels were related to allochtonous organic matter derived from sewage, and other metals such as Cd, Cu and Zn were associated to fertilizers and pesticides used in agriculture as well as domestic wastes. 4.3. Post-depositional processes Although natural processes, such as diagenesis, can produce metal enrichment in surface sediments, the results indicate that the high values for Ag, Cd, Cu, and Zn of the sediments from the Chiricahueto marsh have been generated by anthropogenic activities. The porewater analysis of metals revealed concentrations that suggest a poor mobilization of most metals; in fact, Fe and Mn ions were more enriched with vertical gradients of porewater solutes from 0.33 to 1.9 mg l1 and from 0.21 to 0.75 mg l1, respectively. These gradients represent diffusional effluxes of 0.53 g m2 year1 of Fe and 0.14 g m2 year1 of Mn across the sediment–water interface, while the other metals have significantly lesser fluxes (Soto-Jime´nez, 2002). The relative metal retention in anoxic sediments, likely due to their association with sulfide minerals, indicates that the metal profiles can be used to identify temporal variations in the composition of settling solids. Apparently, the Chiricahueto marsh sediments registered the changes in the input due to anthropogenic activities, which started from the middle of the past century, though unfortunately, independent information about the age of the sediments is not available. Attempts were made to obtain the geochronology of the site by using the radionuclides 210Pb and 137Cs, but neither excess 210Pb nor 137Cs were detected sufficiently in this zone (Ruiz-Ferna´ndez et al., 2001a). On the other hand, the surficial enrichment of Mn in the sediments may indicate diagenetic redistribution controlled by organic matter decomposition, rather than a connection with anthropogenic input. It is well known that bacterial utilization of this metal as an electron acceptor releases Mn ions to porewater solution that are transported upward by diffusion and reincor-
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porated into the sediment, favoring the enrichment of Mn in the upper sediments (Froelich et al., 1979). The identical vertical distribution of Mn and the organic C in the solid phase and the behavior of dissolved Mn+2 -3 in the porewater with a similar trend to NH+ 4 and PO4 profiles produced during organic matter degradation (Soto-Jime´nez et al., 2003), suggest that its behavior is coupled to organic matter oxidation. In addition, the strong association of Pb with Mn suggests that the postdepositional processes produced by the early diagenesis of organic matter can also be responsible, partially or totally, for the enriched Pb concentrations observed in surface layers.
5. Conclusions 1. The results observed in the Chiricahueto marsh sediment core indicate a slight pollution for Ag, Cd, Cu and Zn (and also probably Pb), which are mainly attributed to anthropogenic activities. 2. According to PCA, Cr, Fe, and V (and Co) in the sediment marsh are not derived from the anthropogenic sources, their association with the concentration of Al, Li and mud contents supports their common natural origin. 3. Extensive application of agrochemicals in the Culiaca´n valley is responsible for the enrichment of Cd and partly of Cu and Zn in the sediments from Chiricahueto marsh. Urban wastewater discharges controlled partly the Cd, Cu and Zn inputs, while for Ag and Pb are dominant anthropogenic sources. 4. Although anthropogenic inputs are supposed to be the major source for the increased Ag, Cd, Cu, Pb and Zn concentrations in marsh sediments, such increases, due to diagenetic changes controlled by the redox cycle of Mn could not be excluded in this marsh ecosystem.
Acknowledgements The authors thank H. Bojo´rquez Leyva and R. Garay-Mora´n for their assistance in the laboratory and H.M. Zazueta Padilla, G. Izaguirre-Fierro and G. Ramı´rez-Rese´ndiz for their help during sampling and preparation of figures. Financial support was provided by the grant CONACYT 27953 T from the Consejo Nacional de Ciencia y Tecnologı´a of Mexico.
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Rauret, G., Rubio, R., Lo´pez-Sa´nchez, J.F., Casassas, E., 1988. Determination and speciation of Copper of a river Mediterranean (River Tenes, Catalonia, Spain). Water Research 22, 449–455. Readman, J.W., Liong, W., Kwong, K., Mee, L.D., Bartocci, J., Nilve, G., Rodriguez-Solano, J.A., Gonzalez-Farı´as, F., 1992. Persistent organophosphorus pesticides in tropical marine environments. Marine Pollution Bulletin 24, 398–402. Ruiz-Ferna´ndez, A.C., Hillaire-Marcel, C., Ghaleb, B., Pa´ez-Osuna, F., Soto-Jime´nez, M.F., 2001a. Isotopic constrains (210Pb, 228Th) on the sedimentary dynamics of contaminated sediments from subtropical coastal lagoon (NW Mexico). Environmental Geology 41, 74–89. Ruiz-Ferna´ndez, A.C., Pa´ez-Osuna, F., Hillaire-Marcel, C., SotoJime´nez, M., Ghaleb, B., 2001b. Principal component analysis applied to the assessment of metal pollution from urban wastes in the Culiaca´n river estuary. Bulletin of Environmental Contamination and Toxicology 67, 741–748. Ruiz-Ferna´ndez, A.C., Hillaire-Marcel, C., Ghaleb, B., Soto-Jime´nez, M., Pa´ez-Osuna, F., 2002. Recent sedimentary history of anthropogenic impacts on the Culiaca´n River Estuary, Northwestern Mexico: geochemical evidence from organic matter and nutrients. Environmental Pollution 118, 365–377. Salomons, W., Fo¨rstner, U., 1984. Metals in the Hydrocycle. SpringerVerlag, Berlin. Soto-Jime´nez, M.F., 2002. Procesos geoquı´micos de metales y
nutrientes en ambientes sedimentarios del sistema Altata-Ensenada del Pabello´n, Sinaloa. Tesis Doctoral. ICMyL-UNAM (in Spanish). Soto-Jime´nez, M.F., Pa´ez-Osuna, F., 2001. Distribution and Normalization of Heavy Metal Concentrations in Mangrove and Lagoonal Sediments from Mazatla´n Harbor (SE Gulf of California). Estuarine Coastal and Shelf Science 53, 259–274. Soto-Jime´nez, M.F., Pa´ez-Osuna, F., Bo´jorquez-Leyva, H., 2003. Nutrient cycling at the sediment-water interface and sediments at Chiricahueto marsh: a subtropical ecosystem associated with agricultural land uses. Water Research 37/4, 719–728. Szefer, P., 1990. Mass-balance of metals and identification of their sources in both river and fallout fluxes near Gdansk Bay, Baltic Sea. The Science of the Total Environment 95, 131–139. Szefer, P., Glasby, G.P., Szefer, K., Pempkowiak, J., Kaliszan, R., 1996. Heavy-metal pollution in surface sediments from the southern Baltic Sea off Poland. Journal of Environmental Science and Health 31, 2723–2754. Szefer, P., Kaliszan, R., 1993. Distribution of elements in sediment cores of the Southern Baltic from the point of view of principal component analysis. Studia I Materialy Oceanologiczne NR 64. Marine Pollution 1, 95–102. Szefer, P., Skwarsec, B., 1988. Distribution and possible sources of some elements in the sediment cores of the southern Baltic. Marine Chemistry 23, 109–129.
Geochemical evidences of the anthropogenic alteration of trace metal composition of the sediments of Chiricahueto marsh (SE Gulf of California) M. Soto-Jime´neza, F. Pa´ez-Osunab,*, A.C. Ruiz-Ferna´ndezb b
a Programa de Posgrado en Ciencias del Mar y Limnologı´a, Unidad Acade´mica Mazatla´n, UNAM, Mexico Unidad Acade´mica Mazatla´n, Instituto de Ciencias del Mar y Limnologı´a, UNAM, Apdo. Postal 811, Mazatla´n 82000, Sinaloa, Mexico
Received 19 September 2002; accepted 21 February 2003
‘‘Capsule’’: Principal component analysis allowed separation of natural from anthropogenic origin metals. Abstract Seven sediment cores (60–80 cm) were collected at Chiricahueto marsh, a salt marsh influenced by agrochemical, domestic and industrial effluents. The concentrations of Ag, Al, Cd, Co, Cu, Fe, Li, Mn, Pb, V and Zn were studied in the solid phase at each 1cm section. The profiles of Ag, Cd, Cu, Mn, Ni, Pb and Zn showed a slight recent pollution in the site with enrichment and anthropogenic factors higher than unity; and correlation analysis indicated a direct association with organic carbon. Al, Co, Cr, Fe, Li, and V concentration profiles displayed a negative correlation with organic C and positive with mud content and no consistent enrichment at surface. Based on the principal component analysis and correlation analysis, two principal groups of metals were identified. The first group includes Al, Co, Cr, Fe and Li, that are derived predominantly from the weathering of parent materials in the local bedrock; and the second group include most of the metals, which were relatively enriched at surficial sediments, that are produced mainly by anthropogenic activities such as agriculture (Cd, Cu and Zn), sewage effluents (Ag, Cd, Cu, Ni, Pb and Zn) and in lesser extent atmospheric deposition (Cd and Pb). # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Metals; Agriculture; Sewage; Chiricahueto marsh; Gulf of California
1. Introduction The Sinaloa state lies on the west coast of Mexico beside the Pacific ocean and the Gulf of California. It is the most important national producer in economic activities such as agriculture, fishing and aquaculture. Annually more than 8 million ton of vegetables, fruits and grains are harvested and destined for internal consumption and for exports to the United States. During 1999 total exports from the state were more than 1 billion USD derived principally from the agricultural sector (INEGI, 2000). The Culiaca´n valley represents one third of the agricultural area of the state. Drastic changes in the use of land and water occurred at the middle of the last century, transforming the Culiaca´n valley * Corresponding author. Tel.: +52-669-852845; fax: +52-669-9826133. E-mail address: [email protected] (F. Pa´ez-Osuna).
from a quiet rural hamlet into an area of intensive agriculture. The region was converted in a developing center that resulted in a rapid increasing population growth, from 49,000 inhabitants in the 1950’s to close to 1 million inhabitants at the end of the century (INEGI, 1999). However, as occurred in other countries with emergent economies, the development of the region did not consider the environmental implications with respect to biodiversity and other aspects of ecological change, combined with insufficient environmental protection measures. Pollution problems have been reported in the coastal area adjacent to Culiaca´n valley related to metals, nutrients and pesticides (Pa´ez-Osuna et al., 1992, 1998; Readman et al., 1992; Green-Ruiz and Pa´ezOsuna, 2001; Ruiz-Ferna´ndez et al., 2001a, 2001b, 2002; Soto-Jime´nez et al., 2003). Chiricahueto marsh is a wetland that has been particularly impacted due to excessive conversion of forest in upland areas to cultivated and urbanized lands and
0269-7491/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0269-7491(03)00083-6
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mangrove forest in lowlands to farmlands and shrimp aquaculture facilities. Located on the natural drainage outlet of the agricultural fields (ca. 130,000 ha), Chiricahueto marsh acts as a trap of sediments and other materials, including nutrients and metals, carried through the wastes from culture lands and the polluted runoff from sub-urban and agricultural areas. Additionally, Chiricahueto marsh receives the untreated sewage from Culiaca´n city (14.7103 m3 day1; INEGI, 1999) via a combined sewer system designed to collect rainwater runoff, domestic sewage and industrial wastewater in the same pipe line. The average daily loads of suspended solids of untreated effluents discharged by the Chiricahueto drain are about 3.5 t day1. Another potential source of metals is the discharge of wastewater derived from 14,500 residential septic systems that collect the residues from 127,000 inhabitants established on the drainage basin as well as through atmospheric deposition. The present study investigated the metal pollution in sediments from this anthropogenically altered marsh, using multivariate methods, correlation and principal component analysis (PCA), to define the different origins of these metals (Szefer and Kaliszan, 1993; Callaway et al., 1995; Ruiz-Ferna´ndez et al., 2001b).
2. Material and methods 2.1. Study area Altata-Ensenada del Pabello´n (AEP) is a lagoon system located in the northwestern coast of Mexico (24 200 –24 400 N; 107 300 –107 580 W) with an area of 330 km2 and a drainage basin of 17,700 km2. Chiricahueto lagoon is a wetland marsh type that is part of the AEP (Fig. 1) with an area of ca. 23 km3, characterized by a shallow marsh (< 0.5 m depth) and limited water exchange with the adjacent Ensenada del Pabello´n lagoon. Average annual precipitation values for the region is 673 mm, mostly occurring in the summer months. Wetland water losses to the atmosphere occur from water surfaces and moist soil surfaces (evaporation) and from the interior of the emergent portions of plants (transpiration); they have been estimated as 1500 mm per year (De la Lanza-Espino and Flores-Verdugo, 1998). 2.2. Sample collection and processing Seven undisturbed cores (60–80 cm depth) were taken from an intertidal marsh flat in April–June 1999 (Fig. 1). The cores were immediately transported to the laboratory and the supernatant liquid was collected (about a 5-cm thick layer above the water–sediment interface). Two sediment cores were sectioned at 1-cm intervals, lyophilized (72 h at 40 C and 36103 mBar) and mechanically homogenized with a mortar and pestle.
One core was sectioned at 1-cm intervals and treated with H2O2 for textural analysis. The remaining cores were used for radionuclide examination (Ruiz-Ferna´ndez et al., 2001a) and porewater analysis (Soto-Jime´nez et al., 2003). 2.3. Analytical procedure Grain size was determined separating the fine silt and clay from the coarse fraction (sand) of sediments by wet sieving using a 63 mm mesh stainless steel sieve. The contents of silts and clays were evaluated by pipette analysis (Folk, 1974). Sediment porosity was estimated from the weight loss of the sediment after drying and assuming a specific weight of 2.65 kg dm3. The carbonate content was determined by treating the sediments with 1 M HCl, and the excess of acid was quantified by back-titration with 0.5 M NaOH using phenolphthalein as an indicator (Rauret et al. 1988); organic C content (OC) was analyzed by chromic acid oxidation (Loring and Rantala, 1992). Sediment samples were digested with HCl, HNO3 and HF according to the procedure reported by Loring and Rantala (1992). Samples (0.15 g) of dried, ground and homogenized sediment from each section were transferred into Teflon vessels (100 ml), digested with 1 cm3 of inverted aqua regia (HNO3:HCl, 1:3 v/v) and 6 cm3 HF in a microwave oven (CEM, MSD 2000) (two steps of 30 min at 100% power). Samples digested were transferred into the polypropylene tubes containing 5 g of boric acid and diluted to 50 cm3 with water Milli-Q. Aluminum, Cd, Cr, Co, Pb and V were analyzed by graphite furnace atomic absorption spectroscopy (Varian GTA 110 GFAAS). Metals such as Cu, Fe, Mn, Ni and Zn were determined by flame atomic absorption spectroscopy (Varian SpectrAA 220 AAS). Analytical performance was evaluated with standard reference materials: polluted marine sediment IAEA-356 (IAEA, 1994) and marine sediment BCSS-1 (Loring and Rantala, 1992). Metal recovery depending on element varied between 90 and 104%. Analytical precision of replicates (n=6) varied from 4 to 11% relative to standard deviation. 2.4. Data processing The PIRLA procedure (Binford, 1990) was applied to identify the metal background levels from the elemental profiles. This method involves the calculation of the mean (x) and standard deviation () of the apparently constant values found at the bottom of the core; if the next higher concentration was minor than the mean value plus one standard deviation (x+), then this concentration was included in the set of constant values and the mean and standard deviation were recalculated. The procedure continued until the concentration in the
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Fig. 1. Location of the study area. Chiricahueto salt marsh in the Altata-Ensenada del Pabello´n lagoon system.
next higher level was greater than x+. The asymptotic levels estimated were assumed as the geochemical background concentrations for the different elements in the marsh. Considering that the textural characteristics of sediments can have an influence on the concentration of metals, an enrichment factor (EF) was calculated by using the ratio between the metal concentration of the sample and the average metal concentration of the earth’s crust (Salomons and Fo¨rstner, 1984; Szefer et al., 1996; Soto-Jime´nez and Pa´ez-Osuna, 2001). Additionally, in order to evaluate the human-caused changes registered in the Chiricahueto marsh sediments, an anthropogenic factor (AF) was calculated (Szefer and Skwarzec, 1988; Szefer and Glasby, 1996) as the quotient of the mean metal concentration at the surface (0– 5 cm) and the geochemical background metal con-
centrations estimated with the PIRLA procedure described before. Finally, in order to identify the provenance and distribution of the metal load, a correlation analysis in combination with principal components analysis (PCA) were applied on the entire data by using Statistica for Windows version 4.5 (StatSoft Inc., USA) (Table 1).
3. Results A description of the sedimentary column and the vertical distribution patterns for metals and other sediment proprieties are shown in Figs. 2 and 3. The results obtained correspond to the average of two cores examined, only the 70–80 section was individually analyzed. A very thin brown oxidized layer was identified in the top of the sediment (< 1 cm depth), while in the down-
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Fig. 2. Description of the sedimentary column, and mud, carbonates, Fe, Al, Li, Cr, Co and V profiles in sediment cores from Chiricahueto marsh.
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Fig. 3. Organic carbon content, Ag, Cd, Cu, Mn, Ni, Pb and Zn profiles in sediment cores from Chiricahueto marsh.
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Table 1 Mean metal concentrations in surface (0–5 cm) and bottom layers ( >52 cm) of the profile, average composition of the crust and enrichment (EF) and anthropogenic (AF) factors Metala
Al Ag Cd Co Cr Cu Fe Li Mn Ni Pb V Zn
Sediment concentrations
Factors
Earth crustb
Bottom ( <52 cm)
Surface (0–5 cm)
69 0.8 0.2 13 71 32 36 42 720 49 16 97 127
36.1 5.4 0.2 0.1 0.3 0.1 8.2 0.7 44 7.2 26 5.2 35 2.7 28 4.2 477 31 21 2.3 10 1.0 82 4.2 79 17
354.9 3.90.7 1.20.3 8.91.0 453.9 602.8 292.6 232.8 1256121 423.5 182.6 452.8 13414
EFAl
AF
4.9 6.0 0.7 0.6 1.9 0.8 0.6 1.7 0.9 1.1 0.5 1.1
25.1 4.3 1.1 1.0 2.3 0.8 0.9 2.6 2.0 1.8 0.5 1.7
a Metal concentrations are in mg kg1, except Al and Fe, which are in g kg1. b Martin and Maybeck (1979).
core the prevalence of reducing conditions was evident by the dark gray to black with strong odor of H2S. Observations of the irrigation and bioturbation were neglected because no tube dwellings were observed and no animals were found in the sediment. The sediments were mainly composed of silts and clays (< 63 mm) with a mud content greater than 96% formed by the settlement of fine particulate mainly transported with the material fluvial from agricultural operations. The mud content profile is very regular and does not display any trend with depth, which underlines the good sediments column homogeneity. Low CaCO3 contents (0.3–2.2%) with an irregular downcore increase was observed in the sedimentary column. The organic C in the solid phase ranged from 0.4 to 4%, the highest concentrations were consistently observed in the upper layers showing an exponential decrement with depth. The kinetic of the decomposition and the diagenesis of the organic matter has been discussed by Soto-Jime´nez et al. (2003). It is clear that the diagenetic processes are mainly controlled by the exponential decomposition of organic matter that takes place at the sediment–water interface and in the sedimentary column, under oxic and suboxic-anoxic conditions, respectively. The vertical distribution patterns for Al, Fe, Li, Cr and V showed a slight decrease towards the surface. The profiles of Ag, Cd, Cu, Ni, Pb, and Zn presented a concentration increment from low or background values in the bottom-most layer, followed by fluctuations and surface or subsurface peak values. This pattern is more or less the same as Mn, which showed the highest values in the surface oxidized zone and decreasing concentrations with depth.
PIRLA procedure revealed relatively little variation with depth and low concentration of nutrients (SotoJime´nez et al., 2003) and of most metals in sediments deposited below 52 cm depth. The average concentrations for each element were estimated from these low and relatively constant levels and established as the regional background values (Table 1). Compared with the background levels in the earth’s crust (Martin and Maybeck, 1979), the regional baseline values estimated for metals were significantly lower, except for Cd and Fe (P < 0.05, Table 1). When the baseline levels here proposed are compared with those obtained for rocky samples collected in the drainage basin (Green-Ruiz, 2000), Ag, Cd, Co, Mn and Pb have comparable levels, while Cr, Cu, Fe, Li, Ni, V and Zn showed concentrations more elevated. Above the 52-cm section, the concentrations of most metals and organic C displayed a similar trend to increase upwards with peaks in the subsurface and surface sediments. The metal concentration averages in the surface section (0–5 cm) are also listed in Table 1. Mean enrichment factors in surface sediments using Al as normalizer element show that Fe, Ni, Pb and Zn are close to unity and Co, Cr, Li and V below unity (Table 1). Additionally, the AF values of the Co, Cr, Fe, Li and V were near unity. Both Efs and AFs for surface sediments of the study area were much greater than 1.0 for Ag, Cd, Cu and Mn, suggesting a significant enrichment for these metals in recent times. In the case of Ni, Pb and Zn the enrichment in surficial layers was evident with AFs higher than unity, although the EFs were close to unity due to differences between the background levels in continental crust and natural levels in the region. There is a good correlation between organic C and most of metals (Ag, Cd, Cu, Mn, Ni, Pb and Zn), with high values of Pearson’s correlation coefficients (P < 0.05). Obviously, there is also a very significant relationship among concentrations of these metals (Table 2). A significant negative correlation was observed between Al and Fe with the OC but positive with mud content. Cobalt only correlated directly with Pb and inversely with Ag, Cd and V. In the case of carbonate content there was no significant linear association with any element. After checking the suitability of the dataset for factor analysis, a principal component analysis with Varimax rotation was run. An eigenvalue less than unity was chosen as the break off test for the factors. Based on the loading distribution of the variables in PCA (accounting for 77.2% of total variance), two principal groups were extracted (Fig. 4). The first group includes a strong association between Al, Fe and Li and mud content, whereas Ag, Cd, Cu, Mn, Ni, Pb and Zn together with OC constitute a second group enriched in the surface layers. Co, Cr, V and CaCO3 cannot be clearly included in any of these groups.
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Table 2 Pearson correlation matrix for the metal concentrationsa
OC Ag Al Cd Co Cr Cu Fe Li Mn Ni Pb V Zn a
OC
Ag
1.00 0.95 0.71 0.74 0.34 0.36 0.75 0.70 0.32 0.69 0.81 0.53
1.00 0.69 0.76 0.37 0.40 0.84 0.78 0.32 0.74 0.82 0.55
0.50
0.65
Al
1.00 0.58 0.53 0.46 0.79 0.56 0.68 0.48 0.46
Cd
1.00 0.69
Co
Cr
Cu
Fe
1.00 0.32 0.39
1.00 0.78
Li
Mn
Ni
Pb
V
1.00 0.65 0.36
1.00
Zn
1.00
0.48 0.55 0.29 0.41 0.37
0.58 0.37 0.41
0.48 0.65
0.50
0.70 0.90 0.75 0.46 0.83
1.00 0.40 0.79 0.68 0.66 0.51 0.55
1.00 1.00 0.65 0.70 0.59 0.39
1.00 0.77 0.66
1.00
All correlations are significant at P<0.05.
4. Discussion 4.1. Metals from natural source The significant correlation coefficients found between element concentrations, being in agreement with the principal component analysis results, indicates that there are two main groups of elements in the sediments. The first formed by Al, Fe and Li (and possibly Cr and V) related to mud content, and the other constituted by the rest of metals, except Co, closely associated to organic carbon. These associations between elements were used to establish the major sources that control their profiles into the sedimentary column in Chiricahueto marsh. For example, it is clear that Al, Fe and
Li are elements derived from the weathering of parent materials in the local bedrock and reflect the granular variability in the sediments (Martin and Maybeck, 1979; Szefer, 1990; Soto-Jime´nez and Pa´ez-Osuna, 2001). For this reason, the selection of Al as a reference element for the estimation of enrichment factors was correct. Chromium and V with profiles similar to Al, Fe and Li with the EF < 1, are metals attribute commonly to lithogenic components, but the separation observed for both metals in PCA results may indicate a partial remobilization out of the solid phase controlled by organic matter degradation in reducing conditions (Soto-Jime´nez and Pa´ez-Osuna, 2001). Iron is an element that in the reducing conditions can be mobilized during diagenesis, however, for the sediments studied in this work it was not clearly evidenced as is explained in a posterior section. The levels of Co appeared to be uniformly distributed and the levels were generally low and did not indicate any significant input, thus are derived predominantly from natural source lithogenous material. 4.2. Metals from anthropogenic sources
Fig. 4. PCA results: plot of loading of the three first components obtained in the analysis.
High levels of total Ag in the surface sediments, characterized by EF > > 1, are mainly related to the discharges of untreated urban wastewater (Bryan and Langston, 1992; Hornberger et al., 1999). Any other source is clearly identified for this element. In the case of Cu, two sources can be distinguished, one is due to the application of Cu fungicide, as CupravitR (Cu2(OH)3Cl) or other similar compounds in the tomatoes cultivates (Lycopersicon esculentum Mill) that in the Culiaca´n valley is greater than 20,000 ha. Given that the dose of this product (concentration 85%) varies between 1 and 2 kg ha1 per cycle, the annual input of Cu to the agriculture soils ranges between 0.3 and 1.2 kg ha1, which are
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potentially exposed to be transported to the sediment marsh by fluvial load. Another source is the untreated sewage, as has been previously mentioned in other studies (e.g. Bothner et al., 1994; Alloway, 1995; SotoJime´nez and Pa´ez-Osuna, 2001). The influent Cu concentrations from year 2000 in the municipal effluents ranged from < 0.1 to 0.19 mg l1 (Anonymous, 2001), which give a net input into marsh of about 550–1020 kg each year considering a discharge constant over time in the last recent years. The AF and EF values of Cd for recent sediments of the study area suggest a significant anthropogenic input for this metal, which has been primarily associated to the application of high amounts of phosphate fertilizers produced from phosphate rocks containing from 3 to 150 mg Cd/kg (de Meeuˆs et al., 2002). The application rates of fertilizers as simple or triple super-phosphate calcite to the agricultural lands is a common practice in different crops in the region (INIFAP, 1997): tomatoes, sugar-cane, corn, soybeans, chili-pepper and others, and vary from 30 to 150 kg ha1. For illustrative purposes it was considered that the concentrations in phosphatefertilizers range from 30 to 100 mg Cd kg1 and an annual application rate of 100 kg P2O5, this is equivalent to an applied loading of 3–10 g Cd ha1 year1. Other sources of Cd to marsh sediments include the discharge of domestic effluents ( < 0.066 mg l1) estimated as < 350 kg year1 and the atmospheric deposition that were not evaluated in this work. Another metal derived mainly from agriculture activities is Zn, which results from the direct application of chemicals to the cultivated lands as pesticide and fertilizers (Fo¨rstner and Wittmann, 1979). Zn inputs via the application of fertilizers were estimated between 70 and 220 g ha1 year1, considering a concentration of 1.45 g Zn kg1 P2O5 (Alloway, 1995). In addition the application of the etilen-bis-dithiocarbamate, that contain zinc salts [(C4H6 Mn Zn N2S4)], is common in the region for control of plagues in different crops (INIFAP, 1997). It was estimated that the application rate of 1–2 kg ha1 of a formulation to 80% (20% w/w of Zn), generates an input rate between 160 and 320 g ha1 year1. Similarly, to Ag and Cu, other significant source of Zn is the untreated sewage from Culiaca´n city. This source was estimated as about 970 kg Zn year1 discharged directly to the Chiricahueto marsh, considering an average annual concentration of 0.18 mg Zn l1 in the wastewater (Anonymous, 2001). Lead in the surface sediments (0–1 cm) was found to be enriched relative to crustal material indicating that continental weathering does not directly control its concentrations. Particulate Pb-oxides from combusted fuel in road runoff are likely to be a major source of Pb in the region. Although Chiricahueto marsh is in a rural catchment and is not affected by high road traffic, the Pb enrichment in the sediments could be explained by
mobilization in the combined sewer systems sewage from Culiaca´n city. About 100,000 vehicles of different types circulate in Culiaca´n city and the surrounding areas, which generate road runoff that is lixiviated by fluvial precipitation, mixed together with municipal effluents and partially transported via the Chiricahueto channel toward the saltmarsh. Additionally, Pb is derived from atmospheric input deposited on agriculture lands and lixiviated from agricultural drain. Similar results were obtained by Ruiz-Ferna´ndez et al. (2001b) in a previous study carried out in sediments from Culiaca´n river estuary, in which it was concluded that the metal levels in the column sedimentary are related to the eroding soil of the watershed and anthropogenic sources. High Ag levels were related to allochtonous organic matter derived from sewage, and other metals such as Cd, Cu and Zn were associated to fertilizers and pesticides used in agriculture as well as domestic wastes. 4.3. Post-depositional processes Although natural processes, such as diagenesis, can produce metal enrichment in surface sediments, the results indicate that the high values for Ag, Cd, Cu, and Zn of the sediments from the Chiricahueto marsh have been generated by anthropogenic activities. The porewater analysis of metals revealed concentrations that suggest a poor mobilization of most metals; in fact, Fe and Mn ions were more enriched with vertical gradients of porewater solutes from 0.33 to 1.9 mg l1 and from 0.21 to 0.75 mg l1, respectively. These gradients represent diffusional effluxes of 0.53 g m2 year1 of Fe and 0.14 g m2 year1 of Mn across the sediment–water interface, while the other metals have significantly lesser fluxes (Soto-Jime´nez, 2002). The relative metal retention in anoxic sediments, likely due to their association with sulfide minerals, indicates that the metal profiles can be used to identify temporal variations in the composition of settling solids. Apparently, the Chiricahueto marsh sediments registered the changes in the input due to anthropogenic activities, which started from the middle of the past century, though unfortunately, independent information about the age of the sediments is not available. Attempts were made to obtain the geochronology of the site by using the radionuclides 210Pb and 137Cs, but neither excess 210Pb nor 137Cs were detected sufficiently in this zone (Ruiz-Ferna´ndez et al., 2001a). On the other hand, the surficial enrichment of Mn in the sediments may indicate diagenetic redistribution controlled by organic matter decomposition, rather than a connection with anthropogenic input. It is well known that bacterial utilization of this metal as an electron acceptor releases Mn ions to porewater solution that are transported upward by diffusion and reincor-
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porated into the sediment, favoring the enrichment of Mn in the upper sediments (Froelich et al., 1979). The identical vertical distribution of Mn and the organic C in the solid phase and the behavior of dissolved Mn+2 -3 in the porewater with a similar trend to NH+ 4 and PO4 profiles produced during organic matter degradation (Soto-Jime´nez et al., 2003), suggest that its behavior is coupled to organic matter oxidation. In addition, the strong association of Pb with Mn suggests that the postdepositional processes produced by the early diagenesis of organic matter can also be responsible, partially or totally, for the enriched Pb concentrations observed in surface layers.
5. Conclusions 1. The results observed in the Chiricahueto marsh sediment core indicate a slight pollution for Ag, Cd, Cu and Zn (and also probably Pb), which are mainly attributed to anthropogenic activities. 2. According to PCA, Cr, Fe, and V (and Co) in the sediment marsh are not derived from the anthropogenic sources, their association with the concentration of Al, Li and mud contents supports their common natural origin. 3. Extensive application of agrochemicals in the Culiaca´n valley is responsible for the enrichment of Cd and partly of Cu and Zn in the sediments from Chiricahueto marsh. Urban wastewater discharges controlled partly the Cd, Cu and Zn inputs, while for Ag and Pb are dominant anthropogenic sources. 4. Although anthropogenic inputs are supposed to be the major source for the increased Ag, Cd, Cu, Pb and Zn concentrations in marsh sediments, such increases, due to diagenetic changes controlled by the redox cycle of Mn could not be excluded in this marsh ecosystem.
Acknowledgements The authors thank H. Bojo´rquez Leyva and R. Garay-Mora´n for their assistance in the laboratory and H.M. Zazueta Padilla, G. Izaguirre-Fierro and G. Ramı´rez-Rese´ndiz for their help during sampling and preparation of figures. Financial support was provided by the grant CONACYT 27953 T from the Consejo Nacional de Ciencia y Tecnologı´a of Mexico.
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