Journal of Environmental Radioactivity 69 (2003) 129–143 www.elsevier.com/locate/jenvrad
The loading history of trace metals and nutrients in Altata-Ensenada del Pabello´n, lagoon complex, northwestern Mexico A.C. Ruiz-Ferna´ndez a,∗, F. Pa´ez-Osuna a, M. Soto-Jime´nez c, C. Hillaire-Marcel b, B. Ghaleb b a
ICMyL-UNAM. Calz. Joel Montes Camarena s/n, Cerro del Cresto´n, Playa Sur, 82040 Mazatla´n, Mexico b GEOTOP-UQAM, 201 President Kennedy, PK 7150, Montreal, Qc., H2Y 3X7, Canada c DGEP-UNAM, Calz. Joel Montes Camarena s/n, 82040 Mazatlan, Mexico Accepted 17 March 2003
Abstract This paper summarizes the geochemical investigations about the origin and loading history of some trace metals (Ag, Cu and Zn) and nutrients (N and P) in the coastal lagoon complex of Altata–Ensenada del Pabello´n, Mexico, by using the radioactive chronometers 210Pb and 228 Th and the stable isotopes of C and N. The examination of sediment cores collected at different locations in the lagoon system identified a slight enrichment in metals and nutrients in some points, which was mainly associated to organic matter accumulation. Stable C and N isotope ratios revealed wastewater inputs to the lagoon system and the 210Pb geochronology showed that anthropogenic impact started 50 years ago, with the beginning of the agriculture development and the associated urban growth of the surrounding area. Several atypical 210Pb and 228Th/232Th profiles demonstrated that biological and physical disturbances are common phenomena in these environments, that frequently mask the pollution records; and therefore, considering that the contaminated sediments at some locations in the lagoon system are frequently resuspended and re-oxygenated, the pollutants will continue to be easily remobilized in the food chain. 2003 Elsevier Science Ltd. All rights reserved. Keywords: 210Pb;
∗
13
C;
15
N; Coastal lagoon sediments; Trace metals; Nutrients
Corresponding author. Tel.: +52-669-985-28-45; fax: +52-669-982-61-33. E-mail address:
[email protected] (A.C. Ruiz-Ferna´ndez).
0265-931X/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0265-931X(03)00091-2
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1. Introduction
Coastal waters are particularly vulnerable to pollution by organic wastes from domestic (sewage) and industrial sources, and in some cases, contaminant inputs can be of very serious concern for economically important fisheries. As sediments reflect conditions in the water column, geochemical studies in the sediments can be used for rapid e integrative measurements of pollutants, providing information about temporal trends and provenance of contaminant inputs to aquatic systems. Organic carbon and nitrogen stable isotope ratios (δ13C and δ15N) are natural source indicators of sedimentary particulate organic matter. They can be used to identify organic matter provenance in estuarine and near-shore marine environments (Burnett and Schaeffer, 1980; Sweeney and Kaplan, 1980; Thornton and MacManus, 1994), allowing the discrimination of the origin of those pollutants associated to the sedimentary organic material. 210 Pb (t1/2=22.3 y) generally provides a reliable method of dating sediments deposited over the last 100–150 years (Krishnaswami et al., 1971). 210Pb is found in sediment minerals firstly in a supported form by production in situ from 238U or 226 Ra decay, at equal activity to its parent nuclides which is normally assumed constant down a core profile. The second fraction, the unsupported or excess 210Pb (210Pbxs) has an atmospheric origin, being produced there by decay of 222Rn which emanates from soils after the decay of 226Ra. Atmospheric 210Pb enters to the aquatic environments by direct deposition in rain and also by run-off and erosion from the catchment (Baxter et al., 1981). Geochronology with 210Pb is based on the pattern of radioactive decay of 210Pbxs with depth in the sedimentary column, and the exponential law describes this decay through time as follows (Appleby, 1998): 210
Pb(z) ⫽
Pb(0) e⫺lt,
210
(1)
where 210Pb(0) is the surface activity, 210Pb(z) is the activity at depth z, and l is the decay constant (0.03114 y⫺1). This equation assumes a constant supply of 210Pb and particles to the sediments, as well as a negligible post-depositional migration of 210Pb within the column. However, in areas where the 210Pb distribution is open to alternate interpretations, the use of two or more radionuclides, with different half-lives and different input histories, allow a better characterization of the sediment reworking than it is possible with just one tracer (Robbins and Edgington, 1975; Anderson et al., 1987; Legeleux et al., 1994). 228 Th /232Th activity ratios have also been used to obtain reliable accumulation rates for coastal marine sediments for periods up to a decade, and may be especially valuable in environments of rapid deposition (mm to cm y⫺1). The technique depends upon the removal of a chemically reactive decay product of 228Ra from the overlying water column, and the presence of excess 228Th (228Thxs=228Th/232Th⬎1.0) in the uppermost layers of the deposit is indicative of material accumulation over the last ten years (Koide et al., 1973).
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In most cases, estuaries and coastal lagoons are well suited for the study of the sedimentary record over the last decades to thousands of years, including the history of pollution, providing that they are not disturbed. However, steady-state deposition in coastal environments is often interrupted due to episodic events and the sedimentary patterns become much more complicated. In such conditions, it is difficult to distinguish episodic deposition from other sediment disturbances (i.e. physical mixing or bioturbation) and consequently, to predict the influence of these phenomena on the distribution of the pollutants transported by the sediments. The aim of this paper is to demonstrate the usefulness of radioactive and stable isotopic tracers coupled to pollution studies at the coastal environment in subtropical latitudes, where these techniques have been scarcely used. It summarizes the investigations about the origin and temporal changes in nutrients and trace metal inputs reflected in the sedimentary record at the coastal lagoon system of Altata–Ensenada del Pabello´ n, Mexico.
2. Methods 2.1. Study site The estuarine complex of Altata–Ensenada del Pabello´ n (AEP) is located in the Pacific coast of Mexico (24°18–24°40 N, 107°27–108°00 W) (Fig. 1). AEP extends over an area of 360 km2 and is integrated by two main shallow basins (Altata and Ensenada del Pabello´ n, with 75 and 232 km2, respectively) and the inner lagoons of Caimanero (3 km2), Bataoto (2 km2) and Chiricahueto (23 km2), which are the remnants of ancient estuarine–lagoon environments that developed to the North of Ensenada del Pabello´ n lagoon, and are currently connected with the lagoon system by meandrous channels (Ayala-Castan˜ ares et al., 1994). The AEP complex is surrounded by the valley of the Culiaca´ n River, which is a 17,000 km2 coastal plain containing more than 130,000 ha of intensive and irrigated agricultural lands, that consume large amounts of fertilizers and pesticides and whose wastes drain towards the AEP lagoon system, through several drainage channels and surface runoff. Ensenada del Pabello´ n lagoon is a shallow basin (苲1.5 m depth) with estuarine conditions (salinity ranges from 10 to 28‰) mostly due to the freshwater input of the Culiaca´ n River (3.3×109 m3 y⫺1 mean annual flow) (Carvalho et al., 1996). The lagoon supports an important fishing activity of shrimp (Penaeus spp), oysters (Crassostrea spp), clams (Chione spp) and fish (i.e. Mugil spp, Gerres spp, Lutjanus spp); it sustains a very high biodiversity; serves as a nursery area for shrimp and fish, as a refuge for a large number of migratory birds and some endangered species, as well as provide an attractive scenario for tourism (Carvalho et al., 1996). Ensenada del Pabello´ n lagoon also receives the effluents from at least 10 aquaculture shrimp farming facilities and the sewage from the surrounding villages (100,000 inhabitants) and the cities of Culiaca´ n and Navolato (750,000 and 50,000 inhabitants, respectively). Ensenada del Pabello´ n lagoon was classified by the Mexican government as one of the 15 most polluted basins in the country (PEF, 1996) and currently
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Fig. 1. Sampling stations in the Altata–Ensenada del Pabello´ n lagoon system. Sediment cores were collected in 1999.
has been included among the Marine Priority Areas for Conservation by the Mexican National Commission for Biodiversity. 2.2. Sampling and analytical methods Seven sediment push-cores (up to 80 cm long), were collected with a plastic tube (7 cm inner diameter), between November 1998 and June 1999, in the following locations at the AEP lagoon system: (a) Culiaca´ n River estuary (cores RC and ERC at the upper and lower zones, respectively); (b) el Brinco (BRI); (c) el Castillo (EPC); (d) las Iguanas (EPI); and the inner lagoons (e) Chiricahueto (CHI) and (f) Caimanero (CAI). Sediment samples were freeze-dried and ground in an agate mortar before analysis. 2.3. Geochemical analysis Concentration of carbonates (CaCO3) was analyzed by using a colorimetric procedure in which the sediment sample was treated with HCl 1 N and CaCO3 was
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measured by titration of the acid excess with NaOH 0.5 N (Stuardo and Villarroel, 1976). Organic carbon (OC) was analyzed by using a colorimetric procedure in which the sample was treated with a mixture of K2Cr2O7+Ag2SO4+H2SO4 (El-Rayis, 1985) and the OC content was measured by titration of the excess of the oxidant mixture with Fe(NH4)2(SO4)2 (Loring and Rantala, 1992). Replicate analysis (n=6) provided variation coefficients in a range of 0.4–6.3% for CaCO3 analysis and from 0.5 to 4.8% for OC. Analyses of metals were made by atomic absorption spectrophotometry: (a) Mn, Al and Fe by flame and (b) Ag, Cu and Zn by graphite furnace; after digestion of the sediments with a mixture of HNO3–HCl–HF (Loring and Rantala, 1992). Precision for major elements was between 2 and 5% of the total amount (n=7). 2.4. Radiochemical analysis 210
Pb was determined by alpha counting of 210Po deposited onto silver discs (Flynn, 1968; Schell and Nevissi, 1983; Hamilton et al., 1994) using 209Po as yield tracer. The 226Ra-parent supported 210Pb (210Pbsup) activities in the cores were estimated by mean of three different approaches: (1) Thermal Ionization Mass Spectrometry (TIMS) using 228Ra as yield tracer and chromatographic resins for the chemical isolation of radium (Joffroy et al., 1998); (2) gamma-spectrometry, based on the measurement of 214Bi (609.3 keV) photopeaks, in samples having been stored for 21 days prior to analysis; and (3) alpha-spectrometry, by the analysis of 230Th, assuming secular equilibrium with its daughter 226Ra. 228Th, 230Th, 232Th were estimated by alpha-spectrometry; thorium and uranium isotopes were separated by using chromatographic resins (Edwards et al., 1986) and subsequently, electrodeposited on stainless steel discs in alcoholic media (Gaven, 1982). The sediment dissolution was performed by adding 0.5 g of sediment to a mixture of 1:1:0.5 HNO3+HCl+HF and heating at 200 °C overnight in closed TeflonPFA containers. The acid mixture was evaporated and the residue separated by centrifugation in HCl 0.5 N. 137 Cs was measured by γ-ray spectrometry. The sediment samples were weighed in glass vials (8×40 mm, 1 ml capacity) and each sample was counted on a HPGe well-detector for a minimum of 48 h to obtain an error counting minor to 10%. Replicate analyses (n=12) of the standard reference material IAEA-300 (Radionuclides in Baltic sea sediment) confirm good agreement of activities determined for 210Pb and 137Cs. For 210Po analytical method, accuracy was estimated to be 99%, precision 4.6% and the limit of detection of 0.01 dpm g⫺1.
3. Results and discussion 3.1. Radionuclides Total 210Pb (210Pbtot) profiles obtained from the sediment cores examined are presented in Fig. 2. Excepting the core RC, none of the profiles exhibited a typical exponential decay and, in most cases, the highest 210Pb levels were found at subsur-
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Fig. 2. Total 210Pb stratigraphic profiles for the cores analyzed. Statistical counting uncertainties are smaller than the symbol size (⬍4%).
ficial layers. One interesting aspect of our data is the extremely low 210Pbtot activities (ⱕ4.5 dpm g⫺1) found in all the cores examined. Due to the atmospheric 210Pb fluxes are known to be regulated by the climate features of a region i.e. the scarcity of rainfalls and the oceanic origin (depleted in 210Pb) of the dominant air mass (Preiss et al., 1996), low activities found in the AEP system were related to low atmospheric 210 Pb fluxes, since AEP is located in a semiarid region where rainfalls are highly seasonal and the wind generally blows from the NW. 210 Pbsup activities (dpm g⫺1) determined for each core were: CAI, 1.49±0.07; CHI, 1.59±0.04; ERC, 1.75±0.04; RC, 1.71; EPC, 1.74±0.04; EPI, 1.48±0.04 and BRI, 1.25±0.07. These values were subtracted from the 210Pbtot activities at its corresponding core to estimate the excess 210Pb activities (210Pbxs) at each sediment layer, which were generally lower than 1 dpm g⫺1 for the seven cores studied. As is commonly
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found in the coastal environments, none of the cores exhibited the typical exponential 210 Pbxs profile and most of them showed intermediate layers depleted in 210Pbxs. 210 Pbxs depth distributions of cores CHI and EPC were rather flat, showing values very alike to those determined as 210Pbsup, suggesting the absence of recent sedimentation. For core CHI, the lack of 210Pbxs was explained as the result of Chiricahueto lagoon dried up and filled after the migration of the Culiaca´ n River (Ayala-Castan˜ ares et al., 1994) and due to the lack of a water sheet during nearly all the year, the particles with high content of 210Pb likely just bypassed the area to lower grounds (Chung and Chang, 1995). In the case of core EPC, that was taken in the sub-aquatic deltaic plain of the Culiaca´ n River, the lack of recent accretion was explained on the basis of sea bed scouring resulting from the exposition of sediments to sub-aerial erosion during low tides (Ayala-Castan˜ ares et al., 1994). Core CAI 210Pbxs profile was explained as the result of biological mixing, especially by burrowing fauna, which have the effect of homogenizing the reaches of the sea bed (Nittrouer et al., 1979), although no evidence of organisms were observed. Another possibility could be that CAI 210Pb profile has been created under conditions of a constant sediment accumulation rate but with changes in the specific activity of deposited sediments, due to variations in the 210Pb flux to the sedimentary column, likely in response to quasi-regular fluctuations in sediment type or water mass transport (Carroll et al., 1995). This could be possible considering the cyclic variations in volume and kind of water supply that Caimanero lagoon receives each year, i.e. fresh water runoff during the rainy season, and salt water that fisherman introduce to the lagoon during the dry season, by using a floodgate connected to Ensenada del Pabello´ n lagoon, in order to compensate losses by evapotranspiration to maintain the conditions for the culture of tilapia. ERC and BRI 210Pbxs profiles exhibited a more chaotic distribution, likely the result of resuspension events, with subsurface maximums and some layers showing a deficit of 210Pb, with values sometimes even minor to its corresponding 210Pbsup activity. These profiles are likely promoted by waves and currents, and since resuspension operates on the top of the sedimentary column, probably the most recently deposited sediments were eroded, transported and finally redeposited in the estuary. As the floods proceeded, increasingly older sediments, progressively depleted in 210 Pb, were deposited in the estuary, along with material eroded from the river banks and the land surface. As sediments are depleted in 210Pb, their greater contribution to the core would have “diluted” the 210Pb surficial activities (Hirschberg and Schubel, 1979). In cores ERC and BRI, the highest levels of 210Pbxs were found above of a layer totally depleted in 210Pbxs, which was identified to be composed by reworked old sediments, likely re-deposited during one single resuspension event, possibly triggered by stormy conditions. In order to confirm the presence of fresh, “young” sediments in the sedimentary record, 228Th (t1/2=1.9 y) and 232Th (t1/2=1.4×1010 y) activities were measured (Bruland et al., 1981) in a few samples from cores CAI, CHI and ERC (Table 1). The 228Th/232Th ratios obtained from cores CAI and CHI were very alike and rather close to 1.0. In the case of core CAI, where 210 Pbxs profile accounts for the presence of recent deposited material, the lack of excess of 228Th could be explained on the basis of extensive bioturbation, since
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Table 1 Radiochemical data for some cores collected in AEP lagoon system Depth (cm)
Activity (dpm g⫺1) 210
ERC 0.5 1.5 2.5 3.5 5.5 7.5 9.5 11.5 13.5 15.5 23.5 25.5 29.5 39.5 49.5 59.5 69.5 79.5 CAI 0.5 4.5 9.5 15.5 19.5 25.5 29.5 39.5 49.5 59.5 69.5 79.5 CHI 0.5 1.5 2.5 4.5 5.5 7.5 9.5 14.5 16.5 24.5 29.5 39.5 49.5 59.5
Pbxs
1.06 1.16 1.28 1.44 1.81 2.71 ⫺0.26 1.19 1.00 1.20 0.74 0.51 0.87 0.89 0.96 1.27 ⫺0.20 0.00 1.16 1.10 0.97 1.30 1.40 1.51 1.38 0.65 0.67 0.42 0.84 1.30 ⫺0.05 ⫺0.08 ⫺0.10 0.11 0.15 0.10 ⫺0.27 0.00 0.14 ⫺0.03 ⫺0.24 ⫺0.08 ⫺0.14 0.00
228
Th/232Th
1.46
1.64 1.15
1.13 1.25
1.17
1.09 1.04 1.13
1.18
1.21
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mixing dilutes the observable effects of deposition of young particles (Bruland et al., 1981); while in core CHI, the lack of excess 228Th confirmed that the sediments in this site are rather old. ERC, it was found an excess of 228Th (1.46–1.64) at the upper surface layers, which indicated the presence of fresh sediments. As mentioned above, the highest values of 210Pbxs and 228Th/232Th ratios obtained from the core ERC were found at the 7.5 cm layer, which was placed on top of a layer depleted in 210Pbxs and 228Thxs (Table 1), suggesting that the resuspension event occurred at least 10 years ago (5 half-lives of 228Th). One interesting finding worth mentioning is that neither the cores analyzed in AEP nor those analyzed in other coastal areas at the Pacific coast, such as the saltmarsh of Chiricahueto or the Mitla lagoon (Pa´ ez-Osuna and Mandelli, 1985), showed 137Cs activities above background levels. This could be due to a poor fallout of this artificial radionuclide over low latitude coastal areas, although some studies using estuarine sediments, contaminated by nuclear reactor discharge, have established that 137Cs is desorbed when sediments are exposed to seawater (Patel et al., 1978; Stanners and Aston, 1981) and likely, this is the main process dealing with the absence of 137 Cs in the estuarine sediments in AEP. Cores RC and EPI were the only ones showing profiles that were useful for developing a geochronology of trace metals and nutrients accumulation. None of these 210Pbxs profiles showed ideal exponential distributions and both presented subsurficial peaks, which have been related to the net effects of mixing or diagenesis (Robbins and Edgington, 1975). The suitability of both cores as sedimentary records was confirmed by examining the profiles of Ag, Cu and Zn, which showed the evidence of metal enrichment attributed to anthropogenic influence (see explanation below) and therefore, mixing was considered to exert a negligible effect. By using the model of Constant Rate of Supply dating model (CRS) (Appleby and Oldfield, 1978; Cochran et al., 1998) sediments from RC and EPI were dated up to 19 cm depth (~79 years) and up to 21 cm depth (~90 years) respectively. The sedimentation rates were determined to vary from 0.04 to 1.16 cm y⫺1 at RC site and from 0.09 to 0.88 cm y⫺1 at EPI site, while the mass accumulation rates ranged from 0.08 to 1.63 g cm⫺2 y⫺1 and from 0.10 to 0.86 cm⫺2 y⫺1, correspondingly. In both cases, the 210 Pb geochronologies showed that the highest values and the most abrupt changes in accumulation rates took place during the early 1950s (Fig. 3), which was related to the expansion of the agriculture area of Culiaca´ n Valley, when vast extensions of the catchment area were cleared off to pass from about 35,000 to 94,000 ha of agricultural lands in only one single year. 3.2. Trace metals and nutrients Nutrients (organic carbon, nitrogen and phosphorus) and trace metals (Ag, Cu and Zn) profiles from cores RC and EPI are shown in Fig. 4. In the core RC the organic carbon (OC), N and P profiles showed increasing trends, from relatively low concentrations at the beginning of the century to higher concentrations until the early 1990s. N and OC profiles increased smoothly from 1948 until the present, while the P concentrations start rising from 1970. The increasing nutrient accumulation recorded
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Fig. 3. The time dependent variation of sedimentation and accumulation rates in cores RC and EPI.
in the core is likely due to an increment in primary production or perhaps a higher supply of allochthonous organic material in the site. The metal profiles also showed progressive increases from relatively low concentrations, followed by fluctuations, to subsurface or surface peak values, with the highest concentrations accounting up to 4 times the background levels of Ag in the area and up to 3 times the natural levels of Cu and Zn (Ruiz-Ferna´ ndez et al., 2001). For core EPI, N concentrations are not available; and atypical P and OC profiles showing decreasing trends with the most important depletion taking place during the last two decades. This reduction in OC and P supply to the sedimentary column is likely related to an increment in primary production in the site, which could promote a higher assimilation of the nutrients in the water column and therefore, a minor availability of nutrients attached to the sedimentary matter. The fluctuating profiles of Ag, Cu and Zn showed rather increasing trends, in which background concentrations were increased 2-fold for Ag and Cu and almost 9-fold for Zn. The Principal Component Analysis (PCA) on the geochemical variables analyzed in the core and the significant correlation (P⬍0.05) found between OC and Ag, Cu and Zn, suggested that the sedimentary organic matter is the most important factor of deposition of these pollutants at RC site, which is located in the upper section of the Culiaca´ n River estuary. In the case of the core EPI, which was collected in the lower section of the estuary, characterized by more
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Fig. 4. Chronological variation of trace metals (Ag, Cu and Zn) and nutrients (C, N and P) in cores RC and EPI.
marine conditions, the inverse correlation found between OC and nutrients was explained on the basis of the organic matter decay and a progressive releasing of nutrients from the sediments to supply the nutrients needed for the organisms living in the water column. 3.3. Organic matter provenance One practical way to distinguish between the sources of organic materials that contribute to the sedimentary particulate organic matter the direct measurement of the stable isotopes of C and N. δ13C have been used to distinguish between marine
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Fig. 4.
Continued
and continental plant sources of sedimentary organic matter due to average δ13C for terrestrial higher plants of ⫺27% is distinct from that of phytoplankton of ⫺21% (Gearing et al., 1984) and δ15N has appeared to be an excellent indicator of N derived from human wastes (Sweeney et al., 1978) since the values for human and animal waste nitrate (10-20‰) (Aravena et al., 1993) can be easily distinguished from typical marine planktonic δ15N values (4-10%) and the terrestrial organic matter (⫺1010%) (Le´ tolle, 1980; Gearing, 1988). δ13C and δ15N values obtained from the RC core are shown in Table 2. They were compared to the values obtained from a sediment sample representative of terrestrial origin (the Sanalona dam, several km up Culiaca´ n River) and for the most marine carbon source representative sample available at the AEP complex (the Ensenada del Pabello´ n lagoon) to find out that the sedimentary organic matter accumulated in the core RC had not a marine origin and that the isotopic C signature was also lighter from that obtained from the terrestrial carbon source. The isotopic composition of C and N found in the sediments of core RC was found to be quite similar to that reported elsewhere for sediments enriched by sewage-derived organic matter (Table 3), suggesting that the urban
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Table 2 Isotopic sediment composition at AEP coastal lagoon
δ13C δ15N ∗
Sanalona dam
EP lagoon∗
Sewage
Core RC
Core EPI
⫺24.8 +2.1
⫺25.1 +4.0
n.a. +10.0
⫺29 to ⫺27 +5 to +7
⫺25 to ⫺21 n.a.
Average from bottom values of cores ERC, RC, BRI and EPC. n.a.=not available.
Table 3 Elemental and isotopic C and N organic matter sediment composition C/N
δ13C (‰)
δ15N (%)
Location
Reference
n.a.
⫺26 to ⫺25
n.a.
~13
n.a.
~6
Treatment plants sludge in NY City Sewage in San Pedro, CA
11-13
⫺28.5 to ⫺23
n.a.
Burnett and Schaeffer, 1980 Sweeney and Kaplan, 1980 Andrews et al., 1998
8-17
⫺29 to ⫺27
5–7
Sewage in Kingston, Jamaica Culiaca´ n river estuary
This study
n.a.=not available.
wastes released through the Culiaca´ n River from Culiaca´ n City and other small villages surrounding the Culiaca´ n valley are the main source of pollution in the Culiaca´ n River estuary. The presence of Ag in the sediments of core RC has also confirmed this finding, since Ag has been considered to be a good indicator of allocthonous organic matter (sewage) (Ruiz-Ferna´ ndez et al., 2001). In the case of core EPI, the δ13C values found denoted that the sedimentary organic matter had a predominantly marine nature, although the little depletion observed is also indicative of a certain degree of mixture with terrestrial organic matter.
4. Conclusions This study has showed a rather disturbed sedimentary column, slightly contaminated by trace metals. Considering that the contaminated sediments of the lagoon system are likely frequently resuspended and re-oxygenated, it is concluded that the pollutants will continue to be easily remobilized in the food chain, compromising the health of the environment.
Acknowledgements Thanks to Professor Pavel Povinec for encouraging us to publish this short review about the work that we have recently done in the field of the environmental isotopic
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goechemistry in Mexico. A special thanks to B. Burnett and J.P. Villenueve for their critical review of the manuscript.
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