CIENCIAS NUCLEARES
Tendencia
histórica de la contaminación por metales pesados en sedimentos
de la bahía de Cienfuegos, definida con la geocronología del
y
Historical
trend in heavy metals pollution in the sediments of Cienfuegos bay, defined
by
and
geochronology
Carlos Alonso Hernández, Saumel Pérez Santana, Alain Muñoz Caravaca, Misael Díaz Asencio, Miguel Gómez Batista, Claudia Brunori 2, Roberto Morabito 2, Roberta Delfanti 3, Carlo Papucci 3
1 Centro de Estudio Ambientales de Cienfuegos, Cienfuegos, Cuba
2 ENEA-C.R.E. Casaccia, PROT-CHIM, Roma, Italy
3 ENEA-Marine Environment Research Centre, La Spezia, Italy
carlos@ceac.perla.inf.cu
RESUMEN
En este trabajo
se determinaron los niveles de metales pesados en cores de sedimentos de la
bahía de Cienfuegos (Cuba), y con el uso de radiotrazadores para su datación,
nos permitió reconstruir la historia de su deposición, así
como la identificación de los principales cambios ocurridos en el último
siglo. Durante dos campañas de muestreo (1999 y 2001) se colectaron cores
y sedimentos superficiales en la bahía de Cienfuegos. En este artículo
se presentan los resultados de la distribución horizontal y vertical
de arsénico, níquel y vanadio y de los radionúclidos
(en exceso) y
,
estos últimos usados como herramientas para la datación de los
cores de sedimentos. Los resultados del trabajo evidencian un cambio en el régimen
de sedimentación de la bahía de Cienfuegos en los últimos
cuarenta años, atribuibles al uso de la tierra en el área y a
eventos meteorológicos extremos. La distribución, tanto horizontal
como vertical de los metales analizados, muestra la influencia de la industrialización
reciente en la bahía de Cienfuegos.
ABSTRACT
Investigations
on the concentration level of heavy metals in the sediments of Cienfuegos bay
(Cuba) have been carried out, to reconstruct their “depositional history”
by using radionuclide geochronology, to highlight the major changes occurred
in the last century and to draw hypotheses on their origin. During two sampling
campaigns (1999 and 2001) sediment cores and surface grab samples have been
collected in the two basins of the bay. In this paper results on the horizontal
and vertical distribution of arsenic, nickel and vanadium are discussed, along
with data on concentrations of natural (excess )
and anthropogenic (
)
radionuclides, used as tools to “date” the sediment horizons. Results
evidenced that important changes in the sedimentation regime of the bay occurred
in the last forty years, in relation to changes in land uses and to the effects
of some extreme meteorological events. The distribution of the heavy metals
shows the influence of the recent industrialization of the area, as revealed
by both the spatial and vertical concentration within the sediment horizons.
Key words: cesium 137, Cuba, bays, sediments, metals, radioecological concentration, sensitivity analysis, chemical analysis, absorption spectroscopy, Cienfuegos
INTRODUCCIÓN
The sediments play a key-role in the distribution of heavy metals in a semi-enclosed environment (Fabris et al., 1999; Mason et al., 2004, Tolun, 2001), where the concentrations of heavy metals can reach as much as five orders of magnitude higher than those of the surrounding water (Birch, 1996; Bryan and Langston, 1992; Turner, 2000). The fate of particle-associated contaminants is primarily controlled by the energy released in the environment: fine-grained particles - in particular clay minerals, to which contaminants are preferentially associated. The distribution and bioavailability of metals in a semi-enclosed marine environment also depend on many other processes, that include: geochemical behavior of the element (Algan et al., 2004, Wright and Mason, 1999); the presence of a salinity gradient (Riba et al., 2003, Wang and Liu, 2003); tidal dynamics (Yang, 2004) and hurricanes (Walker, 2001), that can re-suspend surface sediments.
The bay of Cienfuegos,
located in the southern-central part of Cuba (figure 1), is an enclosed bay
with a surface area of 90
and an average depth of 14 m. The bay is divided in two well-defined hydrographic
basins. The northern basin receives most of the anthropic impact from the city
sewage discharge, from the industrial area (thermoelectric power plant, fertilizer
industry, oil refineries) and from the outflow of the Damuji and Salado rivers
that drain an area where intense agriculture has been developed in the last
forty years. The southern basin is subject to a smaller degree of anthropic
pollution, mainly originated from the Caonao and Arimao rivers. The bay represents
the most important natural resource in the Cienfuegos province, and the surrounding
land has been intensively exploited.
The bay of Cienfuegos,
located in the southern-central part of Cuba (figure 1), is an enclosed bay
with a surface area of 90
and an average depth of 14 m. The bay is divided in two well-defined hydrographic
basins. The northern basin receives most of the anthropic impact from the city
sewage discharge, from the industrial area (thermoelectric power plant, fertilizer
industry, oil refineries) and from the outflow of the Damuji and Salado rivers
that drain an area where intense agriculture has been developed in the last
forty years. The southern basin is subject to a smaller degree of anthropic
pollution, mainly originated from the Caonao and Arimao rivers. The bay represents
the most important natural resource in the Cienfuegos province, and the surrounding
land has been intensively exploited.
In the last three
decades some deleterious ecological signals have been observed in the Bay. For
these reasons a large environment research programme has been initiated, to
define the main physical, chemical and biological characteristics of the bay,
whose knowledge will constitute the basis for the correct management of the
area.
As part of this programme, dating techniques based on the natural and antropogenic
radionuclides (
and
)
and mineralogical distribution were applied by Alonso Hernández et al.
(2006) to study of the sedimentation regime in Cienfuegos bay. This study showed
that in the last 40 years significant changes have affected sedimentation processes
in the bay. These results are the base to evaluate the distribution of heavy
metals in the bay.
The aims of the study are to reconstruct the “pollution history”, using natural and anthropogenic radionuclides as tools for “dating” the sediment horizons; to identify the main processes responsible for the spatial and vertical distribution of heavy metals (As, V, Ni) in the sediments, and then to identify the major events related to the management of specific industries located in the bay.
MATERIAL AND METHODS
Sampling
Two sampling campaigns have been carried out in 1999 and 2001. The location of the sampling stations is reported in figure 1. Surface sediments have been collected in 24 stations. Sediment cores (N1999 and S1999) have been collected by a scuba diver carefully inserting a plastic tube, 12 cm in diameter and 100 cm in length, into the sediment. Cores have been quickly sliced into 1-cm intervals. All samples have been put in polyethylene cans and stored at 4°C. Samples were dried at 40°C until constant weight, sieved through 1-mm nylon sieve and grinded. The resulting powder was stored at room temperature in polyethylene cans until analysed. Porosity, Organic Matter, Grain Size, Radiochemical and Mineralogical composition in sediment cores collected in 1999 have been discussed by Alonso Hernández et al. (2006).
Chemical analyses of heavy metals
Analytical procedure, calibration, and quality control of results are described elsewhere (Perez Santana et al., 2004, in press; Ipoly et al., 2002). Briefly, the total dissolution of 0.5 g of sediment was performed using strong acids and a CEM microwave pressurized digestor. Determinations of metals were carried out by ICP-MS (Perkin-Elmer ELAN 6000, equipped with a Ryton cross-flow nebuliser). The calibration standards were prepared diluting stock BDH Aristar solution with double-deionized Milli-Q water. Analyses have been performed on two aliquot of each sample, and on blanks. Performances of the analytical procedure were checked analysing Certified Reference Materials (NIST 1646a, PACS-1, MESS-1). All results were comprised within 90-110% of certified values.
Radiochemical analyses
Sub-samples of
the surface sediments and of all core sections were analysed for ,
and
contents by gamma spectrometry, using low background intrinsic germanium coaxial
detectors (60% efficiency, 2.1 keV resolution at 1333 keV) coupled with a multichannel
analyser. Twenty grams of the dried and homogenised samples were placed in sealed
containers and left for three weeks before counting, to reach the
/
equilibrium.
was determined via its gamma emission at 46.5 keV;
by the 295 keV, 352 keV and 609 keV gamma rays emitted by its daughters
and
;
was measured at 662 keV. Efficiency calibration was performed using a CANMET
Standard U-Ore and QCY44 AMERSHAM Certified Solution. NBS and IAEA Reference
Materials were used to check the accuracy of the results that resulted within
± 10% of the certified values. The excess
was calculated of the difference between the supported
and the
.
RESULTS AND DISCUSSION
Geochronology
The vertical profiles
of
in the cores collected in Northern (N 1999) and Southern (S 1999) basins are
showed in figure 2 and have been used for “dating” specific sediment
horizons. The vertical profiles, concentrations were plotted versus “mass
depth” (g
), instead of depth (“cm”), to avoid correction for sediment compaction.
was introduced into the Caribbean environment mainly by fallout from nuclear
weapon testing in the atmosphere. The input function of
at the latitude of Cuba is well defined (Alonso Hernández, 2004), and
shows a maximum in 1963 due the ban of nuclear testing explosions in the atmosphere
and a sharp decrease in the following years: so it was realistic to attribute
“1963” to the sediment layer corresponding to the maximum
concentration in the vertical profiles. The profiles show also two additional
peaks between the maximum concentration and the surface, that we attributed
to extra-inputs of
-reach
particles into the Bay. In fact, the calculated inventory of
in the sediment cores (0.19 and 0.20 Bq
)
are higher than the cumulative fallout deposition in the area, estimated from
adjacent undisturbed soil (0.12 Bq
,
Sibello Hernández et al., 2002). This likely indicates a significant
transport of
from land to the bay. Moreover, two extreme meteorological episodes have been
actually registered, after 1963, able to produce extra-input from the rivers:
the hurricane “Camille”, that crossed Cuba in 1969 with 500 mm of
rain in 3 days and an exceptional period of heavy rain (1000 mm in 7 days) occurred
in June 1988.
These events provided
an extra-input of suspended load to the bay (carrying an enhanced
content, associated to clay particles), from both the weathering of the catchment
basins and the resuspension & transport of
-rich
sediment initially stored in the river bed. Finally, by crossing the data of
the
vertical profiles with those on the fallout deposition and with the meteo record
for the region we reasonably hypothesised that the two extra-peaks could be
dated at 1969 and 1988.
Results from the
vertical profiles of excess
have been used to integrate those of
dating and to derive information on the sedimentation regime in the last 100
years. The vertical profiles of
suggest significant variations in the sedimentation regime, both spatially and
temporally. A remarkable discontinuity was observed in the profiles (figure
2), corresponding to sedimentation processes occurred in the period 1963-1973.
The CFCS/Constant Flux Constant Sedimentation model (Appleby et al., 1992) was
separately applied to the sediment layers below and above the observed discontinuity.
While from the beginning of 1900 sediments were regularly accumulated at a rate
of around 0.3 g
,
beginning from early Seventies the sediment accumulation rate almost doubled
(0.5 g
)
in the northern basin. In addition, corresponding to the caesium peaks, there
are “packages of sediment” having the same age, supporting the hypothesis
that
peaks are correlated to the meteo events that produced enhanced supply of particles
into the bay, in a very short period of time. More information about the chronological
results was discussed by Alonso Hernández et al. (2006).
Nickel, vanadium, arsenic in sediments
In figure 3 the spatial distributions of nickel, vanadium and arsenic are shown, whereas figure 4 and 5 show the vertical profiles of vanadium, nickel and arsenic in the two cores. All results are reported in table 1, 2 and 3.
In
the case of nickel and vanadium (figure 3a and 3b) particularly high concentrations
were found, as expected, in the surface samples collected in front of the thermoelectric
power plant (figure 1, point “C”). Concentration levels are twice
(nickel) and three times (vanadium) higher than the average value found in the
surface sediments of the northern basin (Ni: 106 mg ,
in respect to 50; V: 400 mg
,
in respect to 137). This area can be considered as zone of preferential accumulation
of the metals emitted by the plant, both in gaseous form and as wastewater.
The plant was activated in 1939, but the substantial increase in power generation
occurred in 1949. Actually the vertical profiles of vanadium and nickel in the
sediment cores (figure 4) reveal enhanced concentrations in strata deposited
after the early sixties. These differences are measured in both basins, demonstrating
that the impact of the power plant has been redistributed on the whole bay territory,
on a fifty years time-scale.
The
spatial distribution of arsenic in 2001 is rather homogeneous in the two basins
of the bay (figure 3), with concentrations around 5 mg .
On the contrary, the vertical profiles in the sediment cores (figure 5) gave
an additional insight on impacts recently produced by the industry in the northern
basin. While the profile gathered in the southern core shows values that can
be considered as “background level”, a significant increase in concentration
– up to 88 mg
,
more than one order of magnitude higher- was found down in the northern core,
in strata corresponding to late-Seventies/early Eighties. This signal marks
the management of a Nitrogen fertilizer industry, including the effect of a
major accident occurred in 1979.
Summarizing,
in fact, the re-distribution in the sediments is correlated to preferential
pathways in wet and dry deposition for nickel and vanadium, initially dispersed
into the atmosphere from the thermoelectric power plant located in the northern
basin, to the water masses circulation and sedimentation regime for the elements
(like Arsenic, for instance) entering directly into the marine environment from
factory and waste treatment. As a matter of fact, significant amounts of heavy
metals have been introduced into the bay, mainly in the northern basin, by authorized
releases - and accidents - from the industrial complex. In addition, as important
changes in land uses and random meteo events produced additional inputs of suspended
particle load (to which particle-reactive contaminants associate), an increased
accumulation in the sediments was observed, both in terms of sediment accretion
rate (g )
and concentration (g
)
of metals.
CONCLUSIONS
The main contribution
of this work has been to demonstrate the power of the nuclear techniques in
environmental studies. Particularly, the use of the
and
in the dating of marine sediments and the possibility of studying the historical
contamination of heavy metals in estuaries and coastal areas.
From the results
of this study can be concluded that:
1. The sedimentation regime in Cienfuegos bay has changed in the last forty
years. Sediment accumulation became faster and irregular. The most important
reasons likely are the changes in land uses, including the development of intensive
agriculture in late Fifties that resulted in enhanced deforestation in the area
drained by the rivers, and in significant increase of the suspended load entering
into the bay; and the period of heavy rain occurred in 1988, as well as during
the hurricanes transits, that also led to enhanced supply of particulate material
transported by rivers.
2. In the bay of Cienfuegos, the pollutants introduced in the atmosphere (i.e.
vanadium and nickel) are homogeneously distributed all over the two basins,
on a time-scale of decades. The highest surface concentrations are found close
to their point source.
3. Arsenic, introduced directly into the sea by authorized releases of a fertilizer
complex, sedimented in the northern basin, close to the point source. The vertical
distribution in the northern sediment core reveals a sharp increase in strata
corresponding to the late-Seventies, when a first major accident occurred with
the sudden release into the bay of liquid wastes with elevated arsenic concentration.
4. The metals tend to remain in the bay environment for a long period: decades,
at least.
5. Results from this study gave a first insight to support adequate strategies
for the management of the bay ecosystem and the surrounding area.
ACKNOWLEDGEMENTS
The work was partially carried out in the framework of the “Seventh Executive protocol of the Cultural and Scientific Agreement, 2001-2004” between Italy and Cuba, Annex 3 / Environment / Project no. 6. The analytical activities have been supported by the International Centre for Theoretical Physics-Programme TRIL and by ENEA fellowships dedicated to developing countries.
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Recibido:
13 de abril de 2006
Aceptado: 26 de abril de 2006
Centro de Estudio Ambientales de Cienfuegos, Cienfuegos, Cuba
carlos@ceac.perla.inf.cu
