Radiocarbon Concentration Measurements in Tree Leaves near SOCOCIM (Rufisque, Senegal), A Cement Factory

Maurice Ndeye¹, Hans-Arno Synal², Matar Séne³

¹Laboratoire Carbone 14, IFAN CH.DIOP, Université Cheikh Anta Diop, Dakar, Sénégal.
²Laboratory of Ion Beam Physics, ETH, Zurich, Switzerland.
³Département de physique, Faculté des Sciences et Techniques, Université Cheilkh Anta Diop, Dakar, Sénégal.

DOI: 10.4236/ojap.2022.111001                

Abstract

Radiocarbon content in biogenic samples is widely used to study the variation of atmospheric CO₂ due to anthropogenic activities. A total of 20 samples of several types of tree leaves, were analyzed for this study. Sampling was carried out at the end of the rainy season in 2017 from the surrounding of the SOCOCIM cement factory in Rufisque town. Rufisque is located on the peninsula of Cape Verde, 25 km east of Dakar, where it is the 《south gate》 of the agglomeration. Reference samples of five different species were collected during the same period (2017) from a clean zone. The ¹⁴C method was used for the determination of Δ¹⁴C values. The data show that the ¹⁴C concentration in the studied sites was significantly lower than the clean area, due to the release of anthropogenic CO₂. To estimate the Suess effect, the fossil fuel fraction was determined based on equations of mass balance for CO₂ concentration, stable isotopic composition of carbon, and ¹⁴C concentration. The results show that selected locations are affected differently according to their distance from the factory and the wind direction.

Keywords

Radiocarbon Concentration, Fossil Fuel Fraction, Tree Leaves, Cement Factory

1. Introduction

Radiocarbon (¹⁴C) is produced in the atmosphere by a reaction of neutrons with atmospheric ¹⁴N that produces ¹⁴C, which is rapidly oxidized into CO₂ and then exchanges with different carbon reservoirs. Natural processes such as solar activity, Earth’s magnetic field, ocean circulation, and rates of exchange between carbon reservoirs all affect the ¹⁴C content. Besides these natural variations, human activities also have an impact on atmospheric ¹⁴C concentration. Two anthropogenic effects are recorded by atmospheric ¹⁴C: first, the Suess effect, which is the addition of carbon dioxide by fossil fuel combustion; and secondly, the increase in ¹⁴C concentration in the atmosphere because of atmospheric nuclear weapons testing [1]. Both anthropogenic factors occur on a global scale, however there are also local Suess effects due to local fossil-fuel sources. So, there are regional discrepancies that are related to the emission zone proximity as well as geographic location [2]. If we compare the global ¹⁴C atmospheric record to regional or local signals, then it is possible to relate this to regional and local changes in human activities such as industrial activity, traffic, domestic use, and land use. [3] highlighted an example of traffic-derived CO₂ in the atmosphere of an urban forest. In this context, the ¹⁴C method has been widely used during the last decades, in various applications such as archaeology [4], forensic studies [5], hydrology [6] [7], and geology, and has gained great interest in ecology and environmental studies [8] [9]. During photosynthesis, tree rings, leaves, and short-lived plants assimilate carbon from the air, and provide changes in atmospheric ¹⁴C concentration. Due to high dead carbon emissions in industrialized and urban areas, ¹⁴C concentrations are diluted [10] [11]. The determination of the ¹⁴C content in atmospheric CO₂ or from biosphere materials makes it possible to estimate the excess CO₂ or the total emission of carbon dioxide of fossil origin [12]. This is based on the differences between radiocarbon concentration in a reference site supposed “clean area” and industrial or urban area. In Senegal we are interested in determining the local variations of ¹⁴C concentration due to fossil fuel combustion caused by the different sources of pollution, such as the transport sector, energy stations, and industries. This work aims to calculate the pollution data obtained in the vicinity of a cement factory (SOCOCIM) situated in Rufisque (33˚37'54"N, 35˚26'70"E). In mining, quarrying, crushing, grinding, and calcining generate large amounts of pollutants, mainly CO₂ [13]. This cement factory located general the cement plant undertakes various processes such as in the town of Rufisque and is created in the 1948’s (before independence) however with the expansion of the city the dwellings have moved closer to the site so this could represent a risk to the environment and therefore to the local population. To evaluate this potential pollution a reference site has been chosen in a rural Village of Ngazobil (14˚12'N, 16˚52'W) which is far away from the cement factory and should not be affected by the CO₂ emissions. Accelerator Mass Spectrometry (AMS) was used to quantify the ¹⁴C concentration of the sample material. Measurements have been performed at the ETH Zurich Laboratory of Ion Beam Physics (Switzerland) using a MICADAS instrument. The radiocarbon isotopic ratio (Δ¹⁴C) and δ¹³C were determined in 20 samples of several species from tree leaves collected at the two sites. Estimation of the fossil fuel fraction was carried out based on equations of mass balance for CO₂ concentration, stable isotopic composition of carbon, and ¹⁴C concentration [14].

2. Materials and Methods

2.1. Samples

For this study a total of 20 samples were collected around the cement factory of Rufisque (Figure 1). In the Colobane district, leaves from Azadirachtaindica, Albicia le bec and Calotropisprocera were collected. Leaves from Prosopischilensis were collected opposite the factory, 50 m from the beach. In a salad field in front of the plant, leaves fromProsopis chilensis, peltiferum, Arkinsonia, cordial, Khay, Lecenia were collected. In a distance of only 10 m from the cement factory samples of Pocéa, aubergine and Goumelia leaves were collected.

These species were chosen because they are widely grown in this region. Thus, the data obtained from this study can be compared to other Δ¹⁴C determined in different zones in the region. All samples were collected in the same period, at the end of the vegetation season in 2017, to avoid possible seasonal variations of ¹⁴C concentration [15]. As reference, leaves from Ngazobil (Figure 1) a site 101.82 km away from the cement factory located in the municipality of Joal-Fadiouth, in the department of Mbour have been chosen. Five tree leaves samples (Kaya Senegalaisis, Ziziphusmucronata, Albizia, Terminaliacatapa, Faderbiaalbidia) were collected from this clean zone in the same sampling period.

2.2. Sampling Site

The climate of the study sites is characterized by the maritime trade winds from the Azores high, from north to north-east, it is constantly humid in winter. The sea trade winds are constantly wet, cool, or even cold in winter. Also, the 'harmattan, of direction East dominant, finishing branch of the continental trade-wind Sahelian, is characterized by a great drought linked to its long continental course, at last the monsoon, comes from the trade-wind resulting from the Anticyclone of Saint Helena in the South Atlantic. It enters the country during the summer in a south-east - north-west direction. It is marked by a low thermal amplitude, but with temperatures generally higher than those of the maritime trade-winds. The rains that fall come from weakened grain lines. They are very weakened. They are very localized in time, usually occur from July to October. However the times when they start and stop are very fluctuating [16] [17].

Samples were collected from rural areas distributed in the vicinity of a cement factory. The selected sites are distinguished by low population density and agricultural land. They are relatively far from large urban cities, about 25 km from the capital Dakar. The Suess effect could thus be attributed to the potential influence of the cement factory. The first location is situated to the west of the cement factory, while the others are distributed to the north and northeast. Figure 1 presents a map of Senegal showing the location of the studied region and the selected reference zones. Table 1 lists the coordinates of sampling locations, including reference areas, as well as their distance from the factory.

Figure 1. Map of Senegal with the sampling sites indicated (1-sococim, represents the cement plant, 2-Ngazobil is the reference site. The 3rd figure represents the 2 sampling sites in the map of Senegal).

Table 1. Locations of the sampling site.

2.3. Chemical Treatment

Sample material was purified using an acid-base-acid (ABA) protocol. At first the samples were treated with a 0.5 M HCl solution at 60˚C for about 2 hours. Following a washing step in deionized water, they were introduced into 0.1M NaOH solution at 60˚C for one hour. After washing again, the materials were introduced into a 0.5 M HCl solution at 60˚C for one and a half hour. Finally, the cleaned materials were washed and dried. From the purified materials a subsample containing approximately 1 mg of carbon was taken, packed in an Al capsule, and introduced into the automated combustion and graphitization system AGE [18], where a catalytic reduction on 4 mg of iron powder of the produced CO₂ gas with hydrogen gas took place at 650˚C. Finally, the graphite produced was pressed into Al cathodes which can be introduced into the ion source of the ETHZ AMS instrument.

2.4. Measurements and Calculations

Analysis of samples has been conducted in routine measurements campaigns at the LIPMicadas system at the Laboratory of Ion Beam Physics at ETH Zurich (Swiss Federal Institute of Technology) [19].

Two independent measurements were made for each sample, the first on September 5^th, 2021, and the second on October 13^th, 2021. For each sample, individual graphite’s were prepared using the precleaned material of original sample as described above. The consecutive graphitization of the purified materials enabled preparation of individual cathodes for the AMS analysis. The AMS measurement follows the standard procedure at ETH [20]. SRM 4990C (Oxa II) reference material was used for normalization. Phthalic acid and brown coal samples were used as blank materials. Blank materials were processed in the same way as samples to be analyzed. Thus, eventual contamination during sample preparation can be controlled. During the measurements, the blank materials resulted in less than 0.2% fraction modern which is equivalent to 50,000 yrs BP. In both measurement runs, final uncertainties of less than 3‰ (<24 yrs) could be reached and the results obtained are nice agreement. By combining both data sets, average values are calculated and final uncertainties of 2‰ (16 yrs) can be stated.

The conventional radiocarbon age is expressed by the formula above according to [21]:

$\text{Age}\left(\text{BP}\right)=8033\times \ln \left(\frac{1}{1+\frac{\Delta {}^{14}\text{C}}{1000}}\right)$. (1)

From this equation Δ¹⁴C values can be deduced

Δ¹⁴C (‰) = $1000\times \left(\exp \left(-\frac{\text{Age}}{8033}\right)-1\right)$ (2)

The δ¹³C values as given in Table 2 are the result of the AMS measurements. They are representative for fractionation effects which may occur during sample preparation and measurement procedure and are used to extrapolate these fractionation effects on the measured Δ¹⁴C (‰) (calculated from Equation (2)) and are the basis of the applied fractionation correction. They cannot be used to assess the δ¹³C of the original sample material.

2.5. Estimation of Fossil Fuel Component

To estimate the local Suess effect in the studied area, the fraction F(%) of fossil-fuel derived CO₂ that was incorporated by the plant material can be calculated according to Equation (3).

$F=\frac{\left({\Delta }^{14}{\text{C}}_{\text{ref}}-{\Delta }^{14}{\text{C}}_{\text{meas}}\right)}{\left({\Delta }^{14}{\text{C}}_{\text{ref}}-{\Delta }^{14}{\text{C}}_{\text{foss}}\right)}$ (3)

A value of F = 10‰ indicates that 1% of the carbon plant material originates from the CO₂ emission of the cement factory. We use Δ¹⁴C_foss = −1000‰, as fossil fuel CO₂ is totally depleted of ¹⁴C ( [22] ), Δ¹⁴C_ref is the average radiocarbon content observed at the reference site.

Table 2. δ¹³C (‰) and Δ¹⁴C (‰) values for different types of tree leave samples.

3. Results and Discussion

Sampling was done at two sites. The first is the reference site or clean air site which is a village called Ngazobil (N 14˚47'39.9"; Wo 17˚02'49.8"). Ngazobil is located at the Atlantic Ocean coastline, surrounded by sea waters that often cause winds, and the local air conditions are mostly dominated by seabreeze. The only main activity is fishing, so no industrial complexes and traffic could be responsible for any increase in local fossil CO₂ release. Thus, we can regard Ngazobil as free of urban pollution. The Δ¹⁴C values of the 5 samples (Nga01, Nga03, Nga05, and Nga06) are very consistent and scatter within a 3.8‰ range, only. This justifies calculating the average value of Δ¹⁴C_ref = 15.89 ± 0.65‰ and using it as reference to calculate the F value of urban pollution at the Rufisque site. It is lower than the value for atmospheric Δ¹⁴CO₂ (51‰) of 2012 as published by [23]. By extrapolating the trend in the Hua data to 2017, a global atmospheric Δ¹⁴CO₂ value would result in the range of ~20‰. This is still higher than the Ngazobil average value. However, more recent Δ¹⁴CO₂ data from Switzerland observed from tree leaves collected between 2012 and 2016, would suggest a Δ¹⁴CO₂ value of ~15‰, based on the 2016 result and an average annual decline of 4.4‰ (Synal pers. communication). This is in nice agreement with our reference value.

All Δ¹⁴CO₂ data at the Rufisque site are lower than the Ngazobil average value (Figure 2). We observe Δ¹⁴CO₂ values between 1.6‰ and 15.3‰. This clearly indicates the impact of the local cement production and the lower the Δ¹⁴CO₂ is, the higher is the impact of fossil CO₂ releases. In the district of Colobane located 60 m from the cement factory we observe Δ¹⁴C values of 11.96‰, 15.31‰, and 11.88‰, respectively. Even if these relatively high values remain lower than those obtained at the reference site. The lowest Δ¹⁴C (1.57‰) value was observed at a location within a approx. 50 m range from the leaves of a prosopis chilensis tree. In a salad field at the same distance to the factory, samples SOCO06, SOCO07, SOCO08 and SOCO10 gave the Δ¹⁴C values of 12.00‰, 13.38‰, 12.65‰, 9.86‰ and 12.46‰ respectively. Although these ¹⁴C concentrations are relatively close to the reference value, the impact of the fossil CO₂ is significant. On the other hand, four samples (SOCO17, SOCO19, SOCO20 and SOCO22) were taken at a distance of 10m from the factory. They fall into two distinct groups, giving values of Δ¹⁴C 12.63‰, 12.19‰ and 5.99‰, 5.02‰, respectively. The first group shows a week, the second a rather strong impact from the CO₂ emissions. By using Equation (3), these results are converted into fraction fossil fuels F(%.) as shown in Table 3 demonstrating the degree of the Suess effect for each location

Figure 2. Radiocarbon content Δ¹⁴C in different types of tree leaves, collected in 2017 from 15 locations of the cement factory compared with 05 samples from the reference area.

Table 3. Fossil fuel fractions F(%) in the studied areas derived from ¹⁴C content in tree leaves.

due to anthropogenic ¹⁴CO₂ emissions from the cement factory. In general, there is no clear trend between distance from the factory and the observed ¹⁴C depletion a week tendency indicating higher impact at lower distances to the factory may be suggested by the data set. However, very local effect as observed at sampling points SOCO20/22 and SOCO05 may overrule a general tendency. It is remarkable that for these samples between 1% - 1.5% of the organic carbon has originated from the fossil fuel emissions of the cement factory.

So far, sampling was carried out on a single season (2017). Confirmation of the values over a longer time period would be helpful to draw more solid conclusions. For an assessment of the Suess effect, these values should be compared with those obtained at points around the alleged source of pollution.

4. Conclusion

This study falls within the general framework of the determination of air pollution due to fossil CO₂ by the various cement factory installed in Senegal. The SOCOCIM cement factory, which is the oldest in Senegal, was the subject of a collection of samples to determine the excess CO₂ due to the anthropogenic effect. A dilution of the ¹⁴C concentration was determined following the contribution of the fossil CO₂ component emitted by the cement factory. The depletion of ¹⁴C in the studied areas can reach an F-value of up to 1.5% of fossil carbon in the biomass of samples under investigation. This could be attributed to the emissions of cement factory that releases large amounts of CO₂, which could reach the selected sampling sites with the south-east - north-west direction wind that prevails most of the year. Future studies and measurements will be carried out in the Department of Rufisque to determine the effect of Suess in other nearby villages and observe any change in the atmosphere of the concentration of ¹⁴C over time.

Acknowledgements

The authors express their gratitude to Professor Doudou Diop the researcher in the Botanic Laboratory for his help in the identification (Taxonomy) of the tree leave samples. We thank the laboratory technician Mr. Alpha Diallo for the constant help in taking charge of the collection and pretreatment of samples. Our gratitude goes also strongly to the team of the Laboratory of Ion Beam Physics Zurich (Switzerland) for the constant support to this work.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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