A01616

Invention Journal of Research Technology in Engineering & Management (IJRTEM) ISSN: 2455-3689 www.ijrtem.com Volume 1 Issue 6 Ç August. 2016 Ç PP 01-06

The Geochemical Evolution and the Morphological Significance of the Sediments of the Kseub wadi between 1969 and 2015 (central Tunisia) Karim Lahmar1, Jean Pierre Larue2 1

(ISEAH Mahdia, University of Monastir, Tunisia)) 2 (University of Paris- Est, Paris, France)

Abstract : In this paper, we attempt to present the geochemical evolution of sediments deposition in the confluence zone of the Kseub wadi (central Tunisia) at the level of lake Nebhana since the floods recorded in autumn, 1969. Results of the physicochemical analyses of sediment samples reveal a highly disturbed sedimentary column. We also notice strong vertical and horizontal gradients of the geochemical parameters in the confluence zone of both streams. The geochemical tracers measured help to reflect a detailed description of the thickness of the new layer of sediments of the Kseub wadi. It is actually between 95 and 120 cm for the stations of core drilling. The organic matter (OM) in the new sediment shows clearly its continental origin and the absence of organic contaminants. Actually, despite the catastrophic events of 1969, sediment samples have remained visible for approximately 46 years. The geochemical evolution of sediments (between 1969 and 2015) reflects an attenuation of the geochemical anomalies of sediment samples (vertical anomalies). These observations suggest a possible change of OM in sediments. They also show an important sedimentary supply that would in turn preserve chemical facies during a rather important lapse of time.

I. Introduction The taking of the first drilling core box in the Kseub wadi shows an important layer of new sediments (thickness about 20 cm). It differs from already present sediments by its brown color and its high fluidity. This study is meant to provide a minute description of the geochemistry of recently deposited sediments. More specifically, it tries to characterize the geochemistry of sediments deposition between 1969 and 2015. Furthermore, it is conducted in an attempt to estimate the thickness of the new sedimentary layer, to determine the nature of OM present in the sediment, and to investigate the morphological evolution of the stream channel between the aforementioned dates. The chemical analyses of samples also bring to light the sedimentary supply which accompanied the floods of 1969 and the possible morphological modifications which followed this phenomenon.

Site of the study and measured parameters The sediment samples were taken by means of a box corer of Hessler type in the Kseub wadi in September 2015 (fig1). The different parameters measured in sediments are: the porosity, the composition in total carbon (TC), the elementary relationship TC / N and the n-alkanes. The salinity and the dissolved organic carbon (DOC) are measured from extracted interstitial waters. The DOC is measured by detection of ultraviolet ray [3].

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Figure 1 : localisation of the Studied zone

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The Geochemical Evolution and the Morphological‌ Geochemical characteristics of sediments deposition between 1969 and 2015 : The distribution of the porosity and the salinity highlight an important disturbance of the sedimentary column. This disturbance which is considered to be significant in the Kseub wadi before reaching the lake of Nebhana decreases gradually as we go away from the zone of core drilling

Station/ CSP

CPS1

CPS2

Depths/ Cm 0 30 60 90 120 0 30 60 90 120

Total of N- alkanes 52.2 31.75 09.56 04.30 01.75 48.50 30.55 15.45 07.20 01.50

Majority N- alkanes C17 C17 C17 C23 C23 C23 C23 C23 C17 C17

FPM/ HPM 3.81 2.65 0.19 0.14 0.02 2.78 2.01 1.02 0.89 0.76

preferential carbon Index 4.45 3.62 4.32 2.94 1.73 5.02 4.12 3.59 2.38 1.78

Table 1: concentrations in NA and indication of biomarkers of the nature and the origin of MB in sediments stations 1 and 2.

[Majority NA: NA present in high concentration in a given layer of sediments; FPM/HPM: NA of low molecular weight (nbC=20)/NA of high molecular weight (nbC=21); Carbon Preference Index: CPI = 2 (C27+C29)/(C26+2C28+C30)] The geochemical properties of sediments situated in the Kseub wadi are less disrupted than the ones situated in the course of the Kseub wadi (in spite of a deposit of about 120 cm). Certain geochemical tracers: such as % CT, CT / N and DOC (not presented here) depict clear anomalies of distribution to a depth corresponding to the former interface water / sediment, buried under a thick layer of new deposits (new interface / former sediment). The most important anomalies of distribution that are considered unusual for this zone of study are observed for the salinity. The presence of water of lesser salinity (around 25) is observed in disrupted sediments. An important and fast sedimentary supply would, then, trap the fresh water transported by streams. The Importance of the salinity variations on the solubility of OM and contaminants is widely Known (Means, 1995; Turner, 1996; Whitehouse, 1984, Yeats and Bewers, 1976, Yeats, 1990), suggesting a mobility of OM and contaminants (organic and inorganic) already present at the time of the floods of October, 1969 and September, 2015.

II. Discussion The dearth of studies on investigating the sediments of the Kseub wadi paved the way for conducting the current study. Actually, the latter is meant to fill this gap. Sediments integrate the entire topographic surface subject to chemical and/or mechanical erosion.

Thus, they are sensitive to the local chemical variabilities (TrĂŠpanier, on 2009). The geochemical study of sediments is not based on the value of an element regarding its standard mantle lithosphere (continental crust). It bases itself on the supply of these elements in quality and quantity of products found downstream to the trunk sewer (the Kseub wadi). The quantitative study shows a more important supply of elements carried by the Kseub wadi. This could be due to the morphology of the ground and the river system of both streams. The topographic analysis [5] shows that both streams represent an important sedimentary supply. The drainage density is significantly high [5]. In spite of its slow flow and its low drainage density, the wadi, Kseub brings different elements especially heavy minerals which tend to settle more quickly. Then, only a small quantity (-20 %) arrives at the zone of confluence. Although, this study has provided valuable insights about the morphological significance of sediments, it is not without its limitation. Actually, it is considered insufficient to draw a firm conclusion on the possible contamination of the river regarding its banks. Thus, other futuristic studies should further dig out this area. A pedological study, then, as well as a comparative study between the geochemical and pedo-geochemical backgrounds in the watershed should be carried out. The rate of the alkanes, in CPS1, decreases gradually. Its evolution is done in exponential form (fig2). In the same way for the CPS2 (fig3)

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Figure 2: N-alkanes evolution per different depths in CPS1.

Figure 3: N-alkanes evolution per different depths in CPS2.

Concerning preferential carbon index (PCI), its evolution is done inversely proportional to the depth in both CPS1 and 2 [6] (fig 4 and 5).

Figure 4: Preferential carbon index evolution per different deeps in CPS1.

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Figure 5: Preferential carbon index evolution per different deeps in CPS2.

The geochemical properties of the layer of sediments deposited between 1969 and 2015 show first of all the existence of both horizontal and vertical gradients for the studied geochemical tracers. Second, they pinpoint salinity anomalies in sediments at the most disrupted stations showing a higher solubility of OM [1]. Then, they show the presence of a layer of sediments that is relatively short of total carbon [4]. The characterization of the nature of OM deposited during the floods of 1969 indicates the presence of OM mainly terrigenous and the uncontaminated character of OM present in the new layer of sediments [2].

Figure 6: Cut on the level of the Kseub wadi

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III. Conclusion The purpose of the present study is to expand the understanding of the geochemical evolution of sediments samples of the course of the Kseub wadi after the establishment of the lake of Nebhana in 1968. This study has been conducted after the occurrence of certain pollution problems from which the basin of the Kseub wadi suffers. Taking into consideration the element tracers and ground observations, we can conclude that the streams of the Kseub wadi is, in major parts, sources of pollution of the lake of Nebhana. This phenomenon is not only connected to the morphology of the hydrographic basin, but also to the anthropic pressure and the immediate release of litter at the bottom of these two streams.

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A. Turner, Trace-metal partitioning in estuaries: importance of salinity and particle concentration. Mar. Chem. 54, 1996, 27-39.

B Deflandre., Flux vertical de matière organique particulaire dans le bassin nord occidental de la Méditerranée. Diplôme d 'Études Approfondies en Météorologie, Océanographie et Environnement. Option: Chimie marine, Université Pierre et Marie Curie, Paris 6, Villefranchesur- mer, 1994.

B.G. Whitehouse, The effects of temperature and salinity on the aqueous solubility of polynuclear aromatic hydrocarbons. Mar. Chem. 14, 1984, 319-332 K Lahmar , Impacts des aménagements hydrauliques sur la dynamique fluviale dans le Nord et le Sud de la Méditerranée cas de l a Cesse en France Sud-Est et de l’oued Maarouf Tunisie Centrale., University of Paris Est, France, 2010.

M.F. Lapointe. Response of the Ha! Ha! River to the flood of July 1996 in the Saguenay region of Quebec : Large-scale avulsion in aglaciated valley. Wat. Resour. Res. 34, 1998, 2383-2392.

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