Origin and Impact of Trace Elements on Brines of Chotts and Sebkhas (Saline Lakes) in Algerian Low Sahara

Abstract


Introduction
The trace element designation is commonly used to refer to naturally occurring chemical elements with a very low concentration (trace levels: less than 0.1%) in all compartments of the environment (Turekian, 1971;Lameyre, 1986;Elder, 1988;Baker and Walker, 1989;Davies, 1997;Callender, 2003;Wang et al., 2008).The spatial distribution of these trace elements results from the combination of natural and anthropogenic factors.If the concentration at a given location is abnormally high, the environmental impact will be potential.In the Algerian Low Sahara, geochemical studies carried out on the brines of the chotts and sebkhas remain rare except for those of Hacini et al. (2008) on the geochemical evolution of the brines.In this context, we try to determine the potential dispersion of some trace elements, in particular Copper (Cu), Nickel (Ni), Lead (Pb), and Zinc (Zn), and to evaluate their environmental impacts based on comparison with reference values.Unfortunately, in Algeria, there are no special laws for the protection of lake environments.There are only a few framework laws that do not clearly define the protection criteria.For that, the recommendations used in this study are those of U.S. E.P. A. (1987aA. ( , 1998aA. ( ) and Sherbrooke. (2014)).

Study Area
The Algerian Low Sahara is a vast depression (-37 to 300 m altitude), occupied in part by Great eastern Erg.It is bordered by the Saharan Atlas Mountains to the North, the Saharan Ridge and the Great Western Erg to the West and plateaux of Tademaït and Tassilis N'Ajjers to the South.This depression opens up to the northeast on the Gulf of Gabes and the Mediterranean and extends over an area of 720.000 km 2 between 35° and 28° north latitude and 3° to 11° east longitude.It is a region of shallow valleys and low plateaux (Dubief, 1963) (Fig. 1).

Materials and Methods
48 samples were collected and analyzed in the transfer geochemistry laboratory of the CNRS in Paul Sabatier University, Toulouse, France.

Analysis of the Liquid Samples
Liquid samples were analyzed by ICP-MS (Elan 6000, Perkin-Elmer) for determination of trace element levels.This technique is based on the coupling: • A plasma torch generating ions: In contact with argon, the sample is nebulized and then transported to the center of the plasma where the temperatures reach 6000 to 8000°C.The sample is then atomized and then ionized in its entirety as monovalent cations; • A mass spectrometer designed to sort, list and count the ions presented in the sample according to their mass, or rather their mass/charge ratio (m/z) (Guernet et al., 1998;Skoog et al., 2003).pH, electrical conductivity, temperature, and dissolved oxygen are measured in situ after each sampling using portable devices (HI-9829, Multi-3430) (Fig. 2).

Analysis of the Solid Samples
As for the waters, the solutions obtained from acid digestions of the solid salts were analyzed by ICP-MS (Elan 6000, Perkin-Elmer).Scanning Electron Microscopy (SEM) techniques were again used to determine trace elements.The scanning principle consists in exploring the surface of the sample in successive lines and transmitting the signal of the detector to a cathode screen whose scanning is exactly synchronized with that of the incident beam.Scanning microscopes use a very fine beam that sweeps the surface of the sample point by point.Under the impact of the accelerated electron beam, backscattered electrons and secondary electrons emitted by the sample are selectively collected by detectors that transmit a signal to a cathode ray screen whose scanning is synchronized with the scanning of the object.

Brines
The temperature measured in situ is around 23°C; while the electrical conductivity is above 100 mS/cm.The pH is always slightly basic (alkaline).The concentrations of the trace elements are generally higher than the reference values (Table 1).
• 96 to 2474 ppb of (Cu) compared to 3.7 ppb of reference.
• 1677 to 2596 ppb of (Zn) compared to 86 ppb of reference.
• Lead concentrations are generally high, ranging from 10 to 71.3 ppb.
• Nickel concentrations vary according to the intensity of urban discharges: they are very low (6 to 18 ppb) in areas where chott is farther from urban discharges, and very high (from 21 to 60.4 ppb) in areas where chott is in direct contact with these urban discharges.These results make it possible to say that the trace element contents depend on the acidity and the abundance of organic matter.The slightly acidic pH values correspond to high levels of Pb, Cu and Zn.On the other hand, the Ni varies proportionally with the pH of the brines.

Solid Salts
The chemical analysis results for solid salts show that the contents of Ni and Pb are generally lower than the values fixed by the standard (Table 2).
• From 9 to 33 ppb of Ni compared to 84 ppb of reference.
• From 0.5 to 13.7 ppb of Pb compared to 14 ppb of reference, except for five samples where the contents are 22 to 127 ppb.The Zn and Cu contents are clearly higher than the values of reference.
• From 94 to 180 ppb of Zn compared to 70 ppb of reference.
• From 285 to 3938 ppb of Cu compared to 60 ppb of reference.
It seems clear that trace element concentrations are very high in brines than in solid salts; which reflects the important effect of water on pollutants in general and trace elements in particular.The lowering of the pH of the water, for example, leads to an increase in the solubility of some trace elements and mobility.

SEM image analysis
This study was deepened by the punctual analysis of SEM images of some representative samples of solid salts.The SEM images present a summary of the most common minerals.The point analysis of these images shows that primary silicates, clay minerals (Si-O-Na, Al, Ca, Mg, K) and evaporite chlorid-sodium (Na-Cl) minerals predominate in solid salts (Figs. 3,4 and 5).The presence of clays will play the role of trapping for these trace elements.However, their affinity for organic matter has long been recognized.

Origins and Impacts of these Trace Elements
Copper is released in lacustrine systems chotts and sebkhas, by urban discharges loaded with organic matter.Most often, it is not easily destroyed and can accumulate in different components of the lacustrine system.Their presence, therefore, influences the chemical and biological behavior of the lacustrine system.Zinc, like copper, is found in environments potentially polluted by urban discharges.Their presence degrades the lacustrine system.Lead is a bio-accumulator element that is released depending on the acidity of the water.However, most of the concentrations found are caused by human activity (wastewater discharges, incineration of medical equipment).It is considered as a toxic element disturbing the biochemical balance of the lacustrine system.45 to 70% of lead and zinc are dissolved in wastewater and are therefore difficult to eliminate (Miquel, 2001).Nickel is adsorbed by sediment, and higher concentrations can damage the growth of micro-organisms in the lacustrine system (Sukandar et al., 2006).

Conclusions
The chotts and sebkhas, in the Algerian Low Sahara, are saline lakes where intense evaporation during the summer favors the crystallization of salts on the surface.These salts, of a chlorinated, sulfated and gypsum character, overcome the clayey-sandy and gypsum formations of the Plio-Quaternary.These saline lakes are fed by the waters of the underlying aquifers and especially by waste water and drainage discharges.These highly charged waters are chemically chlorinated and sodium sulfated.The analysis of geochemical data has shown that these saline lakes have deterioration indices shown by high levels of trace elements: 96.79 to 2474.16 ppb of Cu; 1677.85 to 2596.2 ppb of Zn; 10 to 71.38 ppb of Pb and 6.31 to 60.46 ppb of Ni.These trace elements are trapped in the clays.
Among the anthropogenic sources of these trace elements: the ashes of incineration of medical wastes (Spiky/sharp waste: hypodermic needles, intravenous or other; auto-disable syringes; syringes with needles; infusion sets; scalpels; knives; from blades...) and sewage effluents.However, it appears that the main anthropogenic source of metals is that produced by sewage effluents.It is identified as one of the first human-induced environmental impacts, and the need to reduce these releases is no longer discussed.In the context of environmental management and in order to effectively take into account the issue of the release of trace elements into the environment, it is necessary to increase the accuracy of emission inventories.

Fig. 1 .
Fig. 1.Location of the study area

Table 2 .
Results of trace elements in ppb for saline lakes of Algerian Low Sahara (extract of solid salts)