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Toxicity of Arsenic in the Ground Water of Comarca-Lagunera (Mexico) 880 than the concentration factor. This is in conformity with the results obtained. The precipitation of calcite will be confirmed with the balance diagram of solution/calcite. With constant CO2 content the reduction of alkalinity should increase the pH, by a reduction of the H + content. In deed the mol concentration of this element increases with water concentration. It should be explained by a gradual concentration of CO2 in water which reflecting a space variability of the water tables. Indeed, when the medium is under water the gases exchanges are reduced. It results a weak content of O2 and a high content of CO2. The data concerning metals like Zn, Pb, Cd and Cu are limited and interpretation will be related primarily to Zn and Pb that are elements better know (Fig.7). The lead concentrations are remarkably stables, the slope of the fitted points is close to 0, reflecting a mechanism of strong chemical control of Pb concentrations. It is probably the result of a mineral precipitation (oxyde/hydroxyde or sulphate kind). The study of balance diagrams will let to identify the kind of this mineral precipitation. Concerning the zinc, its variability is more important than Pb and its concentration tendency is less stable. This must be due to mineral precipitation as well, but less systematic than lead. The concentration diagrams show the global concentration of water from upstream to downstream into hydrological flows. Chemical mechanisms of mineral precipitations are suggested but they must be confirmed by the balance diagrams. Conclusion This work is based on the analysis of 51 groundwater samples of the Comarca Lagunera region of northern arid of Mexico exposed to a health risk because of high arsenic concentration in groundwater used for irrigation and some of them for drinking water under rural and marginal conditions. The methods used from statistics to chemical equilibrium modelling and sanitary risk assessment, allow to quantify the health risk and its variability. The origin of arsenic pollution seems to be from dissolution of deep volcanic rocks in the mountains. The importance of this dissolution is variable according to lithological characteristics of the catchments. This induces geographical differences for the health risk assessment. Moreover the climate aridity causes water evaporation and concentration of soluble salts such as arsenic. The statistical approach shows that climate aridity is the main process related to high arsenic contents observed in this area. This is reflected in a health risk that increases from upstream to downstream where low areas are located. When groundwater goes from upstream to downstream it transit by different ventilation conditions: good conditions upstream and bad conditions downstream. This causes a differential expression of dissolved arsenic forms. Trivalent forms are more toxics than pentavalent forms and become dominants downstream where the highest arsenic concentrations are observed. In summary, it was possible to identify the origin of arsenic pollution, the main process responsible for its variability and the induced risk on the catchments area. Health risk related to arsenic is different upon the catchments area. However, the lack of referenced data concerning specific toxicity of arsenic forms, beyond the simple distinction between pentavalent and trivalent forms, does not let to validate the accurate health risk assessment obtained with thermodynamic models. This point could be a research line for the continuation of this work.
881 Faten Semadi, Vincent Valles and Jose Luis Gonzalez Barrios References [1] Al-Droubi, A., 1976. Geochemistry of salts and the solutions concentrated by evaporation - Model thermodynamic of simulation application on the salted ground of Chad, Th Doc. Sci., Univ. Of Strasbourg, France, 177 pages. [2] Condom, N. 2000. Study and modeling of the transfers of water and aqueous solutions in the sodic grounds and the tablecloths of the Office of Niger to Mali. Thesis ENSAM, 180 pages. Dolique, R. 123PP. 1968-L' arsenic and its compounds. Published by the university presses of France. [3] Gonzalez-Barrios, J.L., 1992. Water of irrigation and salinity of the grounds in Mexican arid region: example in "Comarca Lagunera", Th Doc. Sci., University of Montpellier II, France, 316 pages. [4] Ribolzi, O, Valles, V and Bariac, T, 1996. Comparison of hydrograph deconvolutions using residual alcalinity, chloride and oxygen 18 aces hydrochemical tracers. Resour Toilets. LMBO, 32, 4, 1051-1059. [5] Trolard F, Génin J.M.R., Abdelmoula Mr., Bourrié G, Humbert B and Herbillon A. (1997) Identification of green rust in A reductomorphic soil by Romance Mössbaner and spectroscopiés. Géochimica and Casmochimica Acta, 61, 1107-1111. [6] Valles V and A.M. of Cockborne - 1992- Development of a software of geochemistry applied to the study cde the quality of water. Seminar "Deterioration and restoration of the quality of inland waters" Port-Leucate October 1-2, 1992, 20-23. [7] Valles V, M Rezagui, L Auque, A Semadi, L Roger, H. Zougari, 1997. Geochemistry of saltworks slois in two arid zones of the Mediterranean basin. Geochemistry of the Chott Melghir-Mehrouane watershed in Algeria. Arid Soil Research and Rehabilitation (the USA): 11:71-84.
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