-Stream water quality is dependent on many factors, including the source and quantity of the streamflow and the types of geology and soil along the path of the stream. This study aims to evaluate the origin and the mechanisms controlling the input of ions that effect surface water quality in the sub-basin of the Rio das Cobras, Rio Grande do Norte state, Northeastern Brazil. Thirteen ponds were identified for study: three in the main river and ten in the tributaries between, thus covering the whole area and lithology of the sub-basin. The samples were collected at two different times (late dry and rainy periods) in the hydrological years 2009 and 2010, equating to total of four collection times. We analyzed the spatial and seasonal behavior of water quality in the sub-basin, using Piper diagrams, and analyzed the source of the ions using Guibbs diagram and molar ratios. With respect to ions, we found that water predominate in 82% sodium and 76% bicarbonate water (cations and anions, respectively). The main salinity control mechanism was related to the interaction of the colloidal particles (minerals and organic sediment) with the ions dissolved in water. Based on the analysis of nitrates and nitrites there was no evidence of contamination from anthropogenic sources.Keywords: Desertification. Semiarid. Salinity. MECANISMOS CONTROLADORES DA QUALIDADE DAS ÁGUAS SUPERFICIAIS NA SUB-BACIA HIDROGRÁFICA DO RIO DAS COBRAS DO NORDESTE BRASILEIRORESUMO -A qualidade da água de um rio dependente de vários fatores, incluindo a origem e quantidade de escoamento e a formação geologia dos solos ao longo do fluxo. Objetivou-se avaliar a origem e os mecanismos controladores da entrada dos íons e, consequentemente, da qualidade da água na sub-bacia do rio das Cobras, RN. Foram selecionados 13 açudes, sendo 3 no rio principal e 10 entre os afluentes, abrangendo toda a área e litologias da sub-bacia. As amostras foram coletadas em duas épocas distintas (final dos períodos secos e chuvosos) nos anos hidrológicos de 2009 e 2010, totalizando 4 momentos de coleta. Analisou-se o comportamento espacial e sazonal da qualidade da água na sub-bacia utilizando o diagrama de Piper e a origem dos íons pelo diagrama de Gibbs e razão molar. Constatou-se que em relação aos íons, predominam águas sódicas em 82% e águas bicarbonatadas 76% (cátions e ânions), respectivamente. Os resultados obtidos sugerem que o principal mecanismo controlador da salindiade está relacionado a interação das partículas coloidais (sedimentos orgânicos e minerais) com os íons dissolvidos na água. Também não se observou indícios de contaminação antrópica na área, tendo por base a análise de nitratos e nitritos.Palavras-chave: Desertificação. Semiárido. Salinidade.

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The objective of this work was to evaluate the variation in the electrical conductivity and the mobilization of ions present in reject brine from desalination plant of brackish well water in three types of soil. The mobilization of the contaminant ions in the reject brine was studied in glass percolation columns, which were filled with soil of contrasting textures (eutrophic CAMBISOL, typic dystrophic Red OXISOL, ENTISOL Quartzipsamment). Experiments ware repeated three times each, and the initial and final concentrations of the ion contaminants were analyzed. The pollution potential of this residue was determined by the retardation factor and dispersion-diffusion coefficient of Ca 2+ and Mg 2+ , besides the variation of electrical conductivity along the profile of each soil studied. In the Red Oxisol, Ca 2+ and Mg 2+ ions move with greater ease resulting in possibility largest of potential of ground water contamination. In Entisol Quartzipsamment presented higher Mg 2+ (R) ion advancement speed, that is, higher subsurface contamination power for these ions. The eutrophic Cambisol presented low diffusion-dispersion coefficient in all the evaluated ions and, therefore, lower mobility of the ions in the soil profile, and consequently, a greater possibility of contamination when irrigated with reject brine.

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The desalination of the brackish waters is a technology of coexistence with the semi-arid and its adoption implies in determining the environmental impacts due to the reject brine generated. Therefore, this work evaluated the hydrochemical characteristics of well water (feed water), drinking water (purified) and reject brine from reverse osmosis desalination plants in Western Meso-region, Rio Grande do Norte state, Brazil. During the periods between October 2013 and November 2014, four water samples were taken at seven inland desalination plants in communities and rural settlements. The hydrochemical characteristics of the waters were evaluated using multivariate statistical analysis. The following physico-chemical parameters of water collected were evaluated: electrical conductivity (ECw), pH, concentrations of cations (sodium, potassium, calcium and magnesium) and anions (chloride, carbonate and bicarbonate), Sodium Adsorption Ratio, Langelier Saturation Index, Stability Index of Ryznar, and the Calcium/Magnesium ratio. Well water and reject brine were classified into six groups, while purified water was classified into seven groups with different levels of risk of salinization, sodification and ions toxicity.

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-The conventional method monitoring and adequateness of underground dams requires invasive investigatory actions with in their interior structure and only provides specific information for small volumes. On the contrary, application of non-invasive sensing techniques, such as ground penetrating radar (GPR), makes it possible to investigate and explore underground without affecting their structure. A GRP 3D profile was acquired with the aim to allot and monitor an underground dam in the Brazilian semiarid region in the alluvial sub-watershed of the Riacho Pau Lavrado in the Sertão Central de Lajes region of the state of Rio Grande do Norte in Brazil. The GPR profile records were acquired along an alluvial section, processed using the Reflex 5 software and exported into the Opendetec 4.4.0 software in order to obtain a virtual 3D model. The GPR 3D profile of the alluvial deposits shows irregularities of the crystalline basement and external topographical surface (soil). The interpretation of the alluvial GPR profile allows precise adequateness of the underground dam, which supports an increase in the area of hydric accumulation and promotes low-cost building due to the allocation selected with minor sedimentation.