The enrichment of groundwater with different nutrients (Na+, Ca2+, Mg2+, CL-, CO32-, among others) triggers the salinization of the aquifer and makes it inappropriate for many purposes. In this work, we developed a highly sensitive and selective electrochemical sensor, based on Ni-inorganic films electrosynthetized in situ onto multi-walled carbon nanotubes composite paste electrode (MWCNE), which allows the early detection of salinization. The working sensor (beta-NiOx/MWCNE) was derivatized from nickel hexacyanoferrate modified electrode in strong alkaline medium (pH = 12), producing a hybrid film composed by beta-Ni (OH)(2) and beta-NiO(OH). The electrochemical properties, morphology and chemical composition of the formed beta-NiOx thin films were evaluated by voltammetry, scanning electron microscopy and X-ray spectroscopy. The developed beta-NiOx/MWCNE sensor was highly sensitive to the presence of Isle cation by ion exchange, and the increase of Na+ concentration in the range 4.46 x 10(-7) to 4.93 x 10(-8) mol L-1 inhibited linearly the reversible electrochemical signal of the device, allowing to determine trace concentrations of this ion (LOD = 9.86 x 10(-8) mol L-1) with high correlation coefficient of the data (r = 0.999) and suitable precision/reproducibility of the measurements (RSD < 9%). Using Nal. as salinization marker and, beta-NiOx/ MWCNE as electroanalytical device, we found evidences of groundwater salinization in Grossos, a Brazil coast city, whose inhabitants have hypertension above the national average. The attained results were comparable to those obtained by the standard methods for Na+ analysis (percentage error ranging from 0.5 to 1.6%), confirming the accuracy of the proposed electroanalytical platform. (C) 2017 Elsevier B.V. All rights reserved.