Spatial perspective of hexavalent chromium concentration in superficial waters of the Ciénaga de las Quintas Mangrove Swamp, Cartagena de Indias, Colombia

A spatial characterization was undertaken to demonstrate the presence of hexavalent chromium (Cr (VI)) in surface waters (maximum depth of 0.5 m) in the Cienaga de las Quintas mangrove swamp, located in the city of Cartagena de Indias, Colombia. This characterization was geostatistical in nature, an approach that is little used in such studies, although it offers two‐dimensional profiles of the concentration of relevant contaminants (hexavalent chromium in this case) with concentrations between 3 and 9.2 μg/L. Sampling was carried out systematically on a square grid with a cell size of one ha (100 m × 100 m). To corroborate the relationship between the sample positions and concentrations, the Euclidean distance matrices of each variable were calculated previous step for conducting the Mantel test, which indicated a positive correlation between the distance and concentration of hexavalent chromium (R = 0,1674: p value = 0.1275). The Moran Index was also calculated for Cr (VI). The experimental semivariogram (γ)was also calculated and fit to the exponential theoretical model, with a correlation coefficient of 99.4%. Using this analytical model of variance, predictions of Cr (VI) concentrations were made for unsampled locations, using the Kriging method with a new mesh side of only 10 m. This approach indicated the new mesh points exhibiting the higher concentrations coincided with locations in the swamp that receiving sewage effluents from the Bazurto public grocery market and the Chinatown neighbourhood. The Cr (VI) contamination was of anthropogenic origin because its concentration displays a gradient from the zone of human activities to the Cartagena de Indias bay.