Correlation of High Iron Content in Groundwater of the State of Bahia, Brazil with Climate, Lithology, Soil and Vegetation, Using Multivariable Analyses

Abstract

This work developed a statistical correlation between groundwater’s high iron content in the four hydrogeological domains of the State of Bahia, Brazil, and the environmental attributes of climate, lithology, soil, and vegetation. From 3539 wells, flow test ≥ 1 m 3 ∙h −1 , drilling period 2003-2013, 940 wells with high iron content (>0.3 mg/L) were used in this study. All groundwater samples came from new wells soon after the drilling, well construction, and a long pumping time for their development: 24 hours for sedimentary aquifers and 12 hours for karstic, crystalline, and metasedimentary aquifers. The application of Pearson and Spearman linear regression to seventeen physicochemical parameters (SPSS V.12) resulted in no correlations between iron and fourteen parameters, indicating no common origin between those parameters and iron. Only color and turbidity presented correlations > 0.20 with iron. After spatializing the 940 values of iron concentration (ArcGIS V.9) on the maps of each environmental attribute, grades 1 - 5 were given to the variables of each attribute based on the largest iron concentration value. The grades allowed the application of multivariable methods PCA and FA (SPSS V.12). The PCA indicated two factors explaining 59.52% of the total variance, closely attending the recommended minimum of 60%. The significant factor weights from the application of FA were: in Factor 1, soil, −0.71; vegetation, −0.68; and lithology, −0.52; and in Factor 2, climate, +0.74. Indeed, in the crystalline and metasedimentary domains with mafic-ultramafic rocks rich in iron, percentages of wells, 53.3% - 66.7%, occurred in iron-rich soils; of 49.8% - 59.8% in humid to dry forest and of 55.3% - 86.8% in humid to sub-humid climate. While, for the sedimentary domain (primarily sandstones) and karstic domain (carbonate rocks) poor in iron content percentages of wells, 80.9% - 100% occurred in iron-rich soils, 57.0% - 61.8% in humid to dry forest, and 58.6% - 62.4% in sub-humid to dry and semi-arid climate. These results indicated that, although lithology is a determinant for high dissolved iron content in the state of Bahia groundwater, this attribute alone (factor weight −0.52) cannot explain the whole phenomenon. The present work, using multivariable analysis with geospatial mapping of high iron content on top of environmental attributes, revealed the role of each environmental attribute in groundwater’s high iron content. For the governmental drilling well company and its groundwater managers, this knowledge will result in better well locations and a reduction of both well and economic losses, as the long-term maintenance cost for the treatment process due to high iron content is prohibitive for rural municipalities.