Multivariate statistical and geostatistical techniques for redox zonation assessment in a tropical alluvial aquifer

When high concentrations of Mn and Fe are detected in groundwater, the redox processes are likely occurring in the aquifer. They are due to organic matter oxidation, which can be naturally present in the solid matrix or released in the aquifer by anthropic activities. Thus, the definition of redox zonation within an aquifer can be an effective tool for the conceptual model refinement in remediation strategies planning. In developing countries, either the incorrect management of wastewaters or the land use distribution can affect the environmental quality of surface-water, creating eutrophication conditions in the main streams. In hydrogeological contexts where a multilayer alluvial aquifer is connected with surface-water, also the groundwater quality degrades and redox processes can occur. This issue complicates the groundwater study, its exploitation for drinking purpose and its management. The objectives of the study are to investigate the effect of the surface-water/groundwater interactions of San Pedro Sula alluvial aquifer, located in north-western Honduras, on the redox zonation, performing a partition of the aquifer in homogeneous areas. The datasets available refer to 2 monitoring rounds performed in 2002 (wet and in the dry season) consisting of 94 groundwater samplings. In each sample, chemico-physical parameters (pH, EC, Temp, Turb) and analytes (Na, K, Ca, Mg, HCO3, Cl, SO4, NO3, NH4, PO4, Fe, Mn) have been assessed. To assess hydrogeochemical spatial relationships among the variables involved in the redox processes, a multivariate geostatistical approach was performed, called Factorial cokriging, consisting in three main steps: 1) Gaussian anamorphosis to transform raw variables into Gaussian transformed variables; 2) fitting a Linear Model of Coregionalization, including the variogram models of both direct and cross-variograms; 3) extraction and interpolation of the sets of scale-dependent regionalized factors. The first results show a strong relation of Mn concentration with redox processes, which may be ascribed to the organic matter transfer from heavy polluted surface-water to the aquifer. For Fe, its relation to turbidity can be due to a fine colloidal phase developed when different groundwaters, characterized by distinct redox conditions, mix up in the wells.