New insights on advanced redox zonation of aquifers using multivariate geostatistics: the San Pedro Sula case study

The incorrect land use and wastewater management represent anthropogenic pressures on the environment, which can create heavy euthrophication conditions in surface-water. When surface-water/groundwater relationships exist, the organic matter produced in the euthrophicated streams can be transferred into the aquifer, triggering redox processes (i.e. Terminal Electron Accepting Processes, TEAPs). These hydrogeochemical processes provoke severe groundwater quality modifications (e.g. Mn and Fe solubilization), that complicate its exploitation and management. The definition of the redox zonation of aquifers represents an effective tool for the identification of the pollution sources and for the conceptual model refinement, when remediation strategies and groundwater management plans need to be implemented. The study area is the San Pedro Sula aquifer (north-western Honduras), which is a multi-layer alluvial aquifer characterized by well-known surface-water/groundwater interactions and by heavy euthrophicated streams. Here, high concentrations of Mn and Fe have been found in the aquifer (Di Curzio et al., 2016). Although the redox processes are dynamic reactions, the redox zonation is generally aimed to identify homogenous zones within an aquifer characterized by a predominant TEAP (McMahon & Chapelle, 2008). To overcome this methodological approach, the Multi-Collocated Factorial Kriging (MCFK) (Sollitto et al., 2010) has been applied to chemico-physical parameters and analytes, diagnostic of the redox processes (i.e. temperature, pH, turbidity, Mn, Fe, NO3, NH4, PO4). These parameters have been measured in 93 wells, both in the wet and dry season. In addition, the distance from the surface-water has been selected as an auxiliary variable, essential to perform the MCFK, because the eutrophicated streams have been considered one of the pollution sources. The MCFK results show a short range variability, highlighting a strong relation between Mn concentrations and redox processes, due to the organic matter transfer from heavy polluted surface-water to the aquifer. Simultaneously, the relation between Fe and turbidity can be due to a fine colloidal phase, developed when different redox conditions of groundwater mix up in the wells. At a wider range, Fe seems to be related with redox processes, near the other pollution source detected in the northern San Pedro Sula alluvial plain.