Multidisciplinary approach to assess the seasonal effect on redox processes occurring in an tropical alluvial aquifer

The detection of high concentrations on Mn and Fe in groundwater are usually related with the occurrence of redox processes, which are clearly due to organic matter oxidation (Palmucci et al., 2016). In hydrogeological contexts where multilayer alluvial aquifer are in direct connection with surface-water heavily impacted by eutrophication (Lewandowski et al., 2015), the organic matter load in groundwater could change seasonally, due to the dilution effect related either to the river discharge variations or to the piezometric level fluctuation (i.e. wet season recharge), especially in tropical areas. For this reason, the San Pedro Sula alluvial aquifer, located in the north-western Honduras, was selected as study area. This is a tectonic basin filled by continental deposits whose thickness ranges from a few meters up to 250 m. Here, a multilayer alluvial aquifer is hosted and well known surface-water/groundwater interactions were identified (Di Curzio et al., 2016). The datasets used for this research refer to two monitoring rounds performed in 2002, both in the wet (February) and in the dry season (May), each one consisting of 94 groundwater samplings, 32 surface-water samplings, 56 hydraulic head measurements and 9 river discharge measurements. For the assessment of the seasonal variation of redox processes extent, a multidisciplinary approach was chosen, integrating a Principal Component Analysis (Palmucci et al., 2016) on several chemico-physical parameters and a Mn and Fe speciation, using the modeling tool Phreeqc (Appelo & Postma, 2005), both for the wet and the dry season. The first statistical, chemical and numerical results highlight a strong correlation of Mn concentration with redox processes, either in the wet or in the dry season. This is likely related to the organic matter transfer from heavy polluted surface-water to the aquifer and to the trigger of redox processes that solubilize Mn. For the iron, the correlation with redox processes has also been detected, enhancing its solubility in groundwater. In some cases, when the samples present high turbidity, the high concentration of Fe can be related to a fine colloidal phase, that is formed when different groundwaters mix up in the wells. As unusually expected, this phenomenon increases during the dry season, when discharge of main streams and the precipitation are more abundant. In this condition, surface-water transfers more organic matter to the aquifer and the redox processes play the foremost role on Fe mobilization