Coastal aquifer behaviour related to the textural and mineralogical characteristics of the sands in the eastern coast of the province of Buenos Aires

Abstract

There are many coastal aquifers in South America on which there is little data, even lacking preliminary hydrological studies. When analyzing the regional hydrogeological behaviour of the coastal plain of the province of Buenos Aires, Argentina, the Pleistocene–Holocene evolution is considered to be especially relevant. This is particularly so in the case of the sand formations, as they constitute aquifer units due to their high permeability and storage capacity. It was the objective of this work to undertake a detailed granulometric and mineralogical study of the formations that occur in this coastal aquifer system in order to determine its parameters and hydrochemical behaviour. Hydrologically speaking, the aquifer unit is composed of the Punta Médanos Formation, the Mar de Ajó Facies and the upper section of the Destacamento Río Salado Facies, while the clayey levels act as an aquitard/aquiclude. The decrease in grain size with depth suggests a decrease in effective porosity in the same direction, with the resulting reduction in flow velocity. This was corroborated by the fact that the hydraulic conductivity values obtained depended on the sediment grain-size variations in each formation. The mineralogical study shows abundant magnetic, opaque (sulphides and oxides) and mafic minerals with Fe and Mg–Mn in their composition. These could be the source of the high concentrations of Fe and Mn in the regional groundwater, which exceed the values that are fit for human consumption; this represents a problem, both for water service providers and for inhabitants with no access to a water supply network.