Use of geophysical borehole logging techniques and depth sampling methods to characterize the hydrogeochemical heterogeneity of the aveiro cretaceous multilayer aquifer

The Aveiro Cretaceous aquifer is a multilayered coastal aquifer formed by several hydrostratigraphic units with distinct grain sizes and mineralogical compositions, and with different hydrogeological properties. As is common in the region, most boreholes exploiting the aquifer have increasing depths towards the coast following the aquifer thickening and are multi-screened, pumping water from the different aquifer layers with known distinct hydrochemistry and residence times. The use of conventional methods of groundwater sampling from pumping multiscreened boreholes in the Aveiro Cretaceous multilayer aquifer inevitably leads to water mixing between the different aquifer layers, making the understanding of the water quality patterns more difficult, and eventually, leading to misinterpretations. To better address this problem or this uncertainty, it was decided to select seven non-equipped boreholes within the study area for detailed geophysical logging studies and depth sampling. The integration of lithologic logs, geophysical logs and depth sampling hydrogeochemical results allowed to confirm the borehole construction and to observe the vertical variation in the formation, aquifer and fluid properties (temperature and salinity). Flow logs showed that the Furadouro sandstone formation is contributing with most of the water supplied by the aquifer, with a ratio for the transmitted flow that could be as high as 9:1 when compared to the other aquifer layers. The depth sampling allowed the hydrogeochemical characterisation of each aquifer layer confirming the most permeable and productive layers as the ones with youngest and lowest salinity waters.