Continuous monitoring of Eh, pH and CE of a coastal multilayer alluvial aquifer under different dynamic conditions.

<p>Understanding the behavior of the freshwater-seawater interface and its dynamics is a key issue to characterize seawater intrusion (SWI) as well as submarine groundwater discharge (SGD). A new experimental site intensively monitored and characterized was constructed north of Barcelona city (Spain) to gain insights in both phenomena, their interaction and all processes taking place in coastal aquifers. The site comprises 22 piezometers located between 10 and 90 m from the coastline. 16 piezometers are organized in four nests of four partially penetrating piezometers (2 m screened) at different depths ranging between 10 and 25 m.</p><p>Previous studies indicated that the aquifer, which initially looked like a homogeneous unconfined aquifer 22 m thick, effectively behaves as a multi-aquifer and reactive system with freshwater discharging beneath saltwater at several depths. In order to improve understanding of the biogeochemical processes taking place in the aquifer and their dynamics, the 3 nests following a perpendicular line to the sea were equipped with specific sensors at all screened intervals (6, 12, 18 and 22 m depth). In each piezometer, Eh, electrical conductivity, pH, temperature and pressure was recorded at 15 minutes of temporal resolution. In this presentation we will show the initial results of several months of monitoring, which highlights different dynamics at different depths despite being a theoretically &#8220;homogeneous&#8221; alluvial aquifer.</p><p>&#160;</p><p>Acknowledgments</p><p>This work was funded by the Spanish Government (grant no. PID2019-110212RB-C21 and PID2019-110212RB-C22) and the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency.</p>