Detailed characterization of biogeochemical processes in coastal aquifers: variations along the subterranean estuary-mixing zone at the MEDISTRAES experimental site 

<p>Coastal aquifers are characterized by their unique land-sea interaction and the main difficulty in studying these systems is the complexity of their biogeochemical cycles. Important processes in the &#8220;mixing zone&#8221; of coastal systems are associated with the ecosystem&#8217;s diversity, and supply and exchange of chemical compounds. This coastal body known as a &#8220;subterranean estuary&#8221; (STE), is characterized by a free connection to the sea, creating an interface between freshwater and seawater. The distribution and composition of substances flowing from land to sea change because the submarine groundwater discharges (SGD) considerably dilute the seawater that has invaded the aquifer through the free connection to the sea (SWI, Sea Water Intrusion).</p><p>These processes were studied in an area 100 m long inland from the coastline and 30 m wide, in the alluvial aquifer of Argentona, Matar&#243;, northeast of Barcelona, Catalonia (Spain). This experimental site was established in 2015 and has been equipped for intensive monitoring of coastal aquifer processes such as SWI and SGD. Currently, it is monitored with 25 piezometers (2 meters screened) consisting of 5 nests with 4 piezometers each (at 10m, 20m, 15m, and 25m intervals) and 4 individual piezometers. In this system described as a &#8220;multi-aquifer and reactive system&#8221;, we are characterizing the physicochemical and hydrogeochemical conditions associated with biogeochemical processes at different depths and seasonal variations. For this presentation, we will show new results of hydrogeochemistry, nitrogen isotopes, and microbiological parameters associated with different periods of monitoring that characterize the dynamics in the subterranean estuary.&#160;</p><p>Acknowledgments: This work was funded by the Spanish Government (grant no. PID2019-110212RB-C21 and PID2019-110212RB-C22), the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency, and the SENACYT &#8211; IFARHU &#8211; BID Scholarship.</p>