Combining long term monitoring of distributed temperature and cross-hole ERT with hydraulic tests to characterize the seawater intrusion and SGD dynamics in a coastal aquifers

Understanding the behaviour of the saline water interface and its hydrodynamics is a key issue to characterize submarine groundwater discharge and manage groundwater resources in coastal aquifers. With the objective of testing, comparing and combining different methods of monitoring and characterization, an experimental site was constructed in 2015 in the near shore part of an alluvial aquifer situated north from Barcelona city (Spain). The site, located between 30 and 90 m from the seashore, comprises 16 shallow piezometers organized in nests of three (2 m screened each) with depths ranging from 15 and 25 m and 4 stand-alone piezometers. The deepest piezometers of each nest and the stand-alone piezometers are equipped with electrodes every 75 cm in order to perform cross-hole electrical resistivity tomography (CHERT). All piezometers are equipped with Fiber Optic cable to perform Distributed Temperature Sensing (FO-DTS). Two Fiber Optic cable lines of nearby 600 m length each were installed around all boreholes with a spatial resolution of 1 m. These two techniques allowed to monitor at a great detail the solutes and thermal response from the seawater intrusion to changes in the boundary conditions. Additionally, hydraulic characterization of the site was carried out through pumping test, slug test and tide response analysis. In this work, we combine hydraulic testing information with more than a year data on electrical conductivity and temperature, to identify different dynamics of the seawater intrusion with implications for the interface distribution and the submarine groundwater discharge. The data shows different temporal and spatial scales behaviours of the seawater intrusion, related to seasonal variations and aquifer response to short-term events such as heavy rains and storm surges. Combination of the different methods allowed enhanced understanding of the processes driving seawater intrusion dynamics and submarine groundwater discharge.