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Coastal communities often face unique challenges over access to fresh water.

There are new, better, non-intrusive ways of exploring and safeguarding the groundwater they rely on, as a Universitat de Barcelona study discovered, with the help of ERT, Leapfrog Geo, and AGS Res2DInv.

Surface water resources in coastal regions are scarce, and managing groundwater to ensure a reliable freshwater supply encounters a mosaic of intertwined problems. Seawater intrusion is common and only made worse by urban development, tourism, and agriculture. Which intensifies groundwater extraction, leading to further intrusion, and so on. It’s a self-perpetuating problem, and smaller aquifers are particularly vulnerable.

Water percolation through infiltration ponds is one possible and appealing way to support wastewater reuse as an additional, non-conventional water supply.

But the hydrogeological conditions and the processes involved mean that successfully realising this potential requires an intensely detailed understanding of all the intricacies within the system.

(a) Location sketch of the studied area (b) Port de la Selva (1956) soil used for agriculture vs. current urbanization (c) Main hydrological features and water management facilities

Unlocking the possibilities of soil aquifer treatment

Port de la Selva on the north-eastern coast of the Iberian Peninsula has adopted a pioneering approach to the usage of SAT (Soil Aquifer Treatment) to recycle wastewater for recharging aquifers devoted to human consumption – prompted in part by the area’s leap in population from 1,000 in winter to 10,000 in the holiday months.

To help demonstrate the viability of using the natural treatment capacity of the soil and aquifer (without the need for ozonation or reverse osmosis), a thorough understanding of the hydrogeological conditions was required. In a study by the Universitat de Barcelona, electrical resistivity tomography (ERT) was chosen as a non-invasive technology to provide the detailed subsurface information needed for accurate groundwater modelling.

An interdisciplinary team – scholars, professionals, and students – gathered more than 6300 data points from two electrical resistivity campaigns, and the study turned to a software partnership of Leapfrog Geo and AGS Res2DInv to manipulate and interpret that mass of information.

Speed to insight

“If we didn’t have the ease of integrating geophysical and lithological data from boreholes in Leapfrog Geo software, there’s no way we could have developed the aquifer and saltwater model that fast with a high degree of confidence,” says Alex Sendrós, a postdoctoral researcher at the university.

Alex’s team relied on AGS Res2DInv to invert the electrical resistivity data and Leapfrog Geo to deliver intuitive workflows, rapid data processing, and visualisation tools. Together they forged a comprehensive database of geophysical and lithological information across the site.

Previous modelling studies had only explored aquifer responses to rainfall and pumping. Delineating saltwater intrusion requires complex and time-consuming multiple depth sampling at different water control points.

However, with Leapfrog Geo, the lithological log from boreholes and resistivity data could be progressively and rapidly incorporated into the 3D aquifer geometry and saltwater intrusion models using implicit modelling tools. The ERT studies quickly revealed roadblocks to Port de la Selva’s ambitions. For example, the salinity of a shallow aquifer close to the coastline and a low resistivity anomaly triggered by water pumping. The results facilitated the town council’s decision to take effective action, including using a portable desalination plant at one location and recharging the shallow aquifer with treated water to guarantee water supply using only local water resources.

Resistivities interpolated in 3D, from the data in the section.
Examples of ERT cross-section results where B = bedrock (aquiclude unit), SZ = Saturated shallow aquifer, and black dashed line = inferred hydrogeological contact.

Time management, robust procedures, and winning over stakeholders

During the model elaborations procedure, the research team had to make quick and robust simulations using different threshold parameters. The less time spent on data input, compilation, and results in visualisation, the more time they had to construct more reliable models using the proposed methodology at other sites and on larger scales.

“It’s all about time management and working with robust procedures, especially in the research field where we’re working with a large amount of data”, explains Alex. “We have to deliver top quality results to convince stakeholders and scholars to embrace the approach.”

For the geophysical data analysis phase, one geologist could carry the inversion of gathered subsoil resistivity data (step 1) and include it in the Leapfrog workflow (step 2). With less than one week of training, other team members could follow the two-step procedure using AGS Res2DInv and Leapfrog Geo.

“The ability to create and modify inversion parameters and thresholds is critical in delivering accurate and robust models”, says Alex. “The 3D modelling tools improve their use as a decision tool for groundwater managers in general and managers of SAT projects in particular.”

The research team will continue their work by using the AGS Res2DInv and Leapfrog Geo workflow to develop sustainable water management procedures while improving stakeholders’ commitment to the benefits of SAT and the boost it can bring to water security for coastal communities.

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