Conceptual model of the aquifers lying beneath the Recife urban area, showing the sources and processes of salinisation.
© Cary et al., 2015
Studies of global change impacts are key to understanding and anticipating specific environmental changes in coastal aquifers. The aim is to understand the mechanisms governing the degradation, in terms of both quantity and quality, of these water resources from a societal and systems perspective. This requires a multidisciplinary approach combining the "hard" sciences (geology and hydrogeology, geochemistry and modelling) with the human and social sciences (sociology, anthropology).
The BRGM coordinated this groundbreaking ANR-funded project, which involved 9 partners, in Recife, the capital of Brazil's Nordeste. It also contributed its expertise in analysis and interpretation, with a genuinely innovative strategy of combining geochemical tools and isotopic tracers to better understand how these aquifers function under intense pressure.
Recife : investigating an entire aquifer system !
Public water supplies in Recife depend on its reservoir and an obsolete network from which leakage has worsened with successive droughts to the point where water is now rationed. There are a number of alternative water supply sources, including the coastal aquifer system. However, the water table in the aquifer has dropped by up to 70 metres in 20 years, and the water abstracted is highly saline in places. In short, Recife's entire aquifer system needed to be investigated ! This was the aim of the COQUEIRAL project.
Water rationing and failure of the supply network: how people in Recife have to cope.
© Tadeu Giglio
A multi-tracer approach combining chemistry and isotope analysis
Coastal aquifers are subject to intense human pressure, due to high demand but also to multiple sources of contamination. A whole range of tools are therefore needed to discriminate between these sources and the resulting geochemical processes in the aquifer system.
The research team developed a multi-tracer approach combining chemical and isotopic analyses and based on the intrinsic discriminatory properties of tracers used individually or in combination. The method combining several isotopic tools, which were mostly developed in the BRGM's laboratories and applied for the first time in Brazil, produced breakthrough results. The hydrogeological functioning of Recife's aquifers is now better understood.
Carbon 14 dating and analyses of stable water isotopes and noble gases showed residence times of more than 10 000 years, with recharge occurring in a climate that was cooler than now. Concentrations of CFC-SF6 showed recent infiltrations of water from shallow to deep aquifers, due especially to intense pumping and the multiple wells drilled through the clay layer between the two main aquifers.
The salinity is largely a legacy of Pleistocene marine transgression ; present-day saltwater intrusion as a result of overabstraction is limited to the densely populated zones along the coast (detected by means of boron and strontium isotopes). The boron isotopes also traced a saltwater intrusion from the river into the deeper aquifer. When combined with sulphur and oxygen isotopes, the boron isotopes also showed contamination by wastewater seeping directly into the shallow aquifers : the boron isotope signatures in the wastewater were typical of bleaching agents used in detergents.
In general, this "toolbox" method combining several chemical and isotopic tracers is becoming more and more necessary to understand and manage water resources in coastal areas that are subject to ever-increasing human pressure.