Ancient Waters and Rare Earth Clues Redraw Brazil’s Guarani Aquifer Map

Beneath the rolling hills of São Paulo state lies one of the most consequential water systems on Earth. The Guarani Aquifer System stretches across 1.1 million square kilometers of Paraguay, Uruguay, Brazil, and Argentina, storing an estimated 30,000 cubic kilometers of groundwater—enough to submerge the entire planet in a shallow sea if spread evenly. Now, a new study has delivered a surprising message about the aquifer’s outcrop zone in Brazil: the water hidden deep within it is not the young, freely renewable resource that conventional models assumed, but a paleo-relic recharged under a climate far colder than today’s, trapped behind walls of ancient volcanic rock.

The research, conducted in the headwater basin of the Corumbataí River by scientists at the University of Campinas, integrated three independent lines of geochemical evidence—major ion chemistry, stable isotopes of hydrogen and oxygen, and the concentrations of rare earth elements and yttrium, collectively known as REY. The team collected 39 water samples during wet and dry season campaigns in 2022, drawing from springs, rivers, shallow wells, and deep production wells reaching 136 to 200 meters into the aquifer. The goal was ambitious: to rebuild, atom by atom, the conceptual model that governs how water moves, mixes, and ages in this critical recharge zone.

The first surprise came from the deep wells. Groundwater pumped from the Guarani Aquifer System at depth turned out to be strongly alkaline, with a median pH of 9.7, and dominated by sodium and bicarbonate ions—a chemical signature that previous regional models assigned exclusively to confined zones located 20 to 100 kilometers away from the outcrop area. Here, instead, that same facies was found directly beneath the recharge zone, separated from shallower waters not by kilometers of gradual evolution but by thin sheets of diabase, an intrusive volcanic rock from the Serra Geral Formation that sliced into the sandstones roughly 130 million years ago. These sills behave as aquitards, effectively splitting the aquifer into two hydraulically distinct worlds stacked one atop the other.

Stable isotope analysis sharpened the picture dramatically. Springs and surface waters in the basin cluster tightly along the local meteoric water line, with median δ2H values near –40 per mil and δ18O near –6.7 per mil, consistent with modern rainfall in a humid subtropical climate. The deep Guarani groundwater, by contrast, was markedly depleted, with median δ2H of –64.2 per mil and δ18O of –9.7 per mil—values so low that they point to recharge under climatic conditions perhaps 10 to 15 degrees Celsius colder than the present, echoing the last glacial period. In other words, the water supplying public wells in the outcrop area may have entered the ground more than 10,000 years ago and, on any human timescale, is not being replaced.

The rare earth element data added a dimension that conventional hydrochemistry had never captured. Because the lanthanides and yttrium fractionate predictably as water reacts with rocks and travels through an aquifer, their patterns serve as fingerprints of flow paths and residence times. The deep Guarani samples showed extremely low total REY concentrations, with a median of just 0.02 micrograms per liter, strong depletion of light rare earths, and pronounced negative cerium anomalies—hallmarks of geochemically evolved water that has spent a long time underground. Shallow and surface waters carried substantially higher REY loads and less fractionated patterns, marking them as young and chemically immature.

When the researchers superimposed REY concentrations onto the classic Piper diagram, groupings emerged that no major ion analysis alone had ever revealed. One cluster of mixed chloride-nitrate waters carried anomalously high REY loads above 2.8 micrograms per liter, together with elevated nitrate, pointing to anthropogenic contamination of the shallow unconfined Guarani aquifer. Another group bound the springs tightly to the surface waters, confirming their intimate hydraulic connection. A third group captured the sodium-bicarbonate deep waters with REY concentrations below 0.09 micrograms per liter, sealing their identity as isolated, long-residence groundwater sealed off by the diabase.

Ionic ratio analysis told a complementary story. In springs and rivers, the dominant geochemical process is the weathering of feldspar and ferromagnesian minerals in the sandstones and basalts, releasing calcium, sodium, and silica in proportions that match the local geology. In the deep aquifer, the dominant process instead is cation exchange, which strips calcium from solution and loads the water with sodium—explaining the characteristic Na-HCO3 composition. At two of the deep wells, the data even hinted at plagioclase weathering within the diabase sill itself, showing that the volcanic intrusions are not inert barriers but active participants in the water’s chemistry.

The isotope mixing calculations quantified how differently the basin’s two aquifer systems behave. Using the line-conditioned excess, a sensitive indicator of evaporation and recharge seasonality, the team estimated that springs issuing from the Guarani aquifer draw roughly 71 percent of their flow from groundwater with longer residence times, while springs from the overlying Bauru-type sediments receive about 73 percent of their discharge from recent precipitation. This asymmetry reveals that the Guarani outcrop zone is not a simple sponge soaking up rain and releasing it downhill; it is a layered system in which some springs are fed by deep storage and others by rainfall racing through thin soils.

The management implications are stark. The semi-confined groundwater being tapped by public supply wells in the outcrop area is, according to the isotope evidence, old to very old—likely non-renewable on human timescales. Every liter extracted is, in effect, mined from a paleowater reserve recharged under ice-age climates. The study’s authors argue that groundwater management in the region must be urgently improved and regulated, prioritizing surface water sources and modern springs where possible, while recognizing that those same springs are vulnerable to the urban and agricultural contamination already signaled by nitrate and anomalous REY signatures in the shallow aquifer.

Scientifically, the work demonstrates that rare earth elements, long underused in hydrogeology, can expose aquifer compartmentalization that conventional tools miss entirely. By combining REY fingerprints with stable isotopes and classical hydrochemistry, the researchers showed that hydrochemical facies previously thought to be separated by tens of kilometers can coexist at a single location, divided only by a diabase sill. The finding echoes recent work in Uruguay showing that the Guarani Aquifer System behaves far more complexly than gradual, eastward-evolving flow models predict. For the millions of people who depend on this vast reservoir, the message is clear: the map of their water has just been redrawn, and the deepest layer of it is older, more fragile, and more finite than anyone managing it had assumed.

Subject of Research: Hydrogeological conceptual modeling of the Guarani Aquifer System outcrop area in São Paulo, Brazil, using stable isotopes and rare earth element tracers

Article Title: Hydrogeological conceptual model for the Guarani Aquifer System outcrop area in Brazil: Insights from stable isotopes and rare earth elements

Article References: Bassetto-Ferreira, R., Enzweiler, J., & de Abreu, A. E. S. (2026). Hydrogeological conceptual model for the Guarani Aquifer System outcrop area in Brazil: Insights from stable isotopes and rare earth elements. Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03164-6

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03164-6

Keywords: Guarani Aquifer System, hydrogeology, stable isotopes, rare earth elements, groundwater recharge, diabase sills, paleowater, São Paulo, aquifer connectivity, water management, hydrochemistry, springs