For some 300 kilometres, groundwater flows underground between France, Germany and Switzerland, forming Europe's largest aquifer. But a new study has uncovered severe pollution of these waters.
From Frankfurt to Basel, deep underground lies a treasure that supplies drinking water to more than five million people in the region. It is Europe’s largest groundwater reserve. It stretches for some 300 kilometres below the surface and feeds, among other things, the Upper Rhine and numerous wetlands.
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But this vast groundwater reservoir is under severe strain, as a study published in June has shown. Pesticides, pharmaceutical residues and synthetic industrial chemicals known as per- and polyfluoroalkyl substances (PFAS) are polluting the groundwater; corresponding trace substances have been detected.
This underground basin of 150 billion cubic metres is not only vital for drinking water, it also influences countless animal and plant species. New discoveries are made again and again, such as the groundwater crayfish Parabathynella baden-wuerttembergensis. A journey beneath the surface between Germany, France and Switzerland.
Underground: the invisible river in the Rhine valley
The reservoir stretches from Frankfurt am Main across the French border to Strasbourg and further south to Basel in Switzerland. The volume of water is hard to grasp – 150 billion cubic metres is equivalent to around 60 million Olympic-size swimming pools.
The groundwater reserve in the Upper Rhine Valley is roughly comparable to three interconnected Lake Constances underground. Lake Constance itself contains around 48 billion cubic metres of water.
Groundwater is the term used when the surrounding soil is permanently saturated with water. The layers above are not constantly moist; this is the unsaturated soil zone. Depending on the season and rainfall, the water table can fall or rise.
The groundwater used for drinking water is often only a few to a few hundred metres deep. In some places it flows just about a metre below the surface, slowly heading north. But in the Upper Rhine Graben, drilling has already reached 3,335 metres towards the Earth’s core. The so-called “Frankenthal 10” operation, carried out between Mannheim and Worms, is considered the deepest borehole to date.
The deepest known section of the Upper Rhine Graben is the so-called Heidelberger Loch, where groundwater lies at a depth of more than 500 metres. Researchers say other parts of the system are as little explored as the deep sea.
Pesticides and pharmaceutical waste: pollution from harmful substances
As Europe’s largest groundwater body, the Upper Rhine Valley supplies drinking water to almost five million people in its catchment area. That makes it, compared with other large reservoirs in Europe, one of the most valuable groundwater systems.
For some time, however, the area has been increasingly exposed to human-made pressures. Excessive use of fertilisers and pesticides in particular is putting growing pressure on the groundwater, as shown by the findings of the cross-border Ermes II study (source in German) published in June 2026.
Some 96% of the 1,500 monitoring sites examined – spread along the Rhine between southern Germany, northern Switzerland and eastern France – are contaminated with at least one micro-pollutant. Plant protection products (pesticides) have been identified as the main cause. But in addition to agriculture, nearby towns, industrial facilities and numerous wastewater treatment plants are also contributing to the pollution.
The tests, however, examine raw water. Drinking water is not drawn directly from the groundwater, but undergoes special treatment before distribution. The condition of the groundwater only determines the extent to which the water has to be treated before it meets public health standards.
What the cocktail effect has to do with our drinking water
At 59% of the monitoring sites, at least one drinking-water limit value was exceeded, according to the presentation of the study. One of the most widespread substances is trifluoroacetic acid (TFA), which scientists describe as the “ultimate PFAS”.
PFAS, also known as “forever chemicals”, are man-made compounds used by industry in the production of a host of consumer products such as electronic devices, paints, cars and cosmetics. TFA is the PFAS compound most frequently detected in groundwater; it is therefore considered ubiquitous in the environment.
The experts behind the study also refer to a “cocktail effect”. When certain substances or groups of substances occur together, their toxic effect can increase – even at low concentrations. Precisely what impact this has on health and the environment is still largely unknown.
The aim of the study, which has been running for three years, is to identify the reasons for the deterioration in groundwater quality. On this basis, measures are to be developed to prevent any further decline. Regular analyses have been carried out since 1991.
“The findings show that groundwater protection continues to face major challenges,” said Dr Dirk Grünhoff, president of the state environment agency in Rhineland-Palatinate. The data, he added, provide a solid basis for meeting these challenges and devising concrete measures.
Micro-organisms take on a cleaning role
Until now, nature itself has taken care of part of the clean-up. One of the few animals to bear the name Baden-Württemberg lives in the groundwater reservoir: a tiny species of groundwater crayfish, Parabathynella badenwuerttembergensis. Discovered in groundwater in 2013 by researchers at the University of Koblenz-Landau, it plays a particularly important role. Acting as a kind of clean-up crew, these tiny crustaceans break down organic material and devour bacteria.
This groundwater crayfish is an ancient, primitive and oddly shaped creature from a time more than 200 million years ago, as the Baden-Württemberg state environment agency explained in 2013. Its body structure is thought to have changed little over millions of years. The agency described the groundwater as a “living geological museum” and the animals as “living fossils”.
