Groundwater monitoring
The contamination risk is often not greatest where monitoring is strongest, but where abstraction is happening outside the main line of regulatory sight.
Researchers examining the São Paulo Metropolitan Area argue that groundwater use in and around former industrial districts is still being monitored too weakly, despite the scale of dependence on private wells and the persistence of toxic industrial contaminants in the subsurface. Their paper, published in Environmental Earth Sciences, focuses on sites contaminated by chlorinated solvents and highlights a troubling mismatch between urban groundwater use, legacy contamination, and the fragmented way these risks are managed.
For an environmental monitoring audience, this is not just a Brazil story. It is a sharp example of what happens when groundwater abstraction, contaminated land management, and long-term subsurface monitoring are treated as separate administrative problems even though, hydrogeologically, they are part of the same system.
The São Paulo Metropolitan Area has around 22 million inhabitants and consumes an average of 61.6 cubic metres of water per second. Most public supply comes from surface water, but the study says aquifers still account for roughly 18% of total consumption through about 14,000 private wells, contributing around 347 million cubic metres per year. The key problem is that roughly two-thirds of those wells are not formally registered.
That alone creates a major monitoring blind spot. If regulators do not know where many wells are, they cannot assess whether they sit inside contaminated belts, whether pumping is drawing pollution deeper into the aquifer, or whether users are being exposed to unsafe water. As one of the study authors puts it, for every three wells drilled, two are effectively off the books.
For monitoring professionals, that is the core issue. The problem is not simply contamination in the abstract. It is contamination interacting with hidden abstraction.
The study focuses on former and current industrial areas affected by chlorinated solvents such as perchloroethylene and trichloroethylene, compounds long used as degreasers and in some dry-cleaning operations. These substances are especially difficult from a groundwater perspective because they are persistent, highly toxic and can break down into daughter compounds that may be even more hazardous than the original contaminant.
That makes them a different kind of monitoring challenge from more readily degradable fuel-related pollution. The paper notes that while hydrocarbons leaked from petrol stations often degrade more quickly, chlorinated solvents can remain in the subsurface for long periods and continue migrating. Their potability limits are extremely low, on the order of parts per billion, meaning relatively small dissolved concentrations can compromise very large volumes of groundwater.
This is exactly the kind of contamination that punishes weak monitoring. By the time a problem is detected through ad hoc testing, the plume may already have penetrated deeper parts of the aquifer.
One of the study’s strongest contributions is also one of the simplest: it overlays three kinds of data on the same map — industrial zones, areas officially contaminated by chlorinated solvents and water supply wells. In São Paulo, those layers frequently overlap. The paper cites districts such as Mooca, Jurubatuba, Jaguaré and Vila Prudente, as well as municipalities including Diadema, Mauá and Osasco, as areas where contaminated sites and abstraction wells sit uncomfortably close together.
Under São Paulo state law, if there is a contaminated area within 500 metres of a well, the responsible party must submit water quality reports to the environmental agency. Applying that criterion, the researchers identified 17 clusters where contaminated areas and wells fall within overlapping radii. But even that may understate the problem, because unregistered wells are absent from the mapped picture altogether.
For monitoring readers, this is where the story becomes especially useful. It shows the value of integrating contaminated land registers, abstraction data and geospatial analysis rather than treating each as a separate database exercise. The overlap itself is the warning.
The paper also underlines a mechanism that groundwater specialists will recognise immediately: deep pumping does not just extract water, it alters hydraulic gradients.
According to the authors, many chlorinated-solvent impacts are initially concentrated in shallower zones. But pumping from deeper wells, sometimes around 100 metres, can create a downward gradient that slowly pulls contaminated water deeper into the aquifer. Less permeable layers may provide some partial barrier effect but the study says there is still substantial uncertainty about how protective that remains over long periods.
That point matters because it shifts the story from one of passive contamination to one of monitoring-linked risk amplification. In other words, the well network is helping to redistribute contamination vertically.
The researchers are also critical of how contaminated land is often managed. Many sites are handled at the property scale, with remediation and risk controls defined by plot boundaries. But groundwater plumes do not respect cadastral lines. The paper argues that São Paulo contains belts of multipoint contamination, with intersecting plumes and incomplete remediation, especially in deindustrialised districts now being redeveloped for housing, services and commerce.
By 2020, the study says only 18.6% of chlorinated-solvent-contaminated sites had been classified as “rehabilitated for the declared use”, and even that does not mean the contaminant mass has been fully removed. Among 596 areas identified as having a history of chlorinated solvents, more than half were still under remediation and 26% remained under investigation.
For environmental monitoring professionals, the implication is clear: a compliance model built around individual parcels and isolated reports will struggle when contamination is regionally distributed and abstraction is widespread.
The broader relevance of the paper is that it describes a pattern likely to exist in many large urban areas with industrial legacies, informal abstraction, and patchy environmental records. Former factories become mixed-use developments; groundwater remains attractive to private users; contamination persists underground; and oversight remains fragmented between land, water, and redevelopment authorities.
The authors call for stronger databases, multidisciplinary technical teams, and more systematic regional assessments. More importantly, they argue that these areas need to be treated as integrated hydrogeological systems rather than as a set of isolated contaminated plots.
That is the real monitoring lesson here. Groundwater risk is not being missed because the science is mysterious. It is being missed because the monitoring architecture is too fragmented for the way aquifers actually behave.
IET 36.3 May
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