Beach Sand Harbors More Fecal Bacteria Than the Water at Río de la Plata Beaches

A day at the beach usually ends with sand between the toes, a sunburn, and maybe a lingering worry about whether the water was clean. What rarely crosses the minds of swimmers and sunbathers is that the sand itself may be the dirtier half of the equation. A new study of the recreational coastline of the Río de la Plata, the vast estuary that borders Buenos Aires and its surrounding metropolitan region, has found that fecal indicator bacteria are consistently more abundant in beach sand than in the overlying water, and that in most of the beaches surveyed, microbial counts exceeded the guideline values recommended for recreational waters. The findings, published in the journal Environmental Monitoring and Assessment, add a South American freshwater estuary to a growing list of coastlines worldwide where sand is emerging as an underappreciated reservoir of fecal contamination.

The research was carried out by Jimena Camila Suárez, Ramiro Simonetti, Alejandro Mariñelarena and Joaquín Cochero of the Instituto de Limnología Dr. Raúl A. Ringuelet, a joint institute of the National University of La Plata and Argentina’s national research council CONICET. Their study area stretched along 78 kilometers of the southern coastal fringe of the estuary, a strip of shore that serves simultaneously as an economic resource, a freshwater supply and the principal recreational outlet for the largest population centers in Argentina. Because this fringe absorbs the pressures of millions of residents, untreated and partially treated sewage discharges, stormwater runoff and intense summer visitation, it offers an ideal natural laboratory for asking a deceptively simple question: if you want to know how polluted a beach really is, should you be testing the water, the sand, or both?

To answer it, the team sampled eight beaches and quantified two classic indicators of fecal pollution, Escherichia coli and Enterococcus, in both water and sand. Indicator organisms are used around the world as proxies for the presence of fecal matter and, by extension, for the possible presence of enteric pathogens. Water quality regulations, including the United States Environmental Protection Agency’s 2012 Recreational Water Quality Criteria, are built around thresholds for these very bacteria, which is why beach closures and advisories are typically issued on the basis of water samples alone. The Argentine study set out to test whether that water-only approach captures the true microbial picture of a beach, and the answer, in short, is that it does not.

The central technical result is stark: at all study sites, bacterial abundance was higher in the sediment than in the water column. This pattern is consistent with a substantial body of international literature showing that intertidal and foreshore sands can accumulate and sustain fecal indicator bacteria at densities far exceeding those of adjacent waters. Sand grains provide surfaces for bacterial attachment, shelter from ultraviolet radiation, and pockets of organic matter and moisture that support survival and even growth. In freshwater beaches on Lake Michigan, researchers famously showed that wet foreshore sand could act as a source of E. coli to nearshore water, and studies along the California coast have demonstrated that beach sands function as diffuse sources of fecal bacteria to coastal waters. The Río de la Plata results indicate that the same sand-to-water coupling operates along the Argentine estuary, where the combination of fine sediments, nutrient-rich waters and chronic sewage inputs creates favorable conditions for bacterial persistence in the beach matrix.

What controls where the bacteria concentrate? The study found a consistent spatial signature: microbial abundances in sand were negatively associated with distance from sewage discharges for both E. coli and Enterococcus, meaning that the closer a beach sits to an outfall, the more contaminated its sand tends to be. In the water, the same negative relationship with distance from sewage discharges held for Enterococcus, while E. coli abundance in water was additionally associated with dissolved oxygen, being higher where oxygen levels were lower. That second relationship makes physiological sense, since heavy organic loading from wastewater consumes oxygen as it decomposes, so low dissolved oxygen acts as a chemical fingerprint of sewage influence, coinciding with elevated indicator bacteria. Taken together, the spatial and chemical associations point to sewage outfalls as the dominant driver of the contamination pattern, rather than diffuse or naturalized sources alone.

The two indicator organisms did not behave identically. Across sand and water alike, E. coli was always more abundant than Enterococcus, leading the authors to suggest that E. coli may be a more generalized and sensitive indicator of fecal contamination in this system. The distinction matters for monitoring design. Enterococci are the preferred indicators in marine waters under many regulatory frameworks because they correlate well with swimming-associated illness in ocean settings, but in freshwater and estuarine systems the relative performance of indicators can shift. In the turbid, nutrient-rich, freshwater-dominated environment of the Río de la Plata, E. coli appears to track the fecal signal more faithfully, and a monitoring program that relied exclusively on Enterococcus could underestimate the extent of contamination at some sites.

Perhaps the most sobering finding came from a single site. At the Bagliardi beach, the researchers detected Salmonella choleraesuis and Shigella in water samples. These are not harmless indicator organisms; they are genuine enteric pathogens responsible for salmonellosis and shigellosis, diseases characterized by diarrhea, fever and abdominal cramps, and capable of causing severe illness in vulnerable populations. Their presence in recreational waters confirms what the indicator bacteria implied: where fecal pollution is intense enough, pathogens follow. While the study detected these pathogens at one location rather than across all sites, the finding illustrates the public health stakes of the broader contamination pattern and underscores why indicator-based surveillance, however imperfect, remains essential.

The implications for beachgoers extend beyond swimming. Epidemiological work published over the past two decades has linked sand contact itself to enteric illness among beach visitors. A study following beachgoers found that fecal indicators in sand, and the degree of sand contact, were associated with risk of gastrointestinal illness, independent of water exposure. Children, who dig, bury themselves and inevitably transfer sand to their mouths, are considered particularly exposed. Bacteria trapped in sand can also be resuspended into the water by waves, tides and the disturbance caused by bathers, meaning that a beach can fail water quality tests not only because of direct sewage input but because contaminated sand is continuously seeding the water column. Quantitative studies have shown that bathers themselves release bacteria from their bodies and from the sediment they stir up, complicating the interpretation of water samples taken during peak visitation.

All of this converges on the study’s principal recommendation: sand should be incorporated into routine beach monitoring programs. Current regulatory practice in most countries, including the frameworks that govern recreational water quality in Argentina, evaluates beaches almost exclusively through water sampling. The Río de la Plata results suggest that a water-only approach can paint a misleadingly reassuring picture, because a beach may show acceptable water counts while its sand harbors bacterial loads well above recreational guidelines, ready to be resuspended or picked up on the skin and hands of visitors. Monitoring sand alongside water, particularly at beaches near sewage outfalls, would give health authorities a more complete and precautionary basis for advising the public, and would help identify the sites where sanitation infrastructure investments, such as extending sewer networks or upgrading treatment, would deliver the greatest public health return.

The study also fits into a broader rethinking of how coastal pollution is assessed in urbanized estuaries. The Río de la Plata has previously been the subject of research on microplastics, cyanobacterial blooms and microbenthic communities, all of which point to the same underlying pressure: a colossal metropolitan population discharging its wastes into a shallow, slowly flushing estuary. By demonstrating that the sand itself is a reservoir of fecal bacteria that tracks sewage proximity and exceeds recreational guidelines at most beaches, the new work closes an important gap in that picture. For the millions of people who use these shores each summer, the practical message is nuanced rather than alarmist. The water is not uniformly unsafe, and the risks vary with location and proximity to discharges. But the comforting assumption that clean-looking water means a clean beach is no longer tenable. The ground beneath the towel, it turns out, deserves a place on the laboratory bench beside the sample bottles, and until monitoring programs catch up with the science, the humble sandcastle may be a better indicator of a beach’s microbial secrets than any water test.

Subject of Research: Fecal indicator bacteria contamination in beach sand and water along the Río de la Plata estuary

Article Title: Sand as a source of bacteriological pollution in recreational beaches of the Río de la Plata

Article References: Suárez, J. C., Simonetti, R., Mariñelarena, A., & Cochero, J. (2026). Sand as a source of bacteriological pollution in recreational beaches of the Río de la Plata. Environmental Monitoring and Assessment, 198(11), Article 1143. https://doi.org/10.1007/s10661-026-16012-7

Image Credits: AI Generated

DOI: 10.1007/s10661-026-16012-7

Keywords: Escherichia coli, Enterococcus, beach sand, fecal pollution, Río de la Plata, recreational water quality, sewage discharge, Salmonella, Shigella, water microbiology, public health, estuary