Buried but Not Gone: Microplastics Stack Up in Tropical Beach Sediments

Beneath the postcard-perfect sands of Diu Island, on India’s Arabian Sea coast, a hidden record of plastic pollution is quietly accumulating. A new study of five beaches on the island has found that microplastics are not merely sprinkled across the surface of tropical coastal sediments but are stratified through the entire upper 30 centimeters of the sediment column, with each beach telling a different story about how much plastic it retains and how deeply it buries it. The research, led by Ajit Kumar and Kamalesh Prasad of CSIR–Central Salt and Marine Chemicals Research Institute together with colleagues, goes beyond the standard headcount of particles at the surface and instead treats the beach as a three-dimensional archive of contamination, one whose vertical structure may ultimately determine how long plastic persists and how much ecological risk it carries.

The team sampled five beaches representing a gradient of human pressure: Ghoghla, Jalandhar, Jodhpur, Nagoa, and Vanakbara. At each site, three independent sediment pits were excavated roughly ten meters apart in the mid-intertidal zone, and sediment was collected in six sequential five-centimeter horizons from the surface down to 30 centimeters, yielding 90 depth-specific samples. The numbers revealed striking spatial heterogeneity. Microplastic abundance ranged from a low of 11.33 items per kilogram of dry sediment at Ghoghla to 216.67 items per kilogram at Vanakbara, a nearly twentyfold difference. Jodhpur followed with 164.00 items per kilogram, then Nagoa at 104.67 and Jalandhar at 70.00. Statistical tests confirmed the differences were significant, and a linear mixed-effects model that explicitly accounted for the nested sampling design reinforced the conclusion that beach identity, not random local variation, drove the pattern.

The most consequential finding, however, was vertical. Across every beach, microplastic abundance declined significantly with depth: the surface layer from zero to ten centimeters averaged 87.60 items per kilogram, the middle layer from ten to twenty centimeters averaged 53.80, and the bottom layer from twenty to thirty centimeters averaged just 28.60. A Spearman rank correlation of minus 0.564 quantified the negative relationship between depth and abundance, and the Jonckheere–Terpstra trend test confirmed a monotonic decline. Yet the authors stress that the drop-off was far from uniform. At Vanakbara, surface-to-bottom attenuation was only 63.5 percent, meaning a substantial fraction of the plastic load survived into deeper horizons, while at Ghoghla the decline reached 100 percent and no microplastics at all were detected in the bottom layer. Beaches burdened with similar overall quantities of plastic, in other words, may be storing it in fundamentally different ways.

To capture these differences quantitatively, the researchers deployed three complementary burial metrics alongside an exponential decay model. The Surface Enrichment Factor compared mean abundance in the top ten centimeters with that in the bottom ten, with all evaluated beaches showing surface enrichment values above one, ranging from 2.74 at Vanakbara to 3.26 at Jodhpur. The Persistence Depth Index, an abundance-weighted mean burial depth, ranged from 8.53 centimeters at Ghoghla to 11.95 centimeters at Vanakbara, while the Vertical Attenuation Coefficient was lowest at Vanakbara, indicating the most gradual decline. The exponential model told the same story from a different angle: decay constants ranged from 0.0429 per centimeter at Vanakbara to 0.0944 at Ghoghla, and half-depth values—the depth at which predicted abundance falls to half the surface value—stretched from 7.34 centimeters at Ghoghla to 16.18 centimeters at Vanakbara. The model fit the data well, with pseudo-R-squared values reaching 0.935 at Vanakbara.

What drives particles downward? The authors point to a suite of post-depositional processes: natural sedimentation, biofouling that increases particle density and causes even buoyant polymers to sink, aggregation with organic matter, bioturbation by sediment-dwelling organisms, and hydrodynamic reworking by tides, waves, and storms. Fibers, which made up 47.88 percent of all recovered particles, are particularly prone to this fate because their elongated shape anchors them among sediment grains, and fouled fibers gain enough weight to settle despite starting out buoyant. Fragments, the second most abundant morphotype at 17.18 percent, increased in relative proportion with depth, consistent with progressive burial of weathered debris. The picture that emerges is of beaches as dynamic sedimentary reservoirs rather than passive storage: particles are continually delivered to the surface, worked downward, retained for varying periods, and potentially remobilized when wave energy or storms disturb the bed.

Chemical characterization added another layer of nuance. Using stereomicroscopy followed by ATR–FTIR spectroscopy on 200 representative particles, the team identified four polymer types. Polypropylene dominated at 40.71 percent of identified particles, followed by polyethylene at 30.12 percent, polystyrene at 15.88 percent, and polyethylene terephthalate at 13.29 percent. The low-density polyolefins together accounted for more than 70 percent of the polymer assemblage, a fingerprint of global plastic production patterns, fishing gear, packaging, and household waste arriving from both land and sea. Polymer composition was remarkably consistent across beaches and depth layers, suggesting that differences in contamination among sites reflect quantities of input rather than fundamentally different sources.

The ecological risk assessment, however, delivered a cautionary twist. The Pollution Load Index averaged 1.79, exceeding the literature-based reference concentration of 80 items per kilogram used because no local background exists for Diu Island, and individual Contamination Factors ranged from a low of 0.28 at Ghoghla to a considerable 5.42 at Vanakbara. Yet when the team calculated the Polymer Hazard Index, which weights each polymer’s abundance by its published intrinsic hazard score, the overall value of 9.01 fell in the moderate hazard category—and the ranking of risk looked nothing like the ranking of abundance. Polypropylene, the most common polymer, contributed only 0.41 units, or 4.5 percent of the total hazard, because of its low hazard score. Polystyrene, at just 15.88 percent of particles, contributed 4.76 units, a disproportionate 52.8 percent of total risk, followed by polyethylene at 36.7 percent. In short, the most abundant plastic is not the most dangerous one, and assessments based on counts alone would badly misjudge the hazard.

Multivariate analyses wove these threads together. Principal Component Analysis separated Ghoghla, with its light contamination and shallow-only distribution, from the other beaches, while Jodhpur and Vanakbara clustered together across surface and middle layers despite their different burial profiles. Hierarchical Cluster Analysis grouped the contamination and burial indices—Contamination Factor, Surface Enrichment Factor, Vertical Attenuation Coefficient, and Persistence Depth Index—into one family, while the Polymer Hazard Index remained distinct, reflecting the fact that polymer-associated risk is driven by intrinsic hazard characteristics rather than contamination magnitude. The separation of contamination variables from burial variables indicates that how much plastic a beach holds and how deeply it holds it are partially independent dimensions of the problem, both of which matter for ecological interpretation.

The implications reach well beyond one small island. Sediments are the ocean’s principal long-term sink for microplastics, concentrating far greater masses than the water column, and prior studies—including sediment cores from the Beibu Gulf showing up to fivefold higher abundances at depth than at the surface—have warned that surface-only sampling systematically underestimates pollution. This study adds a subtler warning: even when surface counts match, the vertical structure beneath can differ dramatically, which means two beaches with identical surface readings could differ sharply in storage efficiency, persistence, and remobilization potential. Buried particles also face altered conditions—reduced light and oxygen may slow degradation—yet burial is not permanent, because storms, tidal resuspension, and bioturbation can return particles to biologically active surface layers, prolonging exposure for benthic organisms and, through weathering-enhanced surface area, increasing the opportunity for co-pollutants such as metals and persistent organic pollutants to hitch a ride.

The authors are appropriately candid about the limits of their framework. With no local background concentration available, their contamination indices rest on a borrowed reference value, and the polymer hazard scores carry inherent assumptions. The burial metrics are offered as descriptive, comparative tools rather than universal indices, and the sampling design, though rigorously replicated, was conducted at a single time point on a monsoon-influenced coast where seasonal dynamics could shift the picture. Even so, the integrated approach—coupling abundance with burial metrics, exponential attenuation modeling, polymer identification, hazard indexing, and multivariate statistics—offers coastal managers a far richer diagnostic than a surface scrape. As monitoring programs worldwide grapple with how to track plastic pollution meaningfully, Diu Island’s buried layers deliver a clear message: to understand what beaches are doing with our plastic, we have to stop looking only at the surface and start reading the sediment column like the layered archive it truly is.

Subject of Research: Vertical distribution, burial patterns, and ecological risk of sedimentary microplastics in tropical beach sediments

Article Title: Vertical distribution of sedimentary microplastics reveals potential burial patterns and ecological risk in tropical coastal sediments

Article References: Kumar, A., Sri Krishna, S. S., Venmathi Maran, B. A., & Prasad, K. (2026). Vertical distribution of sedimentary microplastics reveals potential burial patterns and ecological risk in tropical coastal sediments. Frontiers in Environmental Science, 14, Article 1933763. https://doi.org/10.3389/fenvs.2026.1933763

Image Credits: AI Generated

DOI: 10.3389/fenvs.2026.1933763

Keywords: microplastics, coastal sediments, beach pollution, polymer hazard, sediment burial, Arabian Sea, India, ecological risk assessment, exponential decay model, marine pollution, tropical beaches, vertical distribution