Estuarine mixing governs spatiotemporal partitioning of potentially toxic elements between coastal waters and sediments in the western Indian Ocean

The coast is a place where two worlds meet and mix. Freshwater rivers, carrying everything from soil to dissolved minerals, flow into the salty ocean. Where they meet, the water chemistry changes constantly, creating a unique environment that shifts with the tides and the seasons. In these estuaries, tiny particles of mud and sand act like sponges, soaking up metals that flow in from the land. Some of these metals, like iron or manganese, are natural parts of the earth, while others, such as lead or mercury, can be harmful to living things if they build up in high amounts. Scientists have long known that these toxic elements do not stay put; they move between the water and the ground, sometimes washing away with the tide and other times settling into the mud. Understanding exactly how this movement works is vital for protecting coastal ecosystems, especially in tropical regions where data has been scarce. Without knowing how these metals behave, it is difficult to tell if a beach is safe or if a local fishery is at risk.

A team of researchers recently set out to map this hidden movement along the coast of Kenya, in the western Indian Ocean. They focused on four specific locations—Gazi, Mkunguni, Shimoni, and Vanga—where rivers meet the sea. Over the course of a full year, from March 2022 to February 2023, they collected samples of surface water and the top layer of active sediment, the muddy ground where life and chemical reactions happen most intensely. Their goal was to measure ten different metals, including arsenic, cadmium, cobalt, chromium, copper, iron, mercury, manganese, nickel, and lead. To understand what was driving the changes they saw, they also tracked the physical and chemical conditions of the water, such as its temperature, saltiness, acidity, and the levels of nutrients like phosphate and nitrate. By comparing the metal levels in the water and the mud against these changing conditions, the scientists hoped to find the rules that govern where these elements end up.

The results painted a clear picture of a landscape defined by location rather than time. The amount of metal found in the water and the mud varied significantly from one place to another, but it did not change much from month to month. One site, Gazi, consistently held the highest concentrations of nearly every metal. In the water there, iron reached levels of 80.6 micrograms per liter and manganese hit 22.9 micrograms per liter. In the sediment at the same spot, the numbers were far higher, with iron measuring 30,200 milligrams per kilogram and manganese at 905 milligrams per kilogram. At the other end of the spectrum, the site of Shimoni usually recorded the lowest amounts. While the levels of most metals stayed relatively steady throughout the year, chromium was the exception, showing a unique pattern where its behavior changed depending on both the location and the specific month.

The key to understanding this distribution turned out to be the saltiness of the water. The researchers found a strong, consistent link between salinity and the metals trapped in the sediment. As the water became saltier, the amount of metal in the mud dropped. This relationship was so strong that saltiness alone could explain a large portion of the variation seen in metals like manganese, cobalt, iron, cadmium, and lead. In fact, for every metal they studied in the sediment, the amount of salt in the water was the single most important factor predicting how much metal would be found there. The models the scientists built showed that salinity was the dominant force, with other factors playing a much smaller role. This suggests that the mixing of fresh and salt water creates a specific environment where metals settle out of the water and get stuck in the mud, particularly in areas where the water is less salty and the sediment is fine.

These findings challenge the idea that short-term changes, like a single rainy month or a temporary shift in the weather, are the main drivers of metal pollution in these coastal zones. Instead, the study shows that the physical geography of the estuary and the way fresh and salt water mix create permanent, site-specific reservoirs of sediment. The metals accumulate in these low-salinity zones and stay there, creating a stable pattern that does not fluctuate wildly with the seasons. This means that to truly understand the health of these coastal waters, monitoring programs cannot rely on taking a single snapshot of the water at one time of year. They must measure both the water and the sediment, and they must focus their sampling on the specific areas where fresh water meets the sea and where fine mud settles. By targeting these low-salinity retention zones, scientists and managers can get a much clearer, more accurate picture of where potentially toxic elements are hiding and how they are moving through the environment.