Sedimentary Fate, Sources, and Ecotoxicological Risk of Persistent Pesticides in the Kol Ramsar Wetland, Southwest India

In the quiet, water-logged fields of the Kol wetland in southwest India, a complex history is written not in ink, but in the mud. This landscape, recognized globally as a vital sanctuary for birds and fish, is also a working farm where rice has been grown for centuries. The story of this place is shaped by the rhythm of the monsoon, which floods the fields for half the year and recedes to reveal the soil for planting. However, the soil holds more than just nutrients; it acts as a long-term memory bank for the chemicals farmers have used to protect their crops. When pesticides are applied, they do not always wash away completely. Instead, many of them stick to tiny particles of dirt and sink into the sediment, the layer of mud at the bottom of the water. Over time, these chemicals can build up, hiding in the dark layers of the earth long after they were sprayed. Scientists who study these environments are interested in how these hidden chemicals move, how long they last, and what they might do to the creatures living in the water and the mud. Understanding this process is crucial because these wetlands are not just farms; they are delicate ecosystems where the health of the soil directly affects the survival of fish, birds, and the people who depend on them.

A team of researchers from the Cochin University of Science and Technology decided to read this chemical history by looking at the mud itself. They focused on the Kol wetland, a Ramsar site, which is a wetland of international importance. The scientists collected samples from the surface of the mud and also drilled deeper into the ground to get core samples, which are like long cylinders of earth that show layers from different times. They took these samples during two distinct times of the year: the wet season, when the fields are flooded, and the dry season, when the land is being prepared for planting. Their goal was to find out which pesticides were present, where they were located in the mud, and whether they were fresh deposits from recent farming or old residues from decades past. They specifically looked for two main groups of chemicals: organochlorines, which are older, very stable chemicals that were once widely used but are now banned in many places, and organophosphates, which are more modern pesticides still in use today.

The researchers found that the mud was indeed a repository for these chemicals. In the surface layers of the sediment, they detected a mix of organochlorine pesticides, including various forms of HCH and DDT, as well as the herbicide butachlor. They also found organophosphate pesticides like profenofos, ethion, and chlorpyrifos. One of the most striking findings was how the amount of these chemicals changed with the seasons. The total concentration of DDT and its breakdown products was significantly higher during the dry season, reaching 31.74 micrograms per kilogram of sediment, compared to 18.62 micrograms per kilogram during the wet season. This suggests that the dry conditions might concentrate these chemicals or that the farming activities during this time introduce more of them. Furthermore, the specific types of DDT found told a story of age and transformation. The researchers found that the breakdown products, known as DDE and DDD, were much more common than the original DDT. This pattern indicates that the DDT in the mud is not from a recent spill but is a remnant of historical use that has been slowly breaking down over many years.

The study also revealed that these chemicals are not just sitting on the surface; they are moving. By examining the deeper layers of the sediment cores, the team found traces of chemicals like beta-HCH, butachlor, and chlorpyrifos buried several centimeters down. This vertical movement suggests that the seasonal flooding and the physical mixing of the soil during farming are pushing these contaminants deeper into the earth. In some deeper layers, they found beta-HCH, a form of the chemical that is particularly resistant to breaking down. This discovery confirms that pesticides can travel from the surface water into the deeper, older layers of the sediment, effectively burying the evidence of past pollution. The ratio of different forms of HCH found in the mud also provided a clue about current practices. The researchers observed a ratio that suggested fresh, local inputs of lindane, a specific type of pesticide, rather than just old, lingering residues. This points to the possibility that despite bans, certain related chemicals are still being used or introduced into the system locally.

The implications of these findings extend beyond just knowing what is in the mud. The researchers assessed the risk these chemicals pose to the life living in the wetland. They compared the levels of pesticides found in the Kol wetland to established safety guidelines used to protect aquatic life. They found that at several locations, the levels of DDE, a breakdown product of DDT, exceeded the interim safety limits. This means that the sediment in these areas could be harmful to the bottom-dwelling organisms, such as worms and small crustaceans, which form the base of the food web. If these creatures are affected, the risk could ripple up to the fish and birds that eat them. The study also used statistical tools to trace the sources of the pollution. The analysis indicated a mix of sources: some chemicals came from historical deposits that have been sitting in the soil for a long time, while others appeared to be from recent agricultural activities and the seasonal transport of water during the monsoon.

Ultimately, this research paints a picture of a wetland that is under a dual pressure: the legacy of past chemical use and the ongoing impact of current farming. The Kol wetland, with its rich biodiversity and cultural significance, is showing signs of stress from these persistent pollutants. The presence of these chemicals in both the surface and deep layers of the sediment suggests that they are not easily washed away and can remain a threat for a long time. The study concludes that to protect the ecological integrity of this important Ramsar site, there is a need for better monitoring of pesticide use and more careful management of agricultural practices. The mud holds the record of what has been done to the land, and reading that record is the first step toward ensuring that the wetland remains a healthy home for the species that depend on it.