Sorption characteristics of the selected pesticides on the river sediments in the Mid-Himalayan region.

The extensive use of pesticides has resulted in their persistence in several environmental compartments, including soil, water, and air. The majority of pesticides remain confined inside soil and sediment, limiting their dispersion to other parts of the ecosystem. In Himachal Pradesh, an agriculturally and horticulturally rich state of the Indian Himalayas, the regular usage of chemical pesticides poses significant risks to the pristine Himalayan ecosystem. Pesticides applied to crops cultivated on the valley slopes, such as apples, are thought to accumulate in the soil and are then transported to adjacent rivers during the monsoon season by surface runoff. The behaviour and movement of these pesticides mostly depend on their adsorption on soils and river sediments. Therefore, this study aims to examine the adsorption capacities of sediment fractions (coarse, medium, and fine sand and silt-clay) collected from the Beas riverbed, one of the major rivers in the Kullu valley of Himachal Pradesh. Fungicides like carbendazim and thiophanate methyl that are commonly used in this region were selected for adsorption experiments along with coarse, medium and fine sands, and silt-clay fractions separated from the Beas River sediments. One gram of each sediment type was spiked with the pesticide mixture containing 100 mg/L of each fungicide and allowed to adsorb for 24 hours. Following that, the spiked sediments were eluted with deionized water to simulate rainwater flushing in the real conditions. The extracts were analysed using HPLC-DAD to measure the concentration of fungicides eluted with water. The findings indicated that sediment type significantly influenced the desorption of carbendazim and thiophanate-methyl. Approximately 6.2% of thiophanate-methyl and 90.2% of carbendazim were eluted from coarse sand. The elution percentages for carbendazim and thiophanate-methyl using medium sand were 87.21% and 4.5%, respectively. Fine sand exhibited increased elution, with 26.4% thiophanate-methyl and 92.2% carbendazim released. Silt-clay sediments released 37.7% of thiophanate-methyl and 89.7% of carbendazim. The findings indicate that sediment retention of pesticides is contingent upon particle size, affecting the quantity of pesticide that may be released into the water. Additional work on adsorption and desorption of captan (organochloride) along with these two pesticides using the batch equilibrium procedures is underway.