UNDERSTANDING THE CADMIUM (CD) MOVEMENT IN THE SURFACE RUNOFF CONSIDERING VARYING RAINFALL CONDITIONS

Abstract

Changing land use patterns and increasing extreme rainfall events will pollute our natural water resources. One of the most hazardous heavy metals is Cd, which moves through the soil with great force and is readily absorbed by plants, which impacts crop growth and human health. Hence, understanding Cd movement considering local rainfall patterns and soil properties (physical and chemical) is greatly helpful in natural resources management. Therefore, simulated rainfall experiments were conducted to investigate the release of Cd element from agriculturally contaminated soils under varying rainfall intensities and slope gradients. This study aimed to understand the movement of Cd element in both runoff and sediment. A rainfall simulator was used for reproduction of natural rainfall characteristics along with agriculturally contaminated soil to study surface runoff and sediment quantity and quality. This study's primary objective is to reproduce the statistical characteristics of natural rainfall to understand Cd movement under different slopes and rainfall intensity. Before the experiment was started, the soil's physical and chemical properties were measured. After the end of each scenario, the same Cd concentrations (20mg/kg of soil) were added to the topsoil under varying rainfall intensities and slopes. A total of 15 different scenarios were studied combining different rainfall intensities (30, 40, 50, 60, and 70 mm/h) and slope gradients (Flat, 5%, and 10%) and understand the cd transport behavior. The surface runoff and sediment yield were measured using volumetric and oven drying method. Additionally, the sediment and surface runoff collected was used to analyze Cd concentration by ICP-MS. The study results will show that the Cd loss characteristics are impacted by rainfall intensity, its kinetic energy, slope, and initial Cd concentration in the topsoil. The result of the runoff rate and the sediment yield of the experiment increased during the initial 15 minutes of rainfall duration and then gradually decreased and became stable. Generally, as the rainfall intensity and slope gradient increases, the sediment yield obtained from all the experiments increases. The Cd concentration range in runoff sample of different slope gradient (flat, 5%, and 10%) were (3.66–34.53 ppb), (18.13 – 45.41 ppb), and (8.45 – 66.49ppb), respectively. The concentration of Cd in the runoff was increasing when the rainfall intensity was increasing. Higher rainfall intensity leads to increased Cd runoff concentration.