Transport of Cu, Zn and Cd in a small agricultural catchment

Concentrations of dissolved and particulate metals (Cu, Zn and Cd) were determined in water samples collected during several rain events from the River Kleine Aa, a first order tributary to Lake Sempach (Central Switzerland). Metal contents were also measured in soil cores obtained from fertilized and unfertilized areas of the catchment and in liquid manure samples. The contribution of farming activities to the metal loads was evaluated. Because total metal concentrations were linearly related to water discharge, rain events significantly contribute to the yearly metal load. During such events, metals were predominantly associated to particles, but dissolved copper constituted up to half of the Cu load at low flow rates. Dissolved Cu concentrations exceeded dissolved Zn concentrations at low discharge rates, and in experimental water extracts of the grassland soil. The dynamic behavior of dissolved metals and dissolved organic carbon were linked in the river water and in the soil extracts. Metal concentration of suspended particles decreased with increasing discharge to a constant level with Cu and Zn contents similar to those of the grassland topsoil. Their Cd content was, however, lower than in the soil. These observations suggest that the grassland topsoil is the main source of dissolved and particulate trace metals in the river water. Farming activities have caused metal accumulation, since the metal contents in the grassland soil were highest at the surface and higher than in the forest soil at any depth. Based on a metal budget of the drainage area and on metal profiles of forest and grassland soil, it is concluded that frequent application of liquid manure with high metal concentrations was mainly responsible for the high Cu and Zn content of the soil and the elevated loss rates to the river, whereas air pollution mainly explained the elevated Cd load of the drainage basin and the river water.