Impact of river water and sediment properties on the chemical composition of water hyacinth and hippo grass

Abstract

Water hyacinth and hippo grass are aquatic weeds that negatively affect freshwater bodies in sub-tropical and tropical areas. Using these weeds as soil amendments can help to reduce their spread, and improve soil fertility. Materials intended for use as soil amendments must have high levels of nutrients and low contaminant levels. It is important to understand how water and sediment properties influence the chemical composition of these weeds, to assist in choosing weeds that have high nutrient levels and low levels of heavy metals. This study aimed to investigate the effect of water and sediment characteristics on the chemical composition of water hyacinth and hippo grass and to assess the suitability of these aquatic weeds as soil amendments. We evaluated how the chemical parameters of water hyacinth and hippo grass varied across different rivers, and examined the relationship between the chemical composition of the aquatic weeds and the chemical composition of water and sediments in rivers where they occur. Plant, sediment, and water samples were systematically obtained from Kafue, Chongwe, Maramba and Kafubu Rivers in Zambia. These rivers are subject to different influences of anthropogenic activities and were therefore expected to differ in their levels of nutrients and heavy metals. Weeds collected from the Maramba River, which passes through human settlements, contained significantly higher concentrations of nitrogen and phosphorus, while weeds collected from Kafubu and Kafue Rivers which pass through industrial and mining areas contained significantly higher levels of zinc, cobalt, manganese, and copper. However, the concentrations of chromium, zinc, copper, manganese, and lead in the aquatic weeds from all four rivers were lower than the critical EU limits for compost. Concentrations of phosphorus and nitrogen in water hyacinth were positively correlated with levels in river water. In hippo grass, the levels of nitrogen and phosphorus were strongly and positively correlated with concentrations in sediments, but weakly correlated with concentrations in water. The results show that aquatic weeds from different locations vary in their nutrient and heavy metal contents, indicating that careful consideration needs to be taken when choosing sources of aquatic weeds intended for agronomic use. Maramba River was identified to be the best source of aquatic weeds for agronomic use because the weeds from this River had higher levels of macronutrients and lower levels of heavy metals.

Keywords

• Water hyacinth
• Hippo grass
• River water
• River sediments
• Soil nutrients
• Heavy metals