Buffering Capacity Studies in a Rural and Urban Wetlands in Lake Victoria Catchment in Uganda

Abstract

Wetlands are known to filter water from catchments by retaining solid waste, and eroded sediments from catchment areas. Wetlands also reduce the impact of flooding, speed of flow, and hence store water while releasing it slowly. The extent to which the wetlands perform these roles was investigated in two wetlands, Kinawataka wetland with an industrial and heavily populated catchment, and Kisoma wetland with subsistence agricultural catchment between January 1999 to July 2001. Water samples were collected once a month from streams entering the wetlands, along transects within the wetland and at the out flow. Parameters investigated included pH, temperature, dissolved oxygen (DO), electric conductivity (EC), Total Dissolved Solids (TDS), orthophosphates, Total Phosphorus (TP), nitrates, Total Nitrogen (TN) and chemical oxygen demand (COD). Results showed that the urban Kinawataka wetland receives a lot of pollution from its catchment and this is considerably reduced as the water move through it to the out flow. Reductions of TN by 50% and TP by only 10% were noted. The rural Kisoma wetland however receives fewer nutrients from its catchment but releases more in its out flow. There were increases in orthophosphates to about 50%, TP to 40% and nitrates to 22%. In situations where large volumes of water was received especially after a heavy storm or during floods, the wetland capacity to buffer was impaired and the materials from the catchment would pass through it unbuffered. It is suggested that wetland buffering depends on the amount of nutrients and water inflow from the catchment, the wetland-slope, nature of the vegetation, size of the wetland, catchment rainfall and anthropogenic characteristics. The conditions within the wetlands modify the nature of the nutrients as the water flow through them.

Key words: Buffering capacity, wetland buffering, catchment, reservoir, nutrients