Distribution and retention of faecal coliforms in the Nakivubo wetland in Kampala, Uganda.

Nakivubo wetland, which has been receiving wastewater from the capital of Uganda for more than 40 years is a tropical wetland dominated by Cyperus papyrus and Miscanthidium violaceum. Field, pilot and laboratory studies were carried out to assess the distribution of faecal coliforms and factors responsible for their retention in different compartments of the two macrophytes in the wetland. There were higher coliform numbers in the free water column below the mat of zones dominated by Miscanthidium (11.1 +/- 0.6 x 10(5) MPN/100 ml) compared to those dominated by papyrus (8.9 +/- 3.1 x 10(4) MPN/100 ml). The thick (1.3 m) and compact mat of Miscanthidium restricts vertical transport of wastewater into the mat, resulting in flow-through of wastewater under the mat. The papyrus mat is loose, open and thin (0.5 m) and allows easy vertical penetration of wastewater into the mat. The unrestricted interaction between the wastewater in the water column and that in the mat of papyrus in addition to plant debris and detritus continuously sedimenting out of the mat to the wetland bottom are responsible for the retention of coliforms in the papyrus dominated zones. Attachment, sedimentation and natural die-off were found to be important mechanisms responsible for the retention of coliforms in the Nakivubo wetland.