Sediments and nutrients input to a small reservoir in the tropical semi-arid regions

The sediment and nutrients that arc input to surface reservoirs compromise the quality and quantity of the stored water. The aim of this study was to investigate the dynamics of sediment and nutrient input along the drainage basin of a small reservoir in the semi-arid region. The reservoir under study was built in 1956 and has a storage capacity of 5.01 hm(3). Seven trenches were dug along the drainage basin of the reservoir following the river. Sediment samples were collected at increasing depths of 20 cm until reaching the original riverbed. An analysis of particle size, total organic carbon (TOC), total nitrogen (TN), available phosphorus (P), exchangeable aluminium (Al), total iron (TFe) and pH was carried out. Approximately 60% of the sediment in the reservoir is composed of clay and silt, and 40% of sand. The greatest thickness of the sediment layer, as well as the greatest concentration of fine sediment, was registered in the section =west to the dam. The values for TOC (2.47 < TOC < 34.47 g kg(-1)) and TN (0.69 <TN < 3.97 g kg(-1)) showed a positive and significant correlation with each other (r = 0.95) and with the fine sediment (r = 0.85; r = 0.77). The P (12.19 < P < 41.99 mg kg(-1)) showed an inverse relationship with the organic components, fine sediment, TFe and Al. The mean C/N ratio of 8.77 demonstrates the predominance of sources of anthropogenic pollution in the drainage basin of the reservoir. The fine sediment proved to a determinant factor in the retention and availability of TOC, TN and P, with the reservoir acting as a deposit for the input nutrients.