Pesticides and phosphorus dynamics in Lake Victoria Basin

Abstract

1890 field samples comprising of 630 sediments, 630 soils and 630 later samples were collected quarterly over a period of 21 months covering the wet, dry and short rain seasons experienced in Lake Victoria Basin. Sampling sites covered lakeshores, river mouths and effluent discharge points. Physico-chemical parameters, total reactive phosphorus, total hydrolysable phosphorus, total phosphorus and pesticides residues were analysed in water, sediment exchangeable phosphorus and sediment bio-available phosphorus were analysed in sediments, whereas available phosphorus was measured in soil. Pesticides residues were analysed in all the three matrices. Most of the water samples from both the rivers and the lake were found to contain phosphorus levels higher than the recommended guidelines for aquatic life indicating the influence of anthropogenic sources, whereas soils contained 10 to 100 times higher concentrations than sediments and water. The total phosphorus in water samples had mean concentrations of 4.61, 3.43, 2.45 and 2.30 mg/l for wet, short rain and dry seasons 1 and 2, respectively, whereas the total reactive phosphorus had mean concentrations of 2.22, 2.08, 1.12 and 1.61 mgll for the same seasons. Sediment bio-available phosphorus was higher than exchangeable phosphorus, with the highest mean concentrations of 24.45 and 8.22 mg/kg obtained during the dry season and wet season, respectively. The mean concentrations of soil available phosphorus ranged between 639 and 1,076 mglkg. The high levels of phosphorus measured In sediments signified accumulation of phosphorus over time. Therefore, through exchange processes, the trapped phosphorus could continue to replenish the fraction in the water column over a longer period . The concentration of pesticide residues varied from one season to the other in all the matrices analysed. Sediments contained the highest levels followed by soil and water. However, there was no clear temporal trend in terms of reduction of pesticide levels in the samples. This was attributed to the diffused nature of the source of pesticides in the catchment. The most dominant pesticides in the lake basin were p,p'-DDT, endrinaldehyde, P-HCH, dieldrin, methoxychlor, heptachlor, 8-HCH, y-HCH and endrin. Studies of pesticides fate in the catchment revealed a strong influence of soil organic carbon and other physico-chemical properties such as texture, pH and minerals composition, whereas calculation of phosphorus loading revealed that the contribution of phosphorus from the drainage basin into Lake Victoria is higher than the figures reported in the previous studies.