An investigation of nutrient levels along the Mbuluzi River - A background for sustainable water resources management

Abstract: Definition: The Mbuluzi River runs through Swaziland and Mozambique into the Indian Ocean. Due to water shortage, population increase and growing industrial and agricultural sectors, high stress is being put on the water quality. The importance of keeping good water quality is accentuated by the fact that the River is shared between two countries and that it supports both capitals with drinking water. The increased agricultural activities in the basin have raised concerns regarding leakage of nutrients, especially from commercial farming. Because of the general water scarcity in the region, the main issue regarding the Mbuluzi River has been water quantity rather than water quality. This, together with economical constraints has lead to limited monitoring and coordination of water quality programmes. Objectives: The main objective of this study is to describe how the water quality varies (seasonally and spatially) along the Mbuluzi River and to propose improvements for future monitoring. The focus is on nutrients and the threat of eutrophication. Specific questions are: ? Does the intense sugar farming in Swaziland or other pollution sources influence the quality of the water entering Mozambique? If so, to what extent? ? How do the Mnjoli and Pequenos Libombos Dams influence the river water quality? ? How is the River's self-purification capacity? ? Can any seasonal differences be detected in the water quality? ? Is there an impending risk of eutrophication? ? How can water quality monitoring become more efficient? Method: Water quality data was obtained from different actors responsible for monitoring. Also, the water quality was measured at eight sites along the River in September-October 2005. Conductivity, pH, TDS, turbidity, temperature and NO3 - (including NO2 -) were measured in field whereas N-tot and P-tot were analysed in laboratory. Further, flow data was obtained from different stakeholders. For the Mozambiquean sampling locations, PO4 3- was also analysed once. Data was analysed using statistical methods. In order to gather information regarding activities in the area along the Mbuluzi River, the main stakeholders in the area were visited and interviews were conducted. Also, laboratories involved in analysing water samples were visited to collect information on current analyses (methods, storage of samples, handling of data etc). Information about current water quality monitoring was obtained by interviews with the main participants. Conclusions: The sugar farming in Swaziland influences the water quality in the downstream stretches by increased levels of conductivity, phosphate, phosphorous, nitrate and nitrite. However, on the stretch between Goba (at the border) and the Pequenos Libombos Dam, the River seems to have a considerable self-purifying capacity, leading to generally acceptable water quality in the downstream reaches (during the time of this field study). This can for example be seen in decreased levels of phosphorous, nitrogen and nitrate/nitrite. It is, however, likely that the selfpurifying capacity is limited and if pollution loads increases it may lead to severe water quality deterioration. Measurements during this field study show that levels of phosphorous, nitrogen and nitrate are lower before the Pequenos Libombos Dam than after. This indicates that the Dam does not function as a nutrient trap, as would be expected. However, since water is released from the bottom of the Dam, inflowing surface water has been compared to outflowing bottom water. This might be one explanation of the higher downstream values. Although no measurements of oxygen have been made, the bottom water is thought to be anoxic. The anoxic conditions and the fact that the outflowing water has a higher pH than the incoming water indicates that denitrification takes place in the Dam. At the time of this field study, the effect of the denitrification is not seen in the nitrate concentrations since the concentrations in the inflowing water were unusually low and the water released from the Dam originates from a relatively long time period, during which the inflowing concentration probably were higher than at present. However, in a longer time perspective (several years), the net effect of the Dam is thought to be a reduction of nitrogen even though this does not appear during drought conditions. Data from the Mnjoli Dam show similar results. Generally, the concentrations of nutrients are found to be higher during rainy seasons, implying that the effect of nutrient loads is higher than the dilution effect of the higher flows. Data show that in late 2005 (dry season) phosphorous is the growth limiting factor in the River and there were no risks of eutrophication, neither in the Dams nor in the River. This may however change during rainy seasons when nutrient concentrations (both N and P) are likely to increase. Information gives that the alien specie Salvinia Molesta is spreading in the Mozambiquean part of the River and increased amounts of algal blooms have been reported in the Dams, indicating that eutrophic conditions may occur occasionally. To improve water quality monitoring, better coordination between different actors is needed. Further, the objectives of the monitoring programmes need to be better defined. The analysis methods also need to be checked and evaluated on a regular basis. Regarding data storage, a uniform system for all participants involved in the River basin monitoring is recommended to be implemented. (Ett examensarbete utfört vid Avd f Teknisk Vattenresurslära, TVRL)