A geochemical investigation of the aquatic sediments, groundwater and surface water of the Verlorenvlei coastal lake, with special reference to nitrate transformations

Master Thesis


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University of Cape Town

The incorporation of nitrogen in living cells gives rise to cycling between atmospheric, inorganic and organic forms of nitrogen. Nitrogen cycling is largely controlled by microbial respiration and metabolism. In aquatic systems, N-cycling occurs dominantly in sediments. Removal of nitrogen from aquatic sediments occurs through the successive Ntransformation processes of mineralisation (organic N -> NH4+), nitrification (NH4+ -> NO3-) and denitrification (NO3- -> N2). Denitrification, mineralisation and also immobilisation of inorganic N (NO3-, NH4+ ) to organic N occur under reducing conditions. Build-up of the nitrate (NO3- ) concentration in groundwater is a widely-recognised phenomenon. Groundwater nitrate may contribute significantly to the N input to aquatic bodies that receive groundwater flow. The Verlorenvlei coastal lake on the arid south-west coast of South Africa is an important ecological habitat and also a valuable agricultural water resource. Analyses conducted in this study indicate that the groundwater, which flows towards the lake, has a significantly higher NO3- concentration than the lake water. The difference in NO3- concentration is due to: 1. Dilution of groundwater by a larger quantity of low-nitrate water in the lake, and/or, 2. Removal of nitrate from groundwater through microbial processes in the lake sediments. Insufficient information is available concerning the magnitude of groundwater flow into the lake to investigate the importance of option 1. Experiments were conducted on two lake sediments (high organic content and low organic content) to determine the relevance of option 2. Sediment subsamples were incubated under reducing conditions and amended with 25mg/l and 100mg/l NaNO3-N solutions with and without the addition of 40mg/l glucose and with and without irradiation. After 10 days incubation, a period chosen to simulate the residence time of groundwater in the sediments, the added NO3- had all been removed through denitrification or immobilised as organic N. The exact removal pathway could not be determined. The high organic content sediment was found to remove nitrate more efficiently. It was concluded that the decrease in nitrate concentration as high-nitrate groundwater flows into the Verlorenvlei lake can probably be ascribed to denitrification or immobilisation processes under reducing conditions in the sediment lining of the lake.

Bibliography: pages 54-59.