Field and laboratory investigations of the adsorption of nitrogen compounds on estuarine based sediments

• Main Author: Abdulgawad, Fathia
• Published: Cardiff University 2010
• Subjects: 363.738
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.584938

Nitrogen plays a major role in the metabolism of aquatic eco-systems. However, at low concentrations it can affect primary productivity and rise to very high concentrations, leading to eutrophication problems. Nitrogen compound fluxes in the aquatic environment are in the form of nitrate, ammonium or nitrite. Ammonium is soluble in the water column and depending on the type of sediment it can be adsorbed onto the sediments. Ammonium adsorption processes have been the main focus of this study in both deionised and saline waters, and for clean clay and sand and natural sediments taken from the Loughor Estuary, in Wales. An equation for ammonium adsorption onto bed sediments has been developed and included in the DIVAST water quality model, which is based on the QUAL2E US EPA model. The adsorption coefficient was found for both fresh and saline water conditions. It was found that adsorption, and consequently the adsorption coefficient, decreases with increasing salinity. Nitrate adsorption was also studied in this research project and it was confirmed that nitrate can adsorb only to specific types of clay, such as Kaolinite and organic matter. Salinity was found to influence nitrate adsorption on the clay and the types and amount of clay present in the bed sediments were found to have an impact on the nitrate mobility across the sediment water interface. The results of the field study for the Loughor Estuary have shown that ammonium adsorption decreased with increasing salinity. The highest amount of ammonium adsorption was found during March, at site 1b at 80.3 ug/g. Dissolved inorganic nitrogen (DIN) was found to be dominated by nitrate and nitrite during autumn and spring, whereas the DIN was found to be dominated by ammonium in the summer. Ammonium and Nitrite concentrations of 4.25 mg/l and 10.23 mg/l respectively, were found to be the highest recorded in literature for UK estuaries. Nitrate concentrations were present on 30th June at all sampling sites and increased with incoming tide, indicating the coastal water source for nitrates during this time of the year. The suspended sediment present in the Loughor Estuary was found to be independent of the velocities present and not sensitive to the difference in velocities.