Limitations and possibilities for microbial degradation of organic contaminants in aquifers
Abstract: Many factors can influence the rate of microbial degradation of organic contaminants in aquifers. Some of these factors, e.g. sorption, the presence of dissolved organic matter (DOM), biomass, redox conditions, cometabolism, microbial community composition, and activity of degrading microorganisms, are addressed in the thesis, with the ambition to gain insights for bioremediation in aquifers, and to prevent contaminants from spreading to drinking water supplies. The influence of sorption of organic contaminants and bacteria on biodegradation was investigated under both diffusion limited and advective flow conditions. The results demonstrated that degradation by suspended bacteria of dissolved aniline and 2,4-dichlorophenol (2,4-DCP) was faster than the degradation of the same compounds sorbed to the solids. Nevertheless, sorption to the solids was not sufficient to exclude trace contaminants from degradation. In another investigation, the main effect of DOM on microbial phenanthrene degradation in groundwater was stimulation of microbial growth and activity of degrading populations, with hydrophilic DOM having a better effect than hydrophobic. In another study, glyphosate degradation was different in a mixture of sediment and groundwater from two aquifers (Vejen, Denmark and Vomb, Sweden). Laboratory experiments excluded sorption and organic carbon limitation as major sources of the observed differences. Glyphosate degradation was positively correlated to the density of bacteria, but the difference in density between the two sites was too small to account for the degradation difference. Instead, it was found that the differences in metabolic activity of the degrading strains and the microbial community composition of the aquifers were large and coincided with differences in rates of biodegradation. Glyphosate sorption was lower and biodegradation was slower under anaerobic conditions compared with aerobic, and most of the degraded glyphosate was not mineralized but cometabolized to AMPA. The thesis pin-points to the possibilities to increase contaminant biodegradation in aquifers by the addition of i) oxygen to stimulate aerobic degradation, ii) a primary growth substrate, e.g. DOM, to increase biomass and thereby stimulate metabolic or cometabolic degradation, and iii) microorganisms to increase the degrading population and/or genetic capacity.
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