The research is concerned with the leachate collection systems for in-ground repositories for the disposal of low-level radioactive waste. The drainage system can become ineffective in the long-term due to clogging, which can be microbiological, chemical or physical in nature. The objectives of this research were to investigate the mechanisms and the potential for biological and chemical clogging. The role of the sulphates dissolved in the leachate and their contribution to the clogging process was also studied with a particular focus on the competition for organic substrate between the sulphate-reducing and methanogenic bacteria. In addition, a review of previous work was made with the aim of establishing a common framework explaining the significance of different factors on the microbially mediated clogging of landfill drainage systems in general The results indicated that: The development of organic clog material reached a steady state at the low organic loading rate of 0.1 mgCOD/cm³.day, although the operational conditions employed were limited in terms of their time scale and physico-chemistry. The development of chemical clog material, which consisted of low solubility salts as FeS and CaCO₃ can be significant but only in the long term. The accumulation of FeS is influenced by the availability of iron species. The accumulation of CaCO₃ precipitates was significant but only occurred for Ca concentrations exceeding the aqueous solubility limit of the CaCO₃ salt. Polysaccharides, produced by the bacterial populations under nitrogen and phosphorus limited conditions in the columns, and entrapment of gas bubbles in the pore space played a significant role in the clogging of the drainage aggregate. Column dismantling revealed that the clog remains relatively porous and permeable when its nature is mainly biological but becomes cemented and impermeable if accumulation of CaCO₃ precipitates has occurred. The laboratory tests investigated the competition between the sulphate reducing and methanogenic bacteria when acetate is used as substrate. The results indicated that sulphate reduction preceded methanogenesis. The overall results indicated that the clog material will be mainly organic in nature with the entrapment of gas bubbles. For the operational period and conditions employed in the laboratory column experiments the quantity of clog material will reach a steady state leaving the drainage system relatively permeable.