The main objective of this thesis was to investigate the influence of biofilms on colloid transport in model sediment columns, at changing fluid ionic conditions. Sandpacked columns and sandpacked microscopy flow cells were used to determine colloid transport. The artificial clay colloid laponite RD (LRD), and the biofilm forming bacterium, Pseudomonas aeruginosa SG81 were used as model colloid and biofilm forming microorganism, respectively. Changing ionic conditions were simulated by using Ca2+ and Na+ based solutions as influents and then switching to very low ionic strength solutions. Colloid transport parameters as well as microbiological parameters were obtained using a combination of online optical detection methods and offline microbiological and biochemical analytical methods. In absence of biofilms, a sodium chloride concentration of 7 x 10−2 M caused complete retention of LRD within the sand columns. Although at 2000 mg L−1 LRD, massive aggregation was observed and clogging occurred, aggregation alone was not responsible for LRD retention at lower concentrations (i.e., 200 or 20 mg L−1). P. aeruginosa SG81 showed relatively low mobility at all ionic strengths tested and some (albeit reduced) mobility when introduced to the columns in 1 M NaCl, the highest concentration tested. In sterile columns, the presence of Na+ and Ca2+ ions in the influent followed by a low ionic strength solution did not cause LRD retention. The colloid was mobile with collision efficiencies from 0.05 to 0.08 (SE ≤ 20 %; n = 9). In the presence of biofilms and after Na+ exposure, no colloid retention occurred but in some cases altered or enhanced colloid transport was observed. Colloid collision efficiency after 3 weeks of biofilm growth was 0.03 (SE ≤ 10 %; n = 3). In contrast, after Ca2+ ions exposure, colloid retention increased with biofilm age. After 3 weeks, almost complete retention was observed with a collision efficiency of 0.9 (SE ≤ 20 %; n = 3). Similar observations were made in columns packed with material from slow sand filtration units. EPS analysis from Ca2+ treated columns, showed that colloid retention also increased with an increase in EPS content. Protein content was found to increase with time in relation to other EPS components and to be significantly correlated to colloid retention (0.999) at the established confidence level. These data reveal the complex interactions between biofilms, ions and colloid transport. Changes in the electrolyte composition of water percolating the subsurface can frequently occur as well as changes in the relative abundance of microbial biofilms. This has to be considered when modeling colloid transport through the subsurface.