Control of subsurface flow is important for sustainable water management. It is, for example, a useful tool for the handling of contaminated sites or the reduction of unwanted seepage. In this thesis the complexation interactions between DOM and Al are successfully used to reduce the hydraulic conductivity of a sand layer. The complexation of Al and DOM and the resulting floc sizes were first analysed with filtrations and a Malvern Mastersizer. According to the scientific norm approximately 95\% of the organic matter precipitated at Al/C 0.03 (i.e. did not pass through a 0.45 $\mu$m membrane). It was found that the mean floc size drastically grows between Al/C 0.02 and 0.03. At high Al/C ratios ($>$ 0.05) these flocs were also formed in a sand layer at constant saturated flow. In the two dimensional laboratory set-up a narrow (1 cm), dark brow layer grew at the contact front of the Al and DOM solutions. It is thought that hydrodynamic dispersion mixes the solutes and therefore changes the Al/C ratio and that the changed Al/C ratio leads to the immobilisation of Al and DOM. EC measurements showed that the small precipitate layer limits further dispersion of Al, and might therefore limit its own growth. Furthermore it was firmly proven that immobilized Al and DOM cause a significant reduction in hydraulic conductivity. Firstly the hydraulic conductivity was reduced to 38\% of its original value after which is was reduced to 23\% in a second experimental phase.