The control of infiltration and seepage of water is one ofthe most challenging tasks in water management and civil­-engineering and, inan attempt to control this, methods for forming a water­impermeable layer inthe soil have been widely practised in soil engineering (Laumann et al., 2018;Proto et al., 2016). The use of natural processes to modify the engineeringproperties of the subsurface could help to develop cost-­effective, robust andsustainable engineering technologies and is attracting increasing attentionfrom the industry (Zhou, 2020). This research aims to reduce the permeabilityby using aluminium (Al) and organic matter (OM) precipitates mixed ex­-situwith porous media to create a horizontal barrier. The Al­-OM precipitates wereexpected to clog the pore space with a reduction in permeability and hydraulicconductivity as result. To find out if it is feasible to use Al­-OMprecipitates mixed ex­-situ with porous media for a permeability reductivelayer, the Al­-OM precipitates and the permeability of the medium wereresearched.  To characterise the flocculation reaction, experiments onthe flocculation were performed. The yield of the reaction was obtained byadding certain amounts of Al and OM solutions to form particular amounts of drymass of flocs. This experiment confirmed the hypothesis that 85% of the addedmass of OM will result in dry mass of flocs. The concept of a critical metal tocarbon ratio (M/C­-ratio), indicating flocculation regardless of the inputconcentrations, was tested by measuring the pH over an increasing M/C-­ratio.From this titration curve, the found critical M/C ratio is between 0.023­0.031,and the pH stabilises at a level lower than pH 4. This result proves theconcept behind the numerical scenarios describing the titration of OM solutionswith Al3+, with a critical molar M/C ratio independent of the inputconcentrations (Veerkamp, 2018; Zhou, 2020). After determining the yield of thereaction and concluding that the concentration of Al and OM was not ofinfluence, the by-products of the Al­-OM reaction were quantified. The ionicstrength of the supernatant of an increasing density of flocs in solution wasdetermined by measuring the electrical conductivity (EC). The results showedthat the ionic strength increased linearly with an increasing density of flocs.The relationship between the ionic strength and the density of flocs was coupledto the linear relationship between concentrations potassium chloride (KCl) andits EC. From the results, the measured EC can be used as a tracer since the K+and Cl­ are non­reactive.  The hydraulic conductivity measurements were conducted by afalling head test to be able to make an indication of the change inpermeability when adding the Al­-OM precipitates to the sand. To find theoptimal method to mix the Al, OM and porous media, the influence of differentmethods of producing, adding and mixing the materials on the permeabilityreduction is explored. In the first mixing method, the Al and OM were added insolution, the solution containing Al-­OM precipitates was centrifuged until thereduced ionic strength was at an EC value less than 700 µm/cm. The hydraulicconductivity measurements were used to obtain the relationship between thehydraulic conductivity reduction over an increasing concentration of flocsretained by one kilogram of sand. The hydraulic conductivity was reducedexponentially over an increasing concentration of flocs up to a magnitude of 3.The results imply a large variability in the achieved reduction dominated bythe amount of retained flocs. The second method is using Al and OM in powderformat and adding them to the dry sand and adding 500 ml of water to thismixture. This mixing method resulted in a completely different floc structure.The flocs produced by mixing in solution have a shear dependency feature, whiledry mixing created particles that have a constant size. For this method, theincrease of the reduction is linear over an increase of concentration of flocsretained by the soil. The highest reduction for this method was found to be ofa magnitude of two, measured at 50 grams of flocs retained by one kilogram ofsand. Finally, this research gave proof of principle of using Al­-OMprecipitates mixed directly with sand could reduce the permeability up to amagnitude of 3. These results present a new road to research on this Al-­OM­-sandmixture’s strength parameters and compaction over time over an increasing flocdensity, since these parameters are critical for using the layer in practice.