Engineering a horizontal layer of reduced permeability using Al-DOM precipitation

Abstract

Leakage of contaminants to the subsurface is a potential threat to groundwater and is a major problem with storage tanks. When the contaminant reaches the groundwater, it can be subjected to subsurface flow due to which a contaminant plume may form and the contaminant can spread. In this research, it was investigated if a horizontal layer of reduced permeability can be engineered by using the SoSEAL (Soil Sealing by Enhanced Aluminium and DOM Leaching) concept to prevent leakage. Within SoSEAL, Aluminium-Dissolved Organic Matter complexation is used to clog the pore space with a reduction in permeability and hydraulic conductivity as a result. The concept is based on the natural process of podsolization in which a layer of reduced permeability (a hardpan) is formed by decades of organic matter leaching to the subsurface while complexing with metal cations. In this research, the complexation reaction using Aluminium-Chloride Hexahydrate and HUMIN-P 775 (Organic Matter) was investigated. The study roughly consisted of two parts, one concerning a situation of ideal mixing and one where the chemicals mixed inside a porous medium. When considering ideal mixing, eleven samples were created with 40 mL of Organic Matter solution and Aluminium solution in the range of 0 to 4 ml was added. These samples were created with an initial OM pH of 5, 7 and 9 to investigate the pH effect. The precipitation was quantified on the basis of measured pH, EC, UV254 and weight. With UV254 measurements it was found that 85% of the organic matter is able to form precipitates by binding aluminium ions. Depending on the pH, the amount of aluminium which needed to be added to reach this 85% varies. At a lower pH, less aluminium salt is needed to be added to form complexes with the available organic matter.
The precipitation as a result of the complexation was also formed in the three-dimensional experimental set up. The chemicals were injected via horizontal drains and a flow was created towards an overflow drain. A narrow precipitation band of a few millimetres thick was formed at the contact interface of the aluminium and organic matter solutions. The Al-DOM precipitation was visualized with a 3D interpretation of photographs of horizontal cross-sections in Matlab. The results of this interpretation showed that it was difficult to form an area-covering layer of precipitates. Most precipitation was shown near the injection- and overflow drains, this assumes that due to separation of flow no Al-DOM precipitation took place in the middle of the set-up. Due to the lack of an area-covering layer, a significant reduction of hydraulic conductivity could not be measured. An average reduction of about 25-30% was measured. In COMSOL it was assumed that in ideal situations the precipitation band could cause a reduction in permeability of about two order of magnitude. The results of this research show that the SoSEAL concept, using HUMIN-P 775 and aluminium hexahydrate, can be used to form a horizontal layer of reduced permeability, but that it is challenging to create an area-covering layer due to separation of flow and preferential flow paths.