The concept of integratedconstructedwetlands (ICW) uses in situ soils to construct and line their cells. The integrity of soil materials, however, may provide a potential pathway for contaminants to flow into the subsoil. In this study, the rates of infiltration and contaminants loading occurring beneath a full-scale ICW treating domestic wastewater were evaluated over an 18-month period. The ICW is located at Glaslough in Co. Monaghan, Ireland. It consists of two sludge cells and a sequence of five shallow vegetated wetland cells. The ICW cells were lined with 500 mm thick local subsoil materials. Infiltration water was collected from zero-tension pan lysimeters, which were placed within the soil-liners of the first three ICW cells and analysed for water quality parameters such as chemical oxygen demand, ammonia-nitrogen, nitrate-nitrogen, and molybdate reactive phosphate. Infiltration rates were 4.3 × 10−9, 3.7 × 10−9 and 1.0 × 10−8 m s−1 from the first three ICW cells. The variation among locations was small. The apparent hydraulic conductivity of the soil liner, calculated with empirical water budgets, ranged between 3.5 × 10−9 and 9.8 × 10−9 m s−1 across the cells. In the first two cells of the ICW, less than 0.5% of the influent contaminant loading to the cells was lost through infiltration to subsoil. Overall, the amount of infiltration and contaminant loading occurring beneath the ICW cells increased from the proximal cells to the distal one. Higher contaminant loading was recorded in the third cell, which received partially treated wastewater. This implies that each ICW cell has the potential to impact differently on the underlying groundwater and that the potential for groundwater contamination from ICW systems was minimal in the first ICW cells, when compared to the last one.