Technical papers on the quantification of advective flow rates through composite liners assume the existence of a single defect in the geomembrane or of multiple defects that are not hydraulically interacting. In this paper, the results of a numerical study assuming the existence of two identical interacting adjacent square defects in the geomembrane are presented. The composite liners under study incorporate a geosynthetic clay liner (GCL). The impact of various parameters on the leakage flow through defects in the geomembrane is studied among which (1) the distance separating interacting adjacent square defects; (2) the hydraulic head applied on top of the composite liner; (3) the defects width and (4) the interface transmissivity. Results are compared with those obtained for two noninteracting square defects in order to evaluate the hydraulic impact of this interaction. Results obtained in terms of flow rates and wetted areas reveal that (1) the interaction between two adjacent square defects does not result in an important reduction of the flow rate; (2)wetted areas can be approached by an ellipse or by an 8-shape depending on the distance separating adjacent defects. Furthermore, results obtained in terms of flow rates and wetted areas for two interacting adjacent defects were compared to the results obtained for a longitudinal defect bounding those two defects. Except for small distances between the two interacting adjacent defects where results are similar to those obtained with a longitudinal defect, in most cases the flow rates and wetted areas obtained with a longitudinal defect are larger than the one obtained for two interacting adjacent defects. The difference increases as the distance between centres of both interacting adjacent defects increases.