The　cut-off　wall　is　an　effective　countermeasure　to　prevent　contaminant　transport　from　a　contaminated　site.　However,　the　aquitard　beneath　the　cut-off　wall　may　serve　as　a　preferential　path,　leading　to　an　earlier　breakthrough　of　contaminants　in　the　cut-off　wall　system.　In　this　study,　we　present　a　2-D　analytical　model　for　characterizing　the　contaminant　transport　in　the　cut-off　wall　and　aquitard　system.　The　Green　function　method　and　discretization　method　are　used　to　investigate　contaminant　migration　in　a　semi-infinite　domain.　We　show　that　the　one-dimensional　cut-off　wall　model　may　be　insufficient　to　predict　contaminant　transport　when　the　aquitard　hydraulic　conductivity　(k2)　k　2　)　exceeds　5　x　10-10　m/s.　The　contaminant　mass　outflow　from　the　aquitard　for　the　case　with　k2　2　=　2　x　10-9-9　m/s　may　account　for　53　%　of　the　total　contaminant　mass　outflow.　Increasing　k2　2　from　5　x　10-10　m/s　to　5　x　10-9-9　m/s　leads　to　a　2.9-fold　increase　of　cumulative　mass　outflow.　An　approximate　fitting　formula　is　proposed　to　calculate　the　required　minimum　thickness　of　cut-off　wall　under　different　type　of　aquitards.　The　thicker　cut-off　wall　is　required　to　ensure　the　cumulative　mass　outflow　will　not　exceed　the　allowable　value　when　k2　2　is　relatively　high　(e.g.,　1　x　10-9-9　m/s).