The quality of groundwater is essential to ensuring the sustainable use of water resources for irrigation. Deep groundwater samples were collected from the Sambagawa Belt in southeastern Japan, at 1,180 m (GWdeep) and 50 m (GWshallow). Groundwater quality was assessed based on analysis of physicochemical parameters. The sodium adsorption ratio of groundwater fell in the range of 1.61-3.13, while electric conductivity of GWdeep was high ( &gt
75.0 mS m-1). Experiments were performed to determine the effects of deep groundwater on germination and early growth of komatsuna (Brassica rapa var. perviridis). Strong inhibition (44% reduction) of root growth was observed with GWdeep compared to the control treatment. In contrast, shoot length increased with increasing depth of the groundwater source. Comparison of the effects of GWdeep and NaCl solution on growth of komatsuna revealed similar patterns at 1-103 mg L-1 NaCl solution. Our findings suggested that deep groundwater can be used for irrigation of komatsuna, but not during the germination stage of this crop.