Abstract: The groundwater monitoring network design involves the determination of the sampling locations and sampling frequencies to characterize the contaminant plume in an aquifer over space and time.This paper introduces a spatial sampling reduction method,the Delaunay method based on Delaunay triangularization,to identify and eliminate the redundant sampling wells of an existed long-term contaminant monitoring network.Considering that the Delaunay method is applicable to both the heterogeneous aquifers and inserting additional sampling locations in the insufficient monitoring network,a modified Delaunay method coupled with contaminant transport model is developed to deal with these issues.The results of this study indicate that the modified Delaunay method can be used as an efficient tool for the cost-effectively spatial sampling network design mean while keeping the accuracy of the characterization of the contaminant plume.