The evaluation of groundwater presence by means of seismic wave velocities is of great concern for many hydrological related engineering applications. This work introduces a simple basic procedure to accurately and systematically identify groundwater levels using P-wave and S-wave velocities. This procedure is defined as a seismic reflectivity parameter and based on the seismic reflection coefficients of P-wave and S-wave at the unsaturated/saturated interfaces (i.e., groundwater level). The present procedure is applied to 1045 K-NET sites in Japan. Observed velocity structures, accompanied by their bulk densities and detailed lithology, for the upper 20 m depth are obtained from the National Research Institute for Earth Science and Disaster Prevention, Japan. Calculations are carried out considering three distinct lithological conditions – gravel, sand and clay–silt sediments – in order to evaluate the ability of this approach (seismic reflectivity parameter) to detect the groundwater level and assess its practical limitations. The increase of the seismic reflectivity parameter value varies substantially with Poisson's ratio, resembling the relationship between ??/VS and Poisson's ratio in gravel, sand and clay–silt sediments. However, positive seismic reflectivity parameter values indicate groundwater presence, whereas negative seismic reflectivity parameter values indicate lithological variation. The seismic reflectivity parameter can also be used as a relative indicator for groundwater saturation levels. Fair comparisons are established between the seismic reflectivity parameter and the water seismic index technique. However, the seismic reflectivity parameter approach shows consistency with previous theoretical and experimental studies related to variations of ?? and ?? in saturated conditions, whereas the water seismic index shows inconsistency with these previous theoretical and experimental studies and an inability in determining saturation levels and detecting groundwater levels.