Groundwater waves, i.e., water table fluctuations, are a natural phenomenon in coastal aquifers. They represent an important part of the interactions between the occan and aquifer, and affect the mass exchange between them. This paper describes a new groundwater wave equation. Because it indudes the effects of vertical flows and capillarity, the new equation is applicable to both intermediate depth aquifers and high-frequency waves. Compared with the wave equation derived by Nielsen and others (1997), the present equation provides a closer representation of groundwater waves. In particular, it predicts high frequency watertable fluctuations as observed in the field. A validation of the new equation has been carried out by comparng the analytical solutions with predictions from direct simulations using the numerical model SUTRA. The effects of various physical parameters and their relative importance is also discussed.