## 1 Introduction  
In coastal areas, tidal dynamics cause periodic fluctuation of the groundwater response in coastal aquifers. The dynamic behavior of the groundwater flow associated with the fluctuation of the piezometric head plays a crucial role in coastal hydrology [1; 2; 3]. Finding out the interaction between the tidal dynamics as well as the groundwater level may be useful for investigating coastal hydrogeology in large scale which is of importance to solve various coastal hydrogeological problems, such as deterioration of the marine environment, seawater intrusion, and the stability of coastal engineering structures with rapidly development in coastal areas [4; 5]. Accordingly, considerable research has been devoted to understanding the tide-induced groundwater level fluctuations in a coast aquifer [6; 7; 8]. Analytical studies have been found to be common for understanding tidal variation affecting groundwater level in coastal areas [9; 10; 11]. [PERSON] (1997) solved an analytical solution of groundwater response to tidal-induced boundary conditions in an estuary [12]. [PERSON] and [PERSON] (1999) studied the groundwater response that consisted of unconfined and confined aquifers with a semi-permeable layer [13]. [PERSON] and [PERSON] (2001) further proposed an exact solution for the groundwaterresponse in a leaky confined aquifer [14]. [PERSON] et al. (2002) discussed the tidal propagation in a coupled unconfined and confined coastal aquifer system [15]. However, due to the mathematical complexity, it is a great challenge for finding exact analytical solutions of governing equations for different boundary conditions. As a result, the developed analytical solutions are usually limited and are restricted to simple problems with specific boundary conditions. Numerical approaches, such as finite elements and finite differences, approximate equations in discrete steps, both in time and in space [16; 17]. The advantage of these approaches is no limit to the complexity of various coastal hydrogeological problems. However, the accuracy cannot be as accurate as the analytical method due to numerical errors [18]. Despite the success of several numerical methods for finding the solution, the traditional approaches need mesh generation, and hence tedious time calculation, to give an approximation to the solution [19]. There is therefore still a need for numerical schemes in the development of new advanced approaches. Meshfree approaches appear as a rival alternative to mesh generation techniques. The conception of meshfree approaches is to solve a problem by using arbitrary or unstructured points that are collocated in the problem domain without using any meshes or elements [20]. The utilization of the mesh-free approach to acquire approximate solutions is advantageous for problem domain involving arbitrary geometry [21; 22]. The collocation Trefftz method (CTM) is one of meshfree approaches for dealing with boundary value problem (BVP) where solutions can be described as a combination of general solutions which satisfy the governing equation [23; 24; 25]. The CTM may release the difficulties for finding exact solutions of governing equations and remains the characteristics of high accuracy due to the adoption of the general solutions [26; 27; 28]. The original CTM is limited to homogeneous and stationary BVPs. Lately, a spacetime meshfree approach for analyzing transient subsurface flow in unsaturated porous media has been proposed by [PERSON] et al. (2018) [29]. In this study, we present a pioneering work for numerical solutions of tide-induced groundwater response in a coast aquifer using the spacetime meshfree method. This paper presents the numerical solutions of tide-induced groundwater response using the spacetime collocation approach (SCA). The newly developed SCA begins with the consideration of the Trefftz basis functions. The basis functions have to satisfy the governing equation of the tide-induced groundwater response in a coastal confined aquifer with an estuary tidal-induced boundary.