This chapter develops the water balance equation and its solutions for various inputs to study temporal water fluctuations in the groundwater system. This equation is applied to a field aquifer for estimating the parameters and recharge. Subsequently, the well-mixed model for the solute and its analytical solutions for various input forms are developed. Further, the chapter discusses the hydraulic response, water retention, and chemical response time, pertinent to understanding energy propagation, solute advection, and mixing concepts. The application of the groundwater model to highway deicing salt application follows. The model for reactive chemical solutions comes next, considering chemical decay, first-order equilibrium, and nonequilibrium reactions. Their effects on solution output are discussed. We then introduce the Monte Carlo simulation, sensitivity analysis, and first-order analysis based on the well-mixed model to address uncertainty in the model outputs due to unknown parameters and inputs. Finally, its application to groundwater reservoirs’ effects on buffering acid rains in a lake is presented.