In this paper, we present analytical solutions and discuss them for simplified groundwater systems with decaying environmental tracers such as 3H, including the formation of a decay product such as tritiogenic 3He. The developed solutions are applicable for shallow, unconfined groundwater systems, which can be conceptually described by a steady-state, two-dimensional, semiconfined groundwater flow model with constant thickness, recharge rate, and porosity. The prerequisite for the applicability of our solutions is that the pumping wells and observation wells at which tracer information is available are fully screened over the entire aquifer thickness. The sampling by pumping from such wells produces the complete mixing of water of different age and origin. The application of our solution to the Baltenswil (Zurich, Switzerland) groundwater field site shows that the simplified model is able to catch essential dynamics of the transient concentration development of 3H and 3He.