Using Environmental Tracers in Modeling Flow in a Complex Shallow Aquifer System

Using environmental tracers in groundwater dating partly relies on the assumption that groundwater age distribution can be described analytically. To investigate the applicability of age dating in complex multiaquifer systems, a methodology for simulating well specific groundwater age distribution was developed. Using a groundwater model and particle tracking we modeled age distributions at screen locations. By enveloping modeled age distributions and estimated recharge concentrations, environmental tracer breakthroughs were simulated for specific screens. Simulated age distributions are of irregular shapes and sizes without being similar to the assumed age distributions used in the analytical approach. The shape of age distribution to some extent depends on sampling size and on whether the system is modeled in a transient or in a steady state, but shape and size were largely driven by the heterogeneity of the model and by topographical variations as well. Analytically derived groundwater ages are dependent on sampling time. This time dependence relates to the nonlinearity of recharge concentrations and the shape and size of age distribution that has no coherence with the simplified assumptions of traditional approaches. Accordingly, constraining flow models by "age observations" may lead to misrepresentations that are biased depending on sampling time. If environmental tracers are used directly in terms of concentrations instead of ages, spatial as well as temporal variations become useful in constraining models.