Dynamic Groundwater Age Distributions: Exploring Watershed Scale Subsurface Systems

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Residence times in groundwater systems are referred to in terms of groundwater age and used to diagnose both natural and anthropogenically impacted groundwater behavior. Environmental tracers are employed as proxies to estimate groundwater age, but their application almost always assumes a steady, stationary flow. The effects and importance of transient flow are not accounted for and are unknown. This project addresses non-stationary flows in complex watersheds by accounting for the impacts of temporal variability of weather and climate forcings on groundwater flow and age transport. It uses a two-fold approach. Synthetic systems will be modeled using five-dimensional (5-D), generalized groundwater age models (3-space, 1-time, 1-age) in order to build understanding of controlling processes on groundwater age distributions (GWAD). In transient flows, GWADs are functions of both the time of observation and the time at which observed water entered the system. Preliminary theoretical results show that transient flows bring in processes at all scales, with significant impacts on long-term memory. Then two instrumented and monitored natural watershed systems in the upper Rio Grande of Colorado and New Mexico will be studied. One of these sites is located in the Jemez Mountains of north-central New Mexico and this watershed is part of a Critical Zone Observatory (CZO) funded by NSF. The type, time, and location of new chemical and isotopic tracer measurements will be optimized based on their preliminary 5-D groundwater age models. These applications to natural watersheds will involve undergraduates, link models with data, test the underlying theory and its application, and elucidate how best to use the available suite of tracer methods for GWAD estimation in non-stationary systems. Innovative mathematical solution methods to the 5-D generalized age equations will be developed and tested, including the use of Laplace transforms on age in order to reduce dimensionality. Extensions of these concepts and approaches to residence time distributions in other hydrologic applications, including stream ecology, and to atmospheric science and oceanography where similar problems are encountered in the context of environmental tracers and circulation models, are anticipated.