Combining environmental tracers and numerical modeling to estimate mountain-front recharge components in the Tucson Basin

Abstract:

Mountain-block systems are critical to global water supplies. Yet, untangling their contribution to low-lying areas through mountain-front recharge, mountain-block recharge, and surface water runoff remains challenging due to the geologic complexity of these system and sparsity of in-situ observations. Here, we explore the combined value of tracer and modeling techniques to address this challenge. We estimate groundwater age distributions and flow path lengths from isotopic analysis and noble gas thermometry, respectively, and combine this with fluid and energy transport modeling to quantify mountain front recharge in the Tucson Basin. We show that incorporating noble gas radioisotopes krypton-85, argon-39, with routine age tracers (e.g., radiocarbon and tritium) can provide valuable information regarding the dominant flow paths of the system. However, the environmental tracers on their own are not sufficient to characterize the geometry of flow paths and can support faulty conceptual models. For example, assuming a constant recharge temperature lapse rate in noble gas thermometry can result in estimates of recharge elevations which are improbable. Using an integrated hydrologic model, we can illustrate the range of age distributions that can result from common assumptions about mountain block configuration. Combining energy transport modeling with noble gas thermometry demonstrates the sensitivity of water table temperatures to mountain-front recharge in our study area and leads to more realistic recharge elevation estimates. Our relatively simple modeling analysis demonstrates the value added by combining multiple environmental tracers with numerical models to explore and test conceptual models of the system and interrogate assumptions made with the environmental tracer analysis. Such integration is currently underutilized and has immense potential for understanding groundwater-surface water interactions, particularly with advances in high performance computing and particle-tracking routines coupled with integrated hydrologic models.