Dynamic Groundwater Age Distributions: Exploring Watershed Scale Subsurface Systems

地下水系统中的停留时间是指地下水年龄，用于诊断受自然和人为影响的地下水行为。环境示踪剂被用作估算地下水年龄的指标，但它们的应用几乎总是假设水流稳定。瞬变流的影响和重要性没有被考虑在内，也是未知的。该项目通过考虑天气和气候强迫的时间变异性对地下水流动和年龄迁移的影响，处理复杂流域中的非稳定流动。它使用了两种方法。将使用五维(5-D)、广义地下水年龄模型(3-空间、1-时间、1-年龄)对合成系统进行建模，以建立对地下水年龄分布(GWAD)的控制过程的理解。在瞬变流中，GWAD是观测时间和观测到的水进入系统的时间的函数。初步的理论结果表明，瞬变流会在所有尺度上引入加工，并对长时记忆产生显著影响。然后将研究科罗拉多州格兰德河上游和新墨西哥州的两个用仪器测量和监测的自然分水岭系统。其中一个地点位于新墨西哥州中北部的Jemez山脉，这个分水岭是由NSF资助的临界区天文台(CZO)的一部分。新的化学和同位素示踪剂测量的类型、时间和地点将根据其初步的5-D地下水年龄模型进行优化。这些对天然流域的应用将涉及本科生，将模型与数据联系起来，测试潜在的理论及其应用，并阐明如何在非平稳系统中最好地使用可用的示踪剂方法来估计GWAD。将开发和测试5维广义年龄方程的创新数学求解方法，包括使用拉普拉斯变换对年龄进行降维。预计这些概念和方法将扩展到包括河流生态学在内的其他水文学应用中的停留时间分布，以及在环境示踪剂和环流模型方面遇到类似问题的大气科学和海洋学。

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.