A Novel Method for Assessing Impacts of Urbanization on Watershed Processes

我们日益城市化的社会面临的最紧迫的环境问题是城市流域的水质下降和栖息地的丧失。与这些影响相关的主要问题包括：土地利用变化对这些水文系统的影响的时间尺度是什么？城市化引起的土地利用变化影响流域流量和水质的门槛是多少？这种阈值能否作为流域管理的预测性工具？为了直接解决这些基本问题，我们提出了一种新的同位素时间追踪方法。河岸树可以记录其生长年轮内溪水化学的高分辨率时间变化。然而，在一棵树内，有一些生理因素可以1)影响树内元素浓度和生长年轮内元素浓度之间的关系“S生长记录”，2)元素在树的老部分和年轻部分之间的调动或“转移”。因此，可能很难将树木化学成分的时间变化与周围环境的化学变化联系起来。使用自然产生的锶同位素作为示踪剂，可以最大限度地减少或消除第一个复杂的因素。为了解决第二个因素，我们建议开发木材样品的化学处理方法，以最大限度地减少这种二次叠印。经验测试将确定处理的效果，从而确定树轮记录中锶同位素值的时间变异性。在得克萨斯州奥斯汀，市政饮用水和污水中的锶同位素值与天然溪水的值不同。在高度城市化的流域，很大一部分溪水显然来自漏水的基础设施和/或灌溉。一项新的研究表明，城市和城市之间的比例用水？天然的？根据流水锶同位素值，这些城市流域的流水与城市化程度和年龄密切相关。我们建议测量沿这些溪流生长的树木的年轮的锶同位素组成，以确定溪水中87Sr/86Sr的变化年表。初步数据表明，树轮87sr/86sr的历史趋势可以用来确定分水岭内人类干扰的时间和强度。由于这一新方法尚未得到证实，我们建议进行一项概念验证研究，当该方法被证明时，我们将向水文科学提出一项更具包容性的建议。拟议的项目将致力于确定土地利用变化对城市水文系统的影响的时间尺度，确定城市化影响流域流量和水质的时间关联指标的阈值，并评估树木年轮中的放射性同位素测量，以此作为土地利用和流域管理的预测工具。预计拟议研究的结果将对更广泛的水文界有所帮助，并可作为流域管理的潜在工具。这里概述的分析方法将用于其他城市分水岭，在这些分水岭的地质环境中，市政水源和天然溪水之间存在同位素对比。同时，这项研究将开发一种新的方法来研究城市地区土地利用变化影响的时间变化，从而推动树化学在水文科学中的应用。该项目将培训野外和分析地球化学技术方面的研究生和本科生，以及基础实践科学方面的K-12教师和学生。拟议研究的结果和方法将纳入：(1)环境同位素地球化学和分析方法研究生课程；(2)德克萨斯州中部流域的实地和实验室讲习班，德克萨斯大学奥斯汀分水岭的S环境科学研究所负责监督K-12教师和NSF REU本科生研究人员。这些教师中的一些人将参与对学校附近溪流的实地研究。拟议研究的结果将很容易纳入正在进行的以分水岭为特色的几个正在进行的多媒体外联活动。

Among the most pressing environmental problems facing our increasingly urbanized society are the reduction of water quality and loss of habitat in urban watersheds. Principal questions associated with these impacts include: What are the time scales of the impact of land use change on these hydrologic systems? What are the thresholds at which land use change via urbanization impacts watershed flow and water quality? Can such thresholds be used as a predictive tool for watershed management? We propose a novel isotopic temporal tracing approach in order to directly address such fundamental questions. Riparian trees may record high-resolution temporal changes in stream-water chemistry within their growth rings. Within a tree, however, there are physiological factors which can 1) affect the relationship between elemental concentrations within the tree''s immediate environment, and elemental concentrations within a growing tree ring, and 2) mobilize, or ''translocate'' elements between older and younger portions of the tree''s growth record. Consequently, it may be difficult to relate temporal changes in the chemical composition of a tree''s rings to chemical changes in the surrounding environment. The use of naturally occurring Sr isotopes as a tracer can minimize or eliminate the first of these complicating factors. In order to address the second factor, we propose to develop chemical treatment methods for wood samples to minimize this secondary overprint. Empirical tests will determine the efficacy of the treatments and thus the ability to discern temporal variability of Sr isotope values within tree-ring records.Within Austin, Texas, municipal drinking water and sewage have Sr isotope values that are distinct from those of natural streamwater. In heavily urbanized watersheds, much of the streamwater is apparently coming from leaky infrastructure and/or irrigation. A new study shows that the ratio of ?city? water to ?natural? streamwater in these urban drainage basins, based on streamwater Sr isotope values, strongly correlates with degree and age of urbanization. We propose to measure the Sr isotope compositions oftree rings from trees growing alongside these streams, in both urbanized and unurbanized watersheds, in order to determine a chronology of changing 87Sr/86Sr within the streamwater. Preliminary data indicate that historical trends in tree-ring 87Sr/86Sr can be used to determine the timing and magnitude of human disturbance within a watershed. Because this novel method is as yet unproven, we are proposing a proofof-concept study, to be followed by a more inclusive proposal to Hydrologic Sciences when/if the method is proven.Intellectual Merit. The proposed project will work to determine the time scales of the impact of land use change on urban hydrologic systems, determine the thresholds at which urbanization impacts a timelinked proxy for watershed flow and water quality, and assess radiogenic isotope measurements in tree rings as a predictive tool for land-use and watershed management. The results of the proposed study are expected to be of use to the wider hydrologic community and as a potential tool for watershed management. The analytical approach outlined here will be of use in other urban watersheds where the geologic setting creates an isotopic contrast between the municipal water source and the natural streamwater. As well, this study will develop a new method for investigating temporal variations in land-use change impacts in urban areas and, as such, will advance the application of dendrochemistry to the hydrologic sciences.Broader Impacts. This project will train graduate and undergraduate students in field and analytical geochemistry techniques and K-12 teachers and students in basic, hands-on science. The results and methods of the proposed study will be incorporated into: (1) graduate courses in Environmental Isotope Geochemistry and Analytical Methods; and (2) ongoing field and laboratory workshops in central Texas watersheds that UT-Austin''s Environmental Science Institute oversees for K-12 teachers and for NSF REU undergraduate researchers. Some of these teachers will be involved with field studies of the streams near their schools. Results of the proposed study will be readily incorporated into several ongoing multimedia outreach initiatives that feature the watersheds to be studied.