Prediction of hyporheic exchange and solute transport dynamics in a headwater tributary of the Illinois and Mississippi River systems

0408744 Packman项目概述：河流和周围地下之间的水和溶质交换(地下交换)最近被认为是流域中许多重要物质循环的关键过程。水下交换尤其被证明会影响营养和碳的动态，以及污染物在地表水中的去向和迁移。这些问题在美国中西部地区已经上升到了前列，因为来自农业和城市中心的大量硝酸盐负荷导致了墨西哥湾的慢性缺氧。尽管源头溪流的水文过程、去除溪流硝酸盐的反硝化效率和到达墨西哥湾的氮负荷之间存在明显的联系，但我们目前对低坡度沙床溪流中这些过程的理解主要是经验性的。最突出的是缺乏基于基本原理的沙床溪流中潮汐交换的预测模型，这阻碍了在不同流动条件下从一个站点获得的实验观测数据转移到其他站点，甚至转移到同一站点。我们的目标是从低坡度的沙床溪流中收集确定的数据集，并测试几个基本的、基于过程的沙床溪流交换模型的适用性，这些模型已经在实验室水槽中被证明是成功的。我们不仅致力于促进我们对真实水流中潜流交换过程的基本理解，而且还致力于评估在各种流动和地貌条件下对溶质运移预测的改进。选择进行深入研究的河流是糖溪，这是沿伊利诺伊州和印第安纳州边界北部农业区的一条源头溪流。糖溪是农业源头地区许多低坡度溪流的代表，这些河流是伊利诺伊河和密西西比河水系的支流。我们建议进行详细的示踪实验和相关的物理测量，这将是值得注意的，因为它们将以前所未有的详细程度解决渗流路径、溶质传输和控制过程。流中溶液注射将在几个季节进行，在糖溪3至4公里长的范围内进行。河流示踪数据将用于评估代表不同地貌条件的8至10个子河段(50,300米长)的综合大宗运输。这些次级河段中的几个将根据其河床地形和形态、跨海峡水流的可变性、沉积物特征和溶质示踪剂的地下运动等方面进行彻底的表征。这一史无前例的详细程度将使用最先进的测量技术来实现。得到的数据集将支持应用一套基础的、预测性的、基于过程的低渗透交换模型及其对下游溶质传输的影响。这些模型将根据它们不仅预测平均示踪剂浓度数据的能力，而且还预测界面高通量和孔隙水示踪剂浓度的点估计来进行评估。在基础理论的基础上，我们将开发和测试在季节变化的条件下，用来预测整个研究范围内溶质运移的潜水交换速率的方法。总而言之，我们将应用几种不同复杂程度的建模方法，以1)评估我们目前从基本原理预测沙床水流中潜流交换的能力，以及2)开发合理的方法来扩大计算，以便可以在较长的河流范围内以特定的不确定性水平来预测溶液的输送。由于这项研究集中于广泛发生的基本过程，这项工作将具有非常广泛的科学影响，并将用于解决多重紧迫的社会问题。详细了解河流/沉积条件和潮汐交换之间的关系将有助于更好地了解营养动态、碳循环和受污染沉积物的释放。我们的预测目标(在不确定性和空间分辨率方面)与未来在REACH和流域尺度水质模型中的应用相一致。特别是，选择糖溪作为密集研究地点将提供关键的水文信息，以支持对密西西比河流域源头硝酸盐通量和沉积反硝化的补充研究。该项目的更广泛影响也将因国际和平协会的协同活动而增加。这两个国际和平组织都有参与科学组织的良好历史，该项目的主题将反映在重大技术会议的特别会议等活动中。我们将特别寻求通过鼓励跨学科和国际交流以及站点间比较来扩大这项工作的总体贡献。该项目还将通过直接培训学生和利用实地教育示范场地为人力资源开发作出重大贡献。

0408744PackmanProject Summary: Water and solute exchange between streams and the surrounding subsurface(hyporheic exchange) has recently been recognized as a critical process in the cycling of many importantsubstances in watersheds. Hyporheic exchange has particularly been shown to influence nutrient andcarbon dynamics, and the fate and transport of contaminants in surface waters. These issues have risen tothe forefront in the Midwestern U.S. because substantial nitrate loads from agriculture and urban centerscontribute to chronic hypoxia in the Gulf of Mexico. Despite the clear link that exists between hydrologicprocesses in headwater streams, denitrification efficiency in removing stream nitrate, and nitrogen loadsreaching the Gulf, our current understanding of these processes in low-gradient, sand-bed streams islargely empirical. Most glaring is the lack of, fundamentally-based, predictive models for hyporheicexchange in sand-bed streams, which has prevented the transfer of experimental observations gained atone site to other sites, or even to the same site under different flow conditions.Our objective is to collect a definitive data set from a low-gradient, sand-bed stream and test theapplicability of several fundamental, process based models of hyporheic exchange that have provensuccessful in laboratory flumes. We seek to both advance our fundamental understanding of hyporheicexchange processes in real stream, but also to assess the improvements in prediction of solute transportthat are possible for a variety of flow and geomorphic conditions. The stream selected for intensive studyis Sugar Creek, a headwater stream in the agricultural region along the northern Illinois-Indiana border.Sugar Creek is representative of many low-gradient streams in agricultural headwater areas that aretributary to the Illinois and Mississippi River systems. We propose to conduct detailed tracer experimentsand related physical measurements that will be notable because they will resolve hyporheic flow paths,solute transport, and controlling processes with an unprecedented level of detail. In-stream soluteinjections will be undertaken in several seasons in a 3 to 4 kilometer long reach of Sugar Creek. Stream-tracerdata will be used to assess bulk transport integrated over 8 to 10 sub-reaches (50 300 m long)representing variable geomorphic conditions. Several of those sub-reaches will be thoroughlycharacterized in terms of their streambed topography and morphology, cross-channel flow variability,sediment characteristics, and subsurface movement of the solute tracer. This unprecedented level ofdetail will be made possible using state of the art measurement technologies. The resulting data sets willsupport the application of a suite of fundamental, predictive, process-based models of hyporheic exchangeand its effects on downstream solute transport. The models will be evaluated in terms of their ability topredict not only reach-averaged tracer concentration data, but also point estimates of interfacial hyporheicflux and porewater tracer concentrations. Based on fundamental theory, we will develop and testapproaches for upscaling rates of hyporheic exchange to predict solute transport in the entire study reachunder seasonally varying conditions. In sum, we will apply several modeling approaches of differingsophistication in order to 1) evaluate our current ability to predict hyporheic exchange in sand bed streamsfrom first principles, and 2) develop reasonable approaches for upscaling the computations so that solutetransport can be predicted with a specified level of uncertainty for longer stream reaches.Because this study is focused on fundamental processes that occur widely, this work will have verybroad scientific impacts and will be used to address multiple pressing societal concerns. Detailedunderstanding of the relationship between stream/sedimentary conditions and hyporheic exchange willfacilitate improved understanding of nutrient dynamics, carbon cycling, and releases from contaminatedsediments. Our goals for predictions (in terms of uncertainty and spatial resolution) are compatible withfuture applications in reach and basin-scale water-quality models. In particular, the choice of Sugar Creekas the intensive study site will provide critical hydrologic information to support complementary studieson nitrate fluxes and sedimentary denitrification in the headwaters of the Mississippi River basin. Theproject's broader impacts will also be increased by the synergistic activities of the PI's. Both PI's have astrong history of involvement in scientific organizations, and the project's themes will be reflected inactivities such as special sessions at major technical meetings. We will particularly seek to broaden thegeneral contribution of this work by encouraging interdisciplinary and international communication andinter-site comparisons. The project will also contribute considerably to human resource development,both through direct training of students and via the use of the field site for educational demonstrations.