COLLABORATIVE RESEARCH: Development of Environmental Tracers for Water and Solute Transport in Arid Vadose Zones with Applications to Paleohydrology

水和溶质通过渗透带的输送是干旱区水文过程中一个主要的不确定区域。了解干旱地区的这些通量对于开发水文模型、估计水资源可用性和确定废物处理设施的选址至关重要。两种最有前途的示踪技术，氯化物质量平衡和氯-36 （36Cl）的长期变化依赖于未经检验的假设，即它们在陆地表面沉积的一致性。这导致了对这些方法的重大批评。本研究的目的是开发可靠的技术来测量补给和溶质运输利用南部大盆地，其中冲积谷的渗透带厚度通常超过250米作为实地研究实验室。为了利用这些示踪数据重建水汽带的溶质通量历史，需要精确重建降水和干沉降中氯离子和36Cl的通量。我们首先建议使用来自该地区的年龄确定的鼠堆材料制定准确的36Cl年表。Plummer等人（1996）最近收集到的鼠堆数据显示了令人信服的证据，表明36Cl的沉积随磁场强度和气候模式的变化而变化，并且还保存在地下水和土壤水中。将利用这些数据，结合古地磁强度数据，确定晚更新世的长期36Cl产量。36Cl/Cl年代学将用于估计晚更新世氯化物通量的变化。如果使用氯化物质量平衡方法来估计补给和土壤水年龄，那么氯化物通量的知识是至关重要的，这是该方法应用中的主要不确定因素之一，特别是在更新世的干燥和多风时期。36Cl/Cl比值的变化与长期变化不一致，是由于降水和/或干沉积造成的氯化物通量增加稀释了比值。提出了两种方法来计算古氯化物通量，首先忽略干沉积中任何额外的36Cl（最小氯化物通量），其次计算湖相粉尘源中观测到的36Cl/Cl。通过类比40 ka之前的气候条件，氯化物通量年表将扩展到14C年代的材料之外。现代氯化物沉积也将在内华达州和新墨西哥州使用原位采样器进行测量。这些地区的土壤首先会被氯化物浸出，然后重新装入只在顶部打开的柱子中。这些柱将被埋在现场，并允许积累氯化物、溴化物和36Cl，以计算这些示踪剂的综合现代通量。由于通量低，积累期必须超过本项目的长度（5-10年），但将提供现代示踪剂通量的可靠估计。然后，重建的36Cl和Cl年表将用于推断保存在法国平原的三个先前分析过的钻孔和1973年钻探的1200米岩心孔（Ue6e）中的充值和古气候条件，该岩心孔保存并存档于美国地质调查局位于内华达州水星的岩心图书馆，该图书馆穿透了450米的主要由冲积沉积物组成的不饱和带。法国平原的钻孔保存了高达120凯尔的土壤水分，希望在Ue6e发现的更深的渗透带将这一记录进一步推高。对档案岩心进行氯化物和36Cl/Cl比值分析。氯化物分析将用于描绘将分析36Cl的岩心部分。本项目开发的氯化物通量年代学和古沉积物将与美国西南部其他地点（莫哈韦沙漠、新墨西哥州和德克萨斯州）报告的土壤来源的古沉积物进行比较，并与邻近Amargosa河流域正在进行的古水文研究进行比较，以保持一致性，并形成古沉积物的区域图景。美国能源部在法国平原和尤卡平原调查期间获得的数据将在可用时合并。该项目将与美国能源部的钻井活动密切协调，以获取新的岩心样本进行进一步分析，并通过美国能源部获得与Ue6e相邻的浅井眼的钻井支持。本提案中要分析的示踪剂剖面，以前报道过的来自邻近的法国平原的示踪剂剖面，也代表了天然渗透带示踪剂测试，为研究溶质在更大的时间和长度尺度上的迁移提供了一个极好的机会。利用重建的氯化物通量和36Cl通量，本文提出的研究将模拟观测到的水和溶质剖面，以确定干燥冲积沉积物的有效分散特性。根据土壤水中的氯化物浓度，法国平坦钻孔的土壤水通量从今天的不到0.1毫米/年到雨期的高达3厘米/年不等。因此，溶质输运的分析本质上是短暂的，使用受观察到的氯化物浓度限制的通量。传统的平流-色散求解器以及溶质运移的移动-不移动模型将应用于数据集，以确定运移机制，并在长时间尺度和时间尺度上为方法提供信心。本研究将大大增加对干旱区水汽带过程的认识。精确的36Cl和Cl年表的发展限制了示踪剂通量，将大大减少这些广泛使用的土壤水通量和补给估计的主要不确定性。现有的核心数据提供了一个独特的机会，可以量化厚水汽带对主要气候变化的反应，这种气候变化可以追溯到倒数第二个冰期。最后，大规模水汽带示踪试验数据的开发和输运参数的估算将为干旱区水文的基本行为提供新的认识。

9614509 Phillips The transport of water and solute through the vadose zone represents a major area of uncertainty in hydrologic processes of arid regions. The understanding of these fluxes in arid regions is critical in developing hydrologic models, estimating water resource availability and in the siting of waste disposal facilities. Two the most promising tracer techniques, chloride mass balance and secular variations in chlorine-36 (36Cl) rely on untested assumptions of the consistency of their deposition at the land surface. This has led to significant criticisms of these methods. The goal of this research is to develop sound techniques for measuring recharge and solute transport using the southern Great Basin, where the thickness of the vadose zone in alluvial valleys often exceeds 250 meters as field research laboratory. To use this tracer data to reconstruct the water solute flux history in the vadose zone, accurate reconstruction of the flux of Chloride and 36Cl from precipitation and dry deposition are needed. We first propose to develop an accurate 36Cl chronology using age dated pack rat midden materials from the area. Recent pack rat midden data from Plummer et al. (1996) show convincing evidence that 36Cl deposition varies both with magnetic field intensity and climatic patterns, and is also preserved in ground waters and soil waters. The 36Cl chronology will be developed using these data, combined with paleomagnetic intensity data to determine the secular 36Cl production through the late Pleistocene. The 36Cl/Cl chronology will next be used to estimate the variations in chloride flux through the late Pleistocene. Knowledge of the chloride flux is crucial if the chloride mass balance approach is used to estimate recharge and soil water age and represents one of the major uncertainties in the application of the method, particularly during dry and windy periods of the Pleistocene. 36Cl/Cl ratio changes that are not in phase with secular variations are the result of dilution of the ratio by increased chloride flux from precipitation and/or dry deposition. Two methods are proposed to bracket the paleochoride flux, by first ignoring any additional 36Cl in dry deposition (minimum chloride flux) and secondly by accounting for 36Cl/Cl observed in lacustine dust sources. The chloride flux chronology will be extended beyond 14C dated material by analogy with climatic conditions occurring prior to 40 ka. Modern chloride deposition will also be measured using in situ samplers in both Nevada and New Mexico. Soil from these areas will first be leached of chloride and repacked into columns open only at the top. The columns will be buried on site and allowed to accumulate chloride, bromide and 36Cl to calculate the integrated modern flux of these tracers. Due to the low fluxes, the accumulation period must extend beyond the length of this project (5-10 yr), but will provide firm estimates of modern tracer flux. The reconstructed 36Cl and Cl chronologies will then be used to infer the recharge and paleoclimatic conditions preserved in three previously analyzed boreholes in Frenchman Flat and a 1200 meter core hole (Ue6e) drilled in 1973, preserved and archived at the USGS Core Library in Mercury, NV which penetrates 450 meters of unsaturated zone comprised primarily of alluvial sediments. The boreholes in Frenchman Flat have preserved up to 120 kyr of soil water and it is hoped that the deeper vadose zone found at Ue6e will push this record back significantly further. Analysis of chloride and 36Cl/Cl ratios will be conducted on the archive core. Chloride analysis will be used delineate portions of the core where 36Cl will be analyzed. The chloride flux chronology and paleorecharage developed in this project will be compared to soil-derived paleorecharge reported from other sites in the American Southwest (Mojave Desert, New Mexico and Texas) along with ongoing paleohydrology studies in the adjacent Amargosa River drainage for consistency and to develop a regional picture of paleorecharge. Data obtained during DOE investigations of both Frenchman Flat and Yucca Flat will be incorporated as it becomes available. This project will closely coordinate with DOE drilling activities to obtain new core samples for additional analysis and work through DOE to obtain drilling support for a shallow borehole adjacent to Ue6e. The tracer profile to be analyzed in this proposal, previously reported tracer profiles from adjacent Frenchman Flat, also represent natural vadose zone tracer tests and provide an excellent opportunity to study solute transport at much larger time and length scales than are currently available. Using the reconstructed chloride and 36Cl flux, the research proposed herein will simulate the observed water and solute profiles to determine the effective dispersive properties of dry, alluvial sediments. Soil water fluxes in the Frenchman Flat boreholes has varied from less than 0.1 mm/yr today to as high as 3 cm/yr during pluvial times, based on chloride concentrations in soil water. Analysis of solute transport will therefore be transient in nature, using the fluxes constrained by the observed chloride concentrations. Traditional advection-dispersion solvers as well as mobile-immobile models of solute transport will be applied to the data sets to determine the mechanisms of transport and to provide confidence in the methods over long-length and time scales. This proposed research will significantly increase the understanding of vadose zone processes in arid regions. The development of an accurate 36Cl and Cl chronology constrain the tracer fluxes will significantly reduce the principal uncertainty in these wildely used soil water flux and recharge estimators. The core data available represents a unique opportunity to quantify the response of thick vadose zones to major climatic shifts extending back as far as the penultimate glaciation. Finally the development of large scale vadose zone tracer test data and the estimation of transport parameters will provide new insight into the fundamental behavior of arid zone hydrology.