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

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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.