Analysis of radionuclide migration through a 200-m Vadose zone following a 16-year infiltration event

Atomic weapons testing at the Nevada Test Site has introduced many tracers for quantifying subsurface hydrologic transport processes in arid climates. In 1975, groundwater adjacent to the Cambric test, conducted beneath Frenchman Flat 10 years earlier, was pumped steadily for 16 years to elicit information on the migration of residual radioactivity through the saturated zone. Radionuclides in the pumping well effluent, including tritium, ¹⁴C, ³⁶Cl, and ⁸⁵Kr, were extensively monitored prior to its discharge in an unlined ditch, where approximately a third of the flow infiltrated over a distance of 1 km. Radionuclide infiltration through a 220-m thick vadose zone created a second, and rather unique long-term field experiment. Effluent data have been utilized in conjunction with geologic data, new radionuclide measurements, isotopic age-dating estimates, and vadose zone flow and transport models to better understand the movement of radionuclides between the ditch, the water table and a nearby groundwater monitoring well. Detection of tritium in the monitoring well occurred approximately 16 years after its initial discharge into the ditch. Modeling and tritium age dating have suggested 3–5 years of this 16-year transit time occurred solely in the vadose zone. They also suggest considerable recirculation of the pumping well discharge back into the original pumping well. Notably, there have been no observations of ¹⁴C or ⁸⁵Kr in the monitoring well, suggesting their preferential retention or volatilization during transit to the water table.     

A study of the 2.5–25 μm spectrum of H2O ice

We present new absorbance spectra of the 3-, 6- and 12-μm features of amorphous and crystalline H₂O ice obtained between 10 and 140 K. Three sets of measurements have been made. In series I, the ice film was initially deposited on to a CsI substrate at 10 K and successive spectra were then obtained at intermediate temperatures as the ice was warmed up to 140 K. The second set, series II, comprises spectra for ice films deposited and measured at temperatures between 10 and 140 K. In the third set of measurements, series III, spectra were obtained for an ice film deposited at 140 K and then at intermediate temperatures as the film was cooled down to 10 K. The series I and II results show that the ice undergoes an amorphous-to-crystalline phase transition in the 110–120 K range. The 3- and 12-μm bands have similar trends in full width at half-maximum (FWHM) and opposite peak wavelength shifts. The temperature behaviour of the 6-μm band is different, as no clear phase transition temperature can be discerned from its FWHM and peak wavelength position data. In the series III spectra the peak wavelength positions and FWHM of the three bands remain relatively constant, thus demonstrating the stability of the crystalline phase against thermal cycling. A comparison between the laboratory results and astronomical spectra suggests that the identification of the librational band of H₂O ice in OH 231.8 + 4.2 may be incorrect.     

Mexican megadrought

Around 1550 AD a megadrought occurred in Mexico, possibly resulting in the death of over 80% of the Indian population at that time from disease. By any measure this was a major disaster that warrants serious examination. A 10,000-year simulation with the CSIRO Mark2 coupled global climatic model has been used to investigate the existence and genesis of major droughts in the Mexican region. One megadrought event was identified and a number of droughts lasting a decade or longer were also found. Average reductions in rainfall of 20 to 40% over the duration of these droughts were found to occur. While there is a mechanistic link between El Niño events and drought over Mexico, this link is tenuous, as shown by observations and this simulation. Stochastic processes appear to have a considerable influence on Mexican rainfall, and contribute to the weakness of the El Niño influence on such rainfall. It is shown that for both El Niño events and stochastic processes drought is created over Mexico by the reduction of the intensity of the climatological high pressure system in this region. This reduces the influx of moisture associated with the monsoonal system and thus rainfall. While only one 'megadrought' was found in the 10,000-year simulation thirteen droughts lasting over a decade occurred, suggesting that a devastating drought in the Mexican region has a return period of less than 1000 years and represents a non-trivial potential problem for this region.     

Estimating past leaf-to-air vapour pressure deficit from terrestrial plant δ13C

δ¹³C was determined in lignin extracted from present-day cladodes of Phyllocladus alpinus (a small coniferous tree) from seven well-lit sites across New Zealand. The δ¹³C values ranged from −30.9‰ to −23.6‰ and were compared with monthly means of temperature, precipitation, relative humidity and vapour pressure deficit from the nearest recording stations. Of these parameters, the leaf-to-air vapour pressure deficit of the first month of cladode growth and expansion proved to be the most significantly correlated with lignin δ¹³C, over a range of 0.3 to 0.8 kPa, confirming the importance of atmospheric moisture content on stomatal conductance. The carbon isotopic signature of lignin from fossilised cladodes preserved under the Kawakawa Tephra (22.6 k ¹⁴C yr BP) on the North Island is identical to that of the whole tissue, suggesting that for this species at least, fossil material can be used to approximate the lignin δ¹³C. The δ¹³C of species- and organ-specific fossil terrestrial plant material therefore provides an excellent method to quantify past changes in leaf-to-air vapour pressure deficit. Copyright © 1999 John Wiley & Sons, Ltd.     

Revisiting “An Exercise in Groundwater Model Calibration and Prediction” After 30 Years: Insights and New Directions

In 1988, an important publication moved model calibration and forecasting beyond case studies and theoretical analysis. It reported on a somewhat idyllic graduate student modeling exercise where many of the system properties were known; the primary forecasts of interest were heads in pumping wells after a river was modified. The model was calibrated using manual trial-and-error approaches where a model's forecast quality was not related to how well it was calibrated. Here, we investigate whether tools widely available today obviate the shortcomings identified 30 years ago. A reconstructed version of the 1988 true model was tested using increasing parameter estimation sophistication. The parameter estimation demonstrated the inverse problem was non-unique because only head data were available for calibration. When a flux observation was included, current parameter estimation approaches were able to overcome all calibration and forecast issues noted in 1988. The best forecasts were obtained from a highly parameterized model that used pilot points for hydraulic conductivity and was constrained with soft knowledge. Like the 1988 results, however, the best calibrated model did not produce the best forecasts due to parameter overfitting. Finally, a computationally frugal linear uncertainty analysis demonstrated that the single-zone model was oversimplified, with only half of the forecasts falling within the calculated uncertainty bounds. Uncertainties from the highly parameterized models had all six forecasts within the calculated uncertainty. The current results outperformed those of the 1988 effort, demonstrating the value of quantitative parameter estimation and uncertainty analysis methods.     

Experiments on the morphological controls of velocity inversions in bedrock canyons

A better understanding of bedrock incision mechanisms and processes is essential to the study of long‐term landscape evolution. Yet, little is known about flow dynamics in bedrock rivers, limiting our ability to make realistic predictions of local bedrock incision rates. A recent investigation of flow through bedrock canyons of the Fraser River revealed that plunging flows, defined by the downward‐directed movement of near surface flow toward the channel bed, occur in channels that have low width‐to‐depth ratios. Plunging flows occur into deep scour pools, which are often coincident with lateral constrictions and channel spanning submerged ridges (sills). A phenomenological investigation was undertaken to reproduce the flow fields observed in the Fraser canyons and to explore morphological controls on the occurrence and relative strength of plunging flow in bedrock canyons. Our observations show that the plunging flow structure can be produced along a scour pool entrance slope by accelerating the flow at the canyon entrance either over submerged sills or through lateral constrictions. Plunging flow appears to be a function of convective deceleration into a scour pool which can be enhanced by sill height, the amount of the channel width that is constricted, pool entrance slope, discharge, and a reduction in channel width‐to‐depth ratio. Plunging flow greatly enhances the potential for incision to occur along the channel bed and is an extreme departure from the assumptions of steady, uniform flow in bedrock incision models, highlighting the need for improved formulations that account for fluid flow. Copyright © 2017 John Wiley & Sons, Ltd.     

Geomorphological evolution and sediment stratigraphy of numerically simulated alluvial fans

When a sediment laden river reaches a flat basin area the coarse fraction of their sediment load is deposited in a cone shaped structure called an alluvial fan. In this article we used the State Space Soil Production and Assessment Model (SSSPAM) coupled landform–soilscape evolution model to simulate the development of alluvial fans in two- and three-dimensional landforms. In SSSPAM the physical processes of erosion and armouring, soil weathering and sediment deposition were modelled using state-space matrices, in both two and three dimensions. The results of the two-dimensional fan showed that the fan grew vertically and laterally keeping a concave up long profile. It also showed a downstream fining of the sediments along the fan profile. Both of these observations are in agreement with available literature concerning natural and experimental fan formations. Simulations with the three-dimensional landform produced a fan with a semicircular shape with concave up long profiles and concave down cross profiles which is typical for fans found in nature and ones developed in laboratory conditions. During the simulation the main channel which brings sediment to the fan structure changed its position constantly leading to the semicircular shape of the fan. This behaviour is similar to the autogenic process of ‘fanhead trenching’ which is the major mechanism of sediment redistribution while the fan is developing. The three-dimensional fan simulation also exhibited the downstream fining of sediments from the fan apex to the peripheries. Further, the simulated fan also developed complex internal sediment stratification which is modelled by SSSPAM. Currently such complex sediment stratification is thought to be a result of allogenic processes. However, this simulation shows that, such complex internal sediment structures can develop through autogenic processes as well. © 2020 John Wiley & Sons, Ltd.     

The long-term stability of engineered landforms of the Ranger Uranium Mine,Northern Territory,Australia: application of a catchment evolution model

There is a need to assess the long-term stability of engineered landforms associated with the rehabilitation of Ranger Uranium Mine, Northern Territory, Australia, as it is a requirement that mill tailings must be contained for periods in excess of 1000 years. The geomorphic model, SIBERIA, is calibrated on hydrologic and erosion data collected by a combination of monitoring and rainfall simulation experiments on the waste rock dumps of Ranger. Preliminary analysis of Ranger's preferred above-grade option suggests that erosion of the order of 7 to 8 m will occur on the structure in a period of 1000 years. This depth of erosion may be sufficient to compromise the integrity of containment. It is shown that SIBERIA has significant advantages over steady-state erosion models. Suggestions are made for the design that will enhance the stability of the structure and extend the structural life of the containment. © 1998 John Wiley & Sons, Ltd.     

Recent climate forcing and physical oceanographic changes in Northern Hemisphere regions: A review and comparison of four marine ecosystems

As part of a project comparing the structure and function of four marine ecosystems off Norway and the United States, this paper examines the oceanographic responses to climate forcing, with emphasis on recent changes. The four Northern Hemisphere ecosystems include two in the Pacific Ocean (Bering Sea and Gulf of Alaska) and two in the Atlantic Ocean (Georges Bank/Gulf of Maine and the Barents/Norwegian Seas). Air temperatures, wind forcing and heat fluxes over the four regions are compared as well as ocean hydrography and sea-ice conditions where seasonal sea ice is found. The long-term interannual variability in air temperatures, winds and net heat fluxes show strong similarity between adjacent ecosystems and within subregions of an ecosystem, but no significant correlations between Pacific and Atlantic ecosystems and few across the Atlantic. In spite of the lack of correlation between climate forcing and ocean conditions between most of the ecosystems, recent years have seen record or near record highs in air and sea temperatures in all ecosystems. The apparent causes of the warming differ. In the Atlantic, they appear to be due to advection, while in the Pacific temperatures are more closely linked to air-sea heat exchanges. Advection is also responsible for the observed changes in salinity in the Atlantic ecosystems (generally increasing salinity in the Barents and Norwegian Seas and decreasing in the Gulf of Maine and Georges Bank) while salinity changes in the Gulf of Alaska are largely related to increased local runoff.