Water Table Uncertainties due to Uncertainties in Structure and Properties of an Unconfined Aquifer

We consider two sources of geology‐related uncertainty in making predictions of the steady‐state water table elevation for an unconfined aquifer. That is the uncertainty in the depth to base of the aquifer and in the hydraulic conductivity distribution within the aquifer. Stochastic approaches to hydrological modeling commonly use geostatistical techniques to account for hydraulic conductivity uncertainty within the aquifer. In the absence of well data allowing derivation of a relationship between geophysical and hydrological parameters, the use of geophysical data is often limited to constraining the structural boundaries. If we recover the base of an unconfined aquifer from an analysis of geophysical data, then the associated uncertainties are a consequence of the geophysical inversion process. In this study, we illustrate this by quantifying water table uncertainties for the unconfined aquifer formed by the paleochannel network around the Kintyre Uranium deposit in Western Australia. The focus of the Bayesian parametric bootstrap approach employed for the inversion of the available airborne electromagnetic data is the recovery of the base of the paleochannel network and the associated uncertainties. This allows us to then quantify the associated influences on the water table in a conceptualized groundwater usage scenario and compare the resulting uncertainties with uncertainties due to an uncertain hydraulic conductivity distribution within the aquifer. Our modeling shows that neither uncertainties in the depth to the base of the aquifer nor hydraulic conductivity uncertainties alone can capture the patterns of uncertainty in the water table that emerge when the two are combined.     

A new record of Colpocoryphe (Trilobita) from the Skiddaw Group of the Cross Fell inlier,northern England

The trilobite Colpocoryphe sp. is described for the first time from the Kirkland Formation, Llanvirn, of the Skiddaw Group in the Cross Fell inlier. It is considered to be closely related to a form previously recorded from the upper Arenig of the Lake District. This new record supports the previously suggested Gondwanan aspects of the northern England Avalonian fauna during the early Ordovician. Copyright © 2000 John Wiley & Sons, Ltd.     

Creating a Population of 12‐Digit Headwater Basins within the Albemarle‐Pamlico Estuary System

Ecological research within the US Environmental Protection Agency's Office of Research and Development has recently changed its focus to quantifying and mapping ecosystem services provided to humans. Our local research group has been charged to develop a regional assessment of several ecosystem services in the Albemarle‐Pamlico Estuary System (APES). Time, data, and funding constraints precluded explicit modeling of the entire APES, so in Phase 1 of our research plan we chose to model ecosystem services in a random sample of headwater catchments. After observing numerous inconsistencies between the National Hydrography Dataset‐Plus (NHDPlus) stream network and the Virginia/North Carolina Watershed Boundary Dataset 12‐digit HUC coverage, we began by creating modified 12‐digit hydrologic units (HUCs) by aggregating smaller catchments delineated within the NHDPlus. In defining our population of interest (headwater 12‐digit HUCs with perennial, natural, wadeable pour points), we generally excluded HUCs that had multiple pour points, no pour points, or whose pour points were intermittent streams, artificial segments, ditches/canals, or lentic systems (lakes and reservoirs). After taking these actions, 318 HUCs remained and a stratified random sample (Omnerik ecoregions as strata) of 50 HUCs was chosen from this population.     

Guidelines on the use of structure-from-motion photogrammetry in geomorphic research

As a topographic modelling technique, structure-from-motion (SfM) photogrammetry combines the utility of digital photogrammetry with a flexibility and ease of use derived from multi-view computer vision methods. In conjunction with the rapidly increasing availability of imagery, particularly from unmanned aerial vehicles, SfM photogrammetry represents a powerful tool for geomorphological research. However, to fully realize this potential, its application must be carefully underpinned by photogrammetric considerations, surveys should be reported in sufficient detail to be repeatable (if practical) and results appropriately assessed to understand fully the potential errors involved. To deliver these goals, robust survey and reporting must be supported through (i) using appropriate survey design, (ii) applying suitable statistics to identify systematic error (bias) and to estimate precision within results, and (iii) propagating uncertainty estimates into the final data products. © 2019 John Wiley & Sons, Ltd.     

Joint parameter estimation from magnetic resonance and vertical electric soundings using a multi‐objective genetic algorithm

Magnetic resonance sounding (MRS) has increasingly become an important method in hydrogeophysics because it allows for estimations of essential hydraulic properties such as porosity and hydraulic conductivity. A resistivity model is required for magnetic resonance sounding modelling and inversion. Therefore, joint interpretation or inversion is favourable to reduce the ambiguities that arise in separate magnetic resonance sounding and vertical electrical sounding (VES) inversions. A new method is suggested for the joint inversion of magnetic resonance sounding and vertical electrical sounding data. A one‐dimensional blocky model with varying layer thicknesses is used for the subsurface discretization. Instead of conventional derivative‐based inversion schemes that are strongly dependent on initial models, a global multi‐objective optimization scheme (a genetic algorithm [GA] in this case) is preferred to examine a set of possible solutions in a predefined search space. Multi‐objective joint optimization avoids the domination of one objective over the other without applying a weighting scheme. The outcome is a group of non‐dominated optimal solutions referred to as the Pareto‐optimal set. Tests conducted using synthetic data show that the multi‐objective joint optimization approximates the joint model parameters within the experimental error level and illustrates the range of trade‐off solutions, which is useful for understanding the consistency and conflicts between two models and objectives. Overall, the Levenberg‐Marquardt inversion of field data measured during a survey on a North Sea island presents similar solutions. However, the multi‐objective genetic algorithm method presents an efficient method for exploring the search space by producing a set of non‐dominated solutions. Borehole data were used to provide a verification of the inversion outcomes and indicate that the suggested genetic algorithm method is complementary for derivative‐based inversions.     

Simulation of scree‐slope dynamics: investigating the distribution of debris avalanche events in an idealized two‐dimensional model

We present a two‐dimensional model of the development of scree slopes using the discrete‐element method. We concentrate on the dynamics of the accumulating debris at the cliff foot rather than on the failure modes of the cliff‐face or shape of the underlying rock surface. The evolution of this unconsolidated material is intermittent and systematically changing over time, with an early high disturbance regime, dominated by a characteristic event size (where 65% of particles in the debris are in motion to some extent), replaced at later times by many shallow slides interspersed with infrequent large events that involve motion through almost the full scree depth. These large slides lead to a stratigraphy in which the layers of material are stretched almost horizontal near the base of the slope. The scree surface thus shows a gradient in age with most recent rock‐fall close to the cliff and the oldest rock‐fall debris outcropping at the foot. The final surface slope tends to show little curvature, and the final mean slope is well correlated with the angle of internal friction of the particles, although the change is very small over a wide range of friction angles [final slope (in degrees relative to horizontal) ~ 0.043 × internal friction angle + 17.49, with a correlation coefficient of 0.89, p‐value 0.0001]. Some weak size‐segregation of the debris is found, but this seems to have little to do with individual particles bounding down the slope. The shape of the rock core agrees largely with the analytic forms given by Fisher–Lehmann and Bakker–Le Heux expressions, but the original simple Fisher quadratic can give the best fit. Overall the evolution shows a remarkable insensitivity to the model parameters, suggesting that the controls on dry scree‐slope evolution are primarily geometric in character. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantifying the rate and depth dependence of bioturbation based on optically‐stimulated luminescence (OSL) dates and meteoric 10Be

Both the rate and the vertical distribution of soil disturbance modify soil properties such as porosity, particle size, chemical composition and age structure; all of which play an important role in a soil's biogeochemical functioning. Whereas rates of mixing have been previously quantified, the nature of bioturbation's depth dependence remains poorly constrained. Here we constrain, for the first time, the relationship between mixing rate and depth in a bioturbated soil in northeast Queensland, Australia using a novel method combining OSL (optically‐stimulated luminescence) ages and meteoric beryllium‐10 (¹⁰Be) inventories. We find that the best fit mixing rate decreases non‐linearly with increasing soil depth in this soil and the characteristic length scale of 0.28 m over which the mixing coefficient decays is comparable to reported rooting depth coefficients. In addition we show that estimates of surface mixing rates from OSL data are highly dependent on erosion rate and that erosion rate must be constrained if accurate mixing rates are to be quantified. We calculate surface diffusion‐like mixing coefficients of 1.8 × 10^?4 and 2.1 × 10^?4 m² yr^?1 for the studied soil for two different estimates of soil erosion. © 2014 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

Offshore experiments on styrene spillage in marine waters for risk assessment

Within the context of risk evaluation of chemical spillages into the marine environment, this paper reports on an offshore experiment to study the behaviour of styrene spilled into sea under natural conditions and discusses theoretical approaches. Floating structures were used to enclose the spillage and the gaseous cloud formation, and dissolution processes were in situ monitored. The identification of spill risks for man and marine environment through GESAMP's hazard profile is described for styrene: Styrene is rated as a chemical with a significant health hazard that will float but also evaporate. However, monitoring of the water column in the experiments showed that the concentration of styrene in water during the first hour represents 50% of the product spilled. For the potentially exposed public, the GESAMP hazard rating recommends the closure of beaches and evacuation. The risk assessment developed from experimental data confirms this safety advice.