Geomorphologic and Palaeoenvironmental Development of Holocene Perched Coastal Dune Systems in Brittany, France

Two perched dune systems are investigated in the Baie d'Audierne (Finistère, Brittany, northwest France). A maximum age of 4545–4160 cal. yr bp for the onset of sand accretion is provided through radiocarbon (¹⁴C) analysis of an organic-rich basal palaeosol, corresponding closely to deceleration, but before stabilisation, of regional Holocene relative sea levels (RSLs). Molluscan analysis through the dune sand indicates an initial bare sand or sparsely vegetated surface, subsequently maturing and colonised by scrub vegetation, and finally returning to more open dune grassland conditions. These data suggest a three-stage Perched Dune Development Model (PDDM), which may be applicable to perched dunes on at least a regional scale: (1) initial sand sheet inundation as RSL rises; (2) stabilisation of RSL allowing dune to mature; and (3) sea-level stabilisation stimulates cliff formation, perching and progressive landward retreat of the dune shore. This study recognises that perched dunes hold greater potential for establishing chronologies for regional Holocene dune development than the more widely studied lowland dune systems, where evidence for early sand inundation may be obscured or lost by modest rises in sea level.     

Discrete fracture model for coupled flow and geomechanics

We present a fully implicit formulation of coupled flow and geomechanics for fractured three-dimensional subsurface formations. The Reservoir Characterization Model (RCM) consists of a computational grid, in which the fractures are represented explicitly. The Discrete Fracture Model (DFM) has been widely used to model the flow and transport in natural geological porous formations. Here, we extend the DFM approach to model deformation. The flow equations are discretized using a finite-volume method, and the poroelasticity equations are discretized using a Galerkin finite-element approximation. The two discretizations—flow and mechanics—share the same three-dimensional unstructured grid. The mechanical behavior of the fractures is modeled as a contact problem between two computational planes. The set of fully coupled nonlinear equations is solved implicitly. The implementation is validated for two problems with analytical solutions. The methodology is then applied to a shale-gas production scenario where a synthetic reservoir with 100 natural fractures is produced using a hydraulically fractured horizontal well.     

Uncertainties associated with Ra and Ra measurements in water via a Delayed Coincidence Counter (RaDeCC)

The short-lived radium isotopes, ²²³Ra (T_1/2 = 11.4 days) and ²²⁴Ra (T_1/2 = 3.66 days), have been successfully used as tracers of several environmental processes, e.g., submarine groundwater discharge, coastal mixing processes, and water residence times. In this paper, the uncertainties associated with ²²³Ra and ²²⁴Ra measurements using a Radium Delayed Coincidence Counter are determined on a detailed error propagation basis with a confidence interval of 1σ. From the data analyses of several groups of coastal water samples, the calculated relative uncertainties averaged 12% for the ²²³Ra and 7% for the ²²⁴Ra. These uncertainties can decrease for radium-enriched groundwater samples although asymptotic limits have been found at 7% relative uncertainty for ²²³Ra and 4% for ²²⁴Ra. In this paper, the influence of sampling and measurement parameters on the final radium uncertainties is evaluated in order to provide guidance to optimize these factors and obtain more reliable results.     

Determination of arsenic(III) for the investigation of the microbial oxidation of arsenic(III) to arsenic(V)

Recent evaluations of acute and chronical toxicity of arsenic resulted in a reduction of the standard value for total arsenic from 40 μg/L to 10 μg/L in drinking water which will be valid in Germany after a transition period as from January 1996. Arsenic is well known as substance of deep groundwaters, mainly of geogenic origin and normally found as As(III) or As(V). As(V) is well removable by flocculation and filtration after adding iron salts. As(III), however, has to be oxidized first to As(V). Therefore, it is important for treatment techniques to be able to distinguish between As(III) and As(V). A modified determination of As(III) using flow injection analysis was installed and optimized in order to investigate whether As(III) may be oxidized to As(V) by bacteria in natural waters. The results showed that at 4°C, no As(III)-oxidation was observed within 14 days. At room temperature, however, in the bacteria-containing samples, an As(III)-oxidation was found starting after 3 to 7 days. After 14 days, no As(III) was left over. In contrast, in the sterile samples, no As(III)-oxidation could be observed within 14 days. These results demonstrated that microbial processes influence the oxidation of As(III) to As(V) in natural waters.     

Flood frequency analysis of southern Africa: II. Identification of regional distributions

Abstract

Regional frequency analysis of annual maximum flood data comprising 407 stations from 11 countries of southern Africa is presented. Forty-one homogeneous regions are identified. The L-moments of the observed data indicate that the possible underlying frequency distributions are Pearson type 3 (P3), lognormal 3-parameter (LN3), General Pareto (GPA) or General Extreme Value (GEV). Simulation experiments for the selection of the most suitable flood frequency procedure indicate that Pearson type 3/Probability Weighted Moments (P3/PWM) and log-Pearson type 3/Method of Moments (LP3/MOM) are suitable procedures for the region.     

Monitoring wetland ditch water levels in the North Kent Marshes,UK, using Landsat TM imagery and ground-based measurements

Abstract

An understanding of hydrology is a prerequisite for ensuring the successful management, conservation and restoration of wetland environments. Frequently, however, little is known about historical hydrological conditions, such as water levels, within wetlands. Moreover, many channel and ditch systems in wetlands are not routinely monitored, except perhaps for research purposes. A methodology is presented herein which makes use of satellite imagery to indirectly provide remotely sensed observations of water levels within channels and ditches. Using multi-temporal Landsat Thematic Mapper (TM) imagery and simultaneous ground-based measurements of water levels, statistical relationships are established between satellite-derived effective wet ditch widths and measured water levels in the drainage system of the Elmley Marshes, southeast England. These relationships can be used subsequently to estimate historical ditch water levels and to monitor contemporary ditch water levels in the wetland. The study shows that satellite imagery has much to offer in monitoring changes in the hydrological regime of wetlands and in providing complimentary approaches to field monitoring.     

Geostatistics as a groundwater exploration planning tool: case of a brackish-saline aquifer

This paper discusses a geostatistical approach to model a groundwater aquifer in 3-D. The study aims at utilizing geostatistics as a tool for characterizing zones of better-water quality in a brackish-saline aquifer. In particular, the geostatistical model was constructed to characterize the aquifer’s salinity, represented by total dissolved solids (TDS), using logs of porosity and resistivity. Quality-checked estimated TDS vertical profiles were employed to construct and model horizontal and vertical semivariograms. Parameters of semivariogram models were used to develop both the kriging plan and the generated model. Results of this modeling process are shown in the form of horizontal salinity distribution maps. The aquifer was sliced into 20 layers, each 20 m thick, to represent its overall thickness. Salinity layers maps reflect vertical stratification of TDS concentrations in the aquifer and show that water quality deteriorates with depth and toward the northern part of the aquifer. Relatively better-quality water (TDS ≤10,000 mg/l) can be found at depths between 100 and 250 m below the aquifer’s top in both eastern and southeastern parts. Water in the same interval to the western and southwestern parts reflects the presence of higher TDS concentration. From a planning point of view, it is more feasible to target the eastern part of the aquifer for pumping and desalination purposes. In addition, the generated model could be utilized as an initial condition for flow simulation.     

An Improved Perturbation Mechanism for Simulated Annealing Simulation

Simulated annealing (SA) is being increasingly used for the generation of stochastic models of spatial phenomena because of its flexibility to integrate data of diverse types and scales. The major shortcoming of SA is the extensive CPU requirements. We present a perturbation mechanism that significantly improves the CPU speed. Two conventional perturbation mechanisms are to (1) randomly select two locations and swap their attribute values, or (2) visit a randomly selected location and draw a new value from the global histogram. The proposed perturbation mechanism is a modification of option 2: each candidate value is drawn from a local conditional distribution built with a template of kriging weights rather than from the global distribution. This results in accepting more perturbations and in perturbations that improve the variogram reproduction for short scale lags. We document the new method, the increased convergence speed, and the improved variogram reproduction. Implementation details of the method such as the size of the local neighborhood are considered.