Natural acid rock drainage associated with black shale in the Yukon Territory,Canada

Investigated herein are water and sediment geochemistry, and metal attenuation processes associated with natural acid rock drainage originating from black shale formations in the Macmillan Pass area, Clear Lake prospect and Engineer Creek by the Dempster Highway in the Yukon Territory, Canada. The most metalliferous water having pH 3.0, 150 mg/L Zn, 39 mg/L Ni, 2.8 mg/L Cu and 9.1 mg/L As was found in a tributary stream of Engineer Creek with no known mineral deposits occurring in the vicinity. For all three study areas, the water and sediment geochemistry is significantly affected by the local lithology and prevailing metal attenuation processes. Despite their anomalous acidity and metal contents, the natural acid streams contribute only a small fraction of the contaminant loadings to the major water courses because of their low flows. Dilution, neutralization, sorption and co-precipitation are identified as the major mechanisms attenuating aqueous transport of potentially deleterious metals. However, microbial mediation in metal attenuation is also evident in low-flow systems. The wide variation of water and sediment geochemistry along a flow path renders the establishment of background metal values difficult. In assessing environmental impacts, it may be more practical to consider metal loadings on a watershed scale than to rely on a comparison with operationally defined background concentrations.     

Silurian of the Midland Valley of Scotland and Ireland

The Silurian rocks and fossils of the Midland Valley of Scotland, and its extension across the northern parts of Ireland, have fascinated and challenged geologists for over 150 years. The spectacular diversity of facies and faunas, displayed in a series of inliers, is matched by contrasts in scenery from the rugged Atlantic coast of western Ireland to the undulating hills and heather moors on the northern margin of the Southern Uplands. Here, tectonic events near the climax of the Caledonian orogenic cycle served to fashion a mosaic of dynamic sedimentary environments on the Laurentian margin, at an exciting time in Earth history, as life established its foothold on the land.     

Relative permeability of CBM reservoirs: Controls on curve shape

Relative permeability to gas and water for 2-phase flow coalbed methane (CBM) reservoirs has long been known to exhibit a strong control on (gas and water) production profile characteristics. Despite its important control on both primary and enhanced recovery of CBM for coal seams that have not been fully dewatered, relative permeability in coal has received little attention in the literature in the past decade. There are few published laboratory-derived curves; these studies and their resulting data represent a small subset of the commercial CBM reservoirs and do not allow for a systematic investigation of the physical controls on relative permeability curve shape. Other methods for estimation of relative permeability curves include derivation from simulation history-matching, and production data analysis. Both of these methods will yield pseudo-relative permeability curves whose shapes could be affected by several dynamic CBM reservoir and operating characteristics.The purpose of the current work is to perform a systematic investigation of the controls on CBM relative permeability curve shape, including non-static fracture permeability and porosity, multi-layer effects and transient flow. To derive the relative permeability curves, effective permeability to gas and water are obtained from flow equations, flow rates and pressure data. Simulated cases are analyzed so that derived and input curves may be compared allowing for investigation of CBM reservoir properties on curve shape. One set of relative permeability curves that were input into the simulator were obtained from pore-scale modeling. Field cases from two basins are also examined and controls on derived relative permeability curve shape inferred. The results of this work should be useful for future CBM development and greenhouse gas sequestration studies, and it is hoped that it will spark additional research of this critical CBM flow property.     

Development of ground-testable phase fresnel lenses in silicon

Diffractive optics, such as Phase Fresnel Lenses (PFL's), offer the potential to achieve excellent imaging performance in the x-ray and gamma-ray photon regimes. In principle, the angular resolution obtained with these devices can be diffraction limited. Furthermore, improvements in signal sensitivity can be achieved as virtually the entire flux incident on a lens can be concentrated onto a small detector area. In order to verify experimentally the imaging performance, we have fabricated PFL's in silicon using gray-scale lithography to produce the required Fresnel profile. These devices are to be evaluated in the recently constructed 600-meter x-ray interferometry testbed at NASA/GSFC. Profile measurements of the Fresnel structures in fabricated PFL's have been performed and have been used to obtain initial characterization of the expected PFL imaging efficiencies.     

Dark Matter and Chemical Evolution of dSph Galaxies

We calculate chemical evolution models for 4 dwarf spheroidal satellites of the Milky Way (Carina, Ursa Minor, Leo I and Leo II) for which reliable non-parametric star formation histories have been derived. We find that galaxies showing one single burst of star formation (Ursa Minor and Leo II) require a dark halo slightly larger that the current estimates for their tidal radii for the gas heated by supernovae to be retained until the observed stellar population has formed. Systems showing extended star formation histories however (Carina and Leo I), are consistent with the idea that their tidally limited dark haloes provide the necessary gravitational potential wells to retain their gas. This revised version was published online in September 2006 with corrections to the Cover Date.     

Monitoring and Mathematical Modeling of Contaminated Ground-Water Plumes in Fluvial Environments

Ground-water monitoring to delineate a contaminant plume in fluvial hydrostratigraphic units often is uncertain. Fluvial deposits consist typically of interbedded layers of sands, silts and clays, with buried stream channel deposits of sands or gravels. The channel deposits are often interpreted erroneously to be discontinuous between test holes and in cross section due to their sinuosity. Erroneous conclusions pertaining to the areal continuity of these geometrically complex deposits are inevitable unless the investigator thoroughly understands the depositional environment(s). The hydraulic conductivity of buried stream channel deposits may be several orders of magnitude higher than the matrix materials in which they are enclosed. The higher hydraulic conductivity of buried stream channel deposits has potentially significant ramifications with respect to ground-water monitoring to delineate the geometry of a contaminant plume migrating through these deposits. Ground-water monitoring at uranium mill waste disposal sites located in fluvial environments began on a significant scale in about 1977. A uranium mill tailing disposal site located in such an environment in central Wyoming is among the first sites monitored. Thirty-seven monitor wells were constructed at the site to delineate a seepage plume originating from one of the tailing ponds. This case history illustrates the need for a detailed under—standing of the hydrostratigraphy at a waste disposal site in order to interpret the meaning of ground-water quality data effectively. Water quality data from monitor wells located on a hit or miss basis often are misleading. The hydrostratigraphic horizon from which a water quality sample is collected must be well defined before the sample analyses can be interpreted quantitatively.     

Assessment of recharge to groundwater systems in the arid southwestern part of Northern Territory, Australia, using chlorine-36

 The sustainability of community water supplies drawn from shallow aquifers in the arid southwest of the Northern Territory has been evaluated using the radioactive isotope chlorine-36 (³⁶Cl). These aquifers include fractured sandstones of the Ngalia Basin, fractured metamorphic rocks and Cainozoic sands and gravels. ³⁶Cl/Cl ratios for these shallow, regional groundwaters exhibit a bimodal distribution with peaks at 205 (±7) and 170 (±7)×10^–15. The higher ratio probably represents modern (Holocene) recharge, diluted with windblown salts from local playa lakes, and occurs mostly around the margin of the basin. The lower ratio corresponds to a ³⁶Cl "age", or mean residence time, of 80–100 ka, implying that the last major recharge occurred during the last interglacial interval (Oxygen Isotope Stage 5). These values are mainly observed in the interior of the Ngalia Basin. Lower values of the ³⁶Cl/Cl ratio measured near playa lakes are affected by addition of chloride from remobilised salts. Finite carbon-14 (¹⁴C) data for the groundwaters are at variance with the ³⁶Cl results, but a depth profile suggests low recharge, allowing diffusion of recent atmospheric carbon to the water table. The ³⁶Cl results have important implications for groundwater management in this region, with substantial recharge only occurring during favourable, wet, interglacial climatic regimes; most community water supplies are dependent on these "old" waters. Received, September 1997 · Revised, August 1998, March 1999 · Accepted, March 1999