Fracture flow and groundwater compartmentalization in the Rollins Sandstone, Lower Mesaverde Group, Colorado, USA

 This paper presents a site-specific conceptual model of groundwater flow in fractured damage zones associated with faulting in a package of sedimentary rocks. The model is based on the results of field and laboratory investigations. Groundwater and methane gas inflows from fault-fracture systems in the West Elk coal mine, Colorado, USA, have occurred with increasing severity. Inflows of 6, 160 and 500 L s⁻¹ discharged almost instantaneously from three separate faults encountered in mine workings about 460 m below ground level. The faults are about 600 m apart. The δ ²H and δ ¹⁸O compositions of the fault-related inflow waters and the hydrodynamic responses of each fault inflow indicate that the groundwaters discharge from hydraulically isolated systems. ¹⁴C data indicate that the groundwaters are as much as 10,500 years old. Discharge temperatures are geothermal (≈30°C), which could indicate upwelling from depth. However, calculations of geothermal gradients, analysis of solute compositions of groundwater in potential host reservoirs, geothermometer calculations, and results of packer testing indicate that the fractured groundwater reservoir is the Rollins Sandstone (120 m thick) directly beneath the coal seams. The packer test also demonstrates that the methane gas is contained in the coal seams. A geothermal gradient of 70–80°C km⁻¹, related to an underlying intrusion, is probably responsible for the slightly elevated discharge temperatures. Large discharge volumes, as great as 8.2×10⁵m³ from the 14 South East Headgate fault (14 SEHG), rapid declines in discharge rates, and vertical and horizontal permeability (matrix permeability generally <0.006 Darcy) indicate fracture flow. An in-mine pumping test demonstrates that the 14 SEHG fault has excellent hydraulic communication with fractures 50 m from the fault. Aeromagnetic data indicate that the faults are tectonically related to an igneous body that is several thousand meters below the coal seams. Exploratory drilling has confirmed a fourth fault, and two additional faults are projected, based on the aeromagnetic data. The conceptual model describes a series of parallel, hydraulically separate groundwater systems associated with fault-specific damage zones. The faults are about 600 m apart. Groundwater stored in fractured sandstone is confined above and below by clayey layers. Received March 1999 / Revised, November 1999 / Accepted, December 1999     

The Taylor Creek Rhyolite of New Mexico: a rapidly emplaced field of lava domes and flows

The Tertiary Taylor Creek Rhyolite of southwest New Mexico comprises at least 20 lava domes and flows. Each of the lavas was erupted from its own vent, and the vents are distributed throughout a 20 km by 50 km area. The volume of the rhyolite and genetically associated pyroclastic deposits is at least 100 km³ (denserock equivalent). The rhyolite contains 15%–35% quartz, sanidine, plagioclase, ±biotite, ±hornblende phenocrysts. Quartz and sanidine account for about 98% of the phenocrysts and are present in roughly equal amounts. With rare exceptions, the groundmass consists of intergrowths of fine-grained silica and alkali feldspar. Whole-rock major-element composition varies little, and the rhyolite is metaluminous to weakly peraluminous; mean SiO₂ content is about 77.5±0.3%. Similarly, major-element compositions of the two feldsparphenocryst species also are nearly constant. However, whole-rock concentrations of some trace-elements vary as much as several hundred percent. Initial radiometric age determinations, all K–Ar and fission track, suggest that the rhyolite lava field grew during a period of at least 2 m.y. Subsequent ⁴⁰Ar/³⁹Ar ages indicate that the period of growth was no more than 100 000 years. The time-space-composition relations thus suggest that the Taylor Creek Rhyolite was erupted from a single magma reservoir whose average width was at least 30 km, comparable in size to several penecontemporaneous nearby calderas. However, this rhyolite apparently is not related to a caldera structure. Possibly, the Taylor Creek Phyolite magma body never became sufficiently volatile rich to produce a large-volume pyroclastic eruption and associated caldera collapse, but instead leaked repeatedly to feed many relatively small domes and flows.The new ⁴⁰Ar/³⁹Ar ages do not resolve preexisting unknown relative-age relations among the domes and flows of the lava field. Nonetheless, the indicated geologically brief period during which Taylor Creek Rhyolite magma was erupted imposes useful constraints for future evaluation of possible models for petrogenesis and the origin of trace-element characteristics of the system.     

Revisiting the Concentration Observations and Source Apportionment of Atmospheric Ammonia

<正>While China’s Air Pollution Prevention and Control Action Plan on particulate matter since 2013 has reduced sulfate significantly, aerosol ammonium nitrate remains high in East China. As the high nitrate abundances are strongly linked with ammonia, reducing ammonia emissions is becoming increasingly important to improve the air quality of China. Although satellite data provide evidence of substantial increases in atmospheric ammonia concentrations over major agricultural regions, long-t...     

Deformation of poorly consolidated sediment during shallow emplacement of a basalt sill,Coso Range,California

A 150-m-long, wedge-shaped unit of folded and faulted marly siltstone crops out between undeformed sedimentary rocks on the north flank of the Coso Range, California. The several-meter-thick blunt end of this wedge abuts the north margin of a basaltic sill of comparable thickness. Chaotically deformed siltstone crops out locally at the margin of this sill, and at one locality breccia pipes about one meter in diameter crosscut the sill. The sill extends about 1 km south up the paleoslope, where it merges through continuous outcrop with a lava flow that in turn extends 1.4 km to a vent area marked by more than 100 m of agglutinate and scoria. Apparently, lava extruded at this vent flowed onto unconsolidated sediments, burrowed into them, and fed a sill at about 40 m depth within the sedimentary sequence. The sill initially propagated by wedging between sedimentary beds, but eventually began to push some beds ahead of itself, forming a remarkable train of folds in the process. The sediments apparently were wet at the time of sill emplacement, because hydrothermal alteration is common near the contact between the two rock types and because the breccia pipes that crosscut the sill apparently resulted from phreatic explosions of pore water heated at the base of the cooling sill. Comparison of deformation of the host material at the Coso locality with that reportedly caused by emplacement of sills elsewhere indicates that the character of deformation differs greatly among the various localities. The specific response of host material depends upon such parameters as initial properties of magma and host material, rate of sill growth and attendant rate of strain of host material, and depth of sill emplacement. Some properties may change considerably during an intrusive-deformational episode, thus complicating accurate reconstruction of such an event.     

Characteristics of summertime daily rainfall variability over South America and the South Atlantic Convergence Zone

Summary ?This paper presents an objective analysis of the structure of daily rainfall variability over the South American/South Atlantic region (15°–60° W and 0°–40° S) during individual austral summer months of November to March. From EOF analysis of satellite derived daily rainfall we find that the leading mode of variability is represented by a highly coherent meridional dipole structure, organised into 2 extensive bands, oriented northwest to southeast across the continent and Atlantic Ocean. We argue that this dipole structure represents variability in the meridional position of the South Atlantic Convergence Zone (SACZ). During early and later summer, in the positive (negative) phase of the dipole, enhanced (suppressed) rainfall over eastern tropical Brazil links with that over the subtropical and extra-tropical Atlantic and is associated with suppressed (enhanced) rainfall over the sub-tropical plains and adjacent Atlantic Ocean. This structure is indicative of interaction between the tropical, subtropical and temperate zones. Composite fields from NCEP reanalysis products (associated with the major positive and negative events) show that in early and late summer the position of the SACZ is associated with variability in: (a) the midlatitude wave structure, (b) the position of the continental low, and (c) the zonal position of the South Atlantic Subtropical High. Harmonic analysis of the 200 hPa geopotential anomaly structure in the midlatitudes indicates that reversals in the rainfall dipole structure are associated primarily with variability in zonal wave 4. There is evidence of a wave train extending throughout the midlatitudes from the western Pacific into the SACZ region. During positive (negative) events the largest anomalous moisture advection occurs within westerlies (easterlies) primarily from Amazonia (the South Atlantic). In both phases a convergent poleward flow results along the leading edge of the low-level trough extending from the tropics into temperate latitudes. High summer events differ from those in early and late summer in that the rainfall dipole is primarily associated with variability in the phase of zonal wave 3, and that tropical-temperate link is not clearly evident in positive events. Received May 31, 2001; revised October 17, 2001; accepted June 13, 2002     

A GIS‐based variable source area hydrology model

Effective control of nonpoint source pollution from contaminants transported by runoff requires information about the source areas of surface runoff. Variable source hydrology is widely recognized by hydrologists, yet few methods exist for identifying the saturated areas that generate most runoff in humid regions. The Soil Moisture Routing model is a daily water balance model that simulates the hydrology for watersheds with shallow sloping soils. The model combines elevation, soil, and land use data within the geographic information system GRASS, and predicts the spatial distribution of soil moisture, evapotranspiration, saturation‐excess overland flow (i.e., surface runoff), and interflow throughout a watershed. The model was applied to a 170 hectare watershed in the Catskills region of New York State and observed stream flow hydrographs and soil moisture measurements were compared to model predictions. Stream flow prediction during non‐winter periods generally agreed with measured flow resulting in an average r² of 0·73, a standard error of 0·01 m³/s, and an average Nash‐Sutcliffe efficiency R² of 0·62. Soil moisture predictions showed trends similar to observations with errors on the order of the standard error of measurements. The model results were most accurate for non‐winter conditions. The model is currently used for making management decisions for reducing non‐point source pollution from manure spread fields in the Catskill watersheds which supply New York City's drinking water. Copyright © 1999 John Wiley & Sons, Ltd.     

Epibiont shadowing: a hitherto unrecognized way of preserving soft-bodied fossils

Outlines of soft-bodied sessile epibionts that grew attached to the surfaces of calcareous shells in ancient seas are sometimes preserved by a process termed ‘Epibiont Shadowing’. Processes that altered the surface of the shell were prevented from happening immediately beneath the attached epibiont, leaving a shadow of its attachment site following its death and decay. Microboring around the perimeter of the epibiont by presumed endolithic cyanobacteria gave rise to Endolithic Shadows, and dissolution of the calcareous substrate, maybe beneath larger smothering organisms, produced Solution Shadows of smaller organisms that protected their sites of attachment from such etching effects. Recognition of this type of preservation allows the stratigraphic range of certain soft-bodied groups to be extended. Details of the shadows may yield information about the morphological construction of the groups in question.     

Molecular band variations as a probe of the vertical structure of a Jovian atmosphere

A variational technique is used to compute synthetic spectra for models of cloudy Jovian planetary atmospheres which incorporate abrupt changes in their vertical structure. The dependence of the center-to-limb variations in equivalent widths of molecular bands upon the properties of the various scattering layers in the model is examined. A range of theoretical models are delineated on the basis of their ability to reproduce observational results for the specific case of Jupiter.     

Identifying connections in a fractured rock aquifer using ADFTs

Fractured rock aquifers are difficult to characterize because of their extremely heterogeneous nature. Developing an understanding of fracture network hydraulic properties in these aquifers is difficult and time consuming, and field testing techniques for determining the location and connectivity of fractures in these aquifers are limited. In the Clare Valley, South Australia, well interference is an important issue for a major viticultural area that uses a fractured aquifer. Five fracture sets exist in the aquifer, all dipping > 25 degrees . In this setting, we evaluate the ability of steady-state asymmetric dipole-flow tests (ADFTs) to determine the connections between a test well and a set of piezometers. The procedure involves dividing a test well into two chambers using a single packer and pumping fluid from the upper chamber to the lower chamber. By conducting a series of tests at different packer elevations, an "input" signal is generated in fracture zones connected to the test well. By monitoring the "output" response of the hydraulic dipole field at piezometers, the connectivity of the fractures between the test well and piezometers can be determined. Results indicate the test well used in this study is connected in a complex three-dimensional geometry, with drawdown occurring above and below areas of potentiometric buildup. The ADFT method demonstrates that the aquifer evaluated in this study cannot be modeled effectively on the well scale using continuum flow models.