Ground water flow analysis of a mid-Atlantic outer coastal plain watershed, Virginia, U.S.A

Models for ground water flow (MODFLOW) and particle tracking (MODPATH) were used to determine ground water flow patterns, principal ground water discharge and recharge zones, and estimates of ground water travel times in an unconfined ground water system of an outer coastal plain watershed on the Delmarva Peninsula, Virginia. By coupling recharge and discharge zones within the watershed, flowpath analysis can provide a method to locate and implement specific management strategies within a watershed to reduce ground water nitrogen loading to surface water. A monitoring well network was installed in Eyreville Creek watershed, a first-order creek, to determine hydraulic conductivities and spatial and temporal variations in hydraulic heads for use in model calibration. Ground water flow patterns indicated the convergence of flow along the four surface water features of the watershed; primary discharge areas were in the nontidal portions of the watershed. Ground water recharge zones corresponded to the surface water features with minimal development of a regional ground water system. Predicted ground water velocities varied between < 0.01 to 0.24 m/day, with elevated values associated with discharge areas and areas of convergence along surface water features. Some ground water residence times exceeded 100 years, although average residence times ranged between 16 and 21 years; approximately 95% of the ground water resource would reflect land use activities within the last 50 years.     

An overview of toxicant identification in sediments and dredged materials

The identification of toxicants affecting aquatic benthic systems is critical to sound assessment and management of our nation's waterways. Identification of toxicants can be useful in designing effective sediment remediation plans and reasonable options for sediment disposal. Knowledge of which contaminants affect benthic systems allows managers to link pollution to specific dischargers and prevent further release of toxicant(s). In addition, identification of major causes of toxicity in sediments may guide programs such as those developing environmental sediment guidelines and registering pesticides, while knowledge of the causes of toxicity which drive ecological changes such as shifts in benthic community structure would be useful in performing ecological risk assessments. To this end, the US Environmental Protection Agency has developed tools (toxicity identification and evaluation (TIE) methods) that allow investigators to characterize and identify chemicals causing acute toxicity in sediments and dredged materials. To date, most sediment TIEs have been performed on interstitial waters. Preliminary evidence from the use of interstitial water TIEs reveals certain patterns in causes of sediment toxicity. First, among all sediments tested, there is no one predominant cause of toxicity; metals, organics, and ammonia play approximately equal roles in causing toxicity. Second, within a single sediment there are multiple causes of toxicity detected; not just one chemical class is active. Third, the role of ammonia is very prominent in these interstitial waters. Finally, if sediments are divided into marine or freshwater, TIEs perforMed on interstitial waters from freshwater sediments indicate a variety of toxicants in fairly equal proportions, while TIEs performed on interstitial waters from marine sediments have identified only ammonia and organics as toxicants, with metals playing a minor role. Preliminary evidence from whole sediment TIEs indicates that organic compounds play a major role in the toxicity of marine sediments, with almost no evidence for either metal or ammonia toxicity. However, interpretation of these results may be skewed because only a small number of interstitial water (n = 13) and whole sediment (n = 5) TIEs have been completed. These trends may change as more data are collected.     

Studies of ultracompact H ii regions — II. High-resolution radio continuum and methanol maser survey

High spatial resolution radio continuum and 6.67-GHz methanol spectral line data are presented for methanol masers previously detected by Walsh et al. (1997). Methanol maser and/or radio continuum emission is found in 364 cases towards IRAS -selected regions. For those sources with methanol maser emission, relative positions have been obtained to an accuracy of typically 0.05 arcsec, with absolute positions accurate to around 1 arcsec. Maps of selected sources are provided. The intensity of the maser emission does not seem to depend on the presence of a continuum source. The coincidence of water and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. However, there is a striking difference between the general proximity of methanol and water masers to both cometary and irregularly shaped ultracompact (UC) H  ii regions, indicating that, in other cases, there must be differing environments conducive to stimulating their emission. We show that the methanol maser is most likely present before an observable UC H  ii region is formed around a massive star and is quickly destroyed as the UC H  ii region evolves. There are 36 out of 97 maser sites that are linearly extended. The hypothesis that the maser emission is found in a circumstellar disc is not inconsistent with these 36 maser sites, but is unlikely. It cannot, however, account for all other maser sites. An alternative model which uses shocks to create the masing spots can more readily reproduce the maser spot distributions.     

Grain-size trends, basin subsidence and sediment supply in the Campanian Castlegate Sandstone and equivalent conglomerates of central Utah

Reconstructions of grain-size trends in alluvial deposits can be used to understand the dominant controls on stratal architecture in a foreland basin. Different initial values of sediment supply, tectonic subsidence and base-level rise are investigated to constrain their influence on stratal geometry using the observed grain-size trends as a proxy of the goodness of fit of the numerical results to the observed data. Detailed measurements of grain-size trends, palaeocurrent indicators, facies and thickness trends, channel geometries and palynological analyses were compiled for the middle Campanian Castlegate Sandstone of the Book Cliffs and its conglomerate units in the Gunnison and Wasatch plateaus of central Utah. They define the initial conditions for a numerical study of the interactions between large-scale foreland basin and small-scale sediment transport processes. From previous studies, the proximal foreland deposits are interpreted as recording a middle Campanian thrusting event along the Sevier orogenic belt, while the stratal architecture in the Book Cliffs region is interpreted to be controlled by eustatic fluctuation with local tectonic influence. Model results of stratal geometry, using a subsidence curve with a maximum rate of ≈45 m Myr^?1 for the northern Wasatch Plateau region predict the observed grain-size trends through the northern Book Cliffs. A subsidence curve with a maximum rate of ≈30 m Myr^?1 in the Gunnison–Wasatch Plateaus best reproduces the observed grain-size trends in the southern transect through the southern Wasatch Plateau. Eustasy is commonly cited as controlling Castlegate deposition east of the Book Cliffs region. A eustatic rise of 45 m Myr^?1 produces grain-size patterns that are similar to the observed, but a rate of eustatic rise based on Haq et al. (1988) will not produce the observed stratal architecture or grain-size trends. Tectonic subsidence alone, or a combined rate of tectonic subsidence and a Haq et al. (1988) eustatic rise, can explain the stratal and grain-size variations in the proximal and downstream regions.     

Polar constituents isolated from Green River oil shale

This paper interprets the mass spectrometric, i.r. absorption and NMR data for 22 compounds obtained from a polar fraction of Green River shale. The major constituents analyzed are believed to be of the following compositional types: C_nH_2nO (cyclohexanols and chain isoprenoid ketones), C_nH_2n?10O (tetralones and indanones), C_nH_2n?7N (tetrahydroquinolines), C_nH_2n?11N (quinolines), C_nH_2n?1NO (alkoxypyrrolines), C_nH_2n?5NO₂ (maleimides), C_nH_2n?8 (tetralins), C_nH_2n?12 (naphthalenes) and C_nH_2n?14 (benzylbenzenes). This work expands the present information about nitrogen, oxygen and aromatic constituents indigenous to Green River shale.     

The socioeconomic impacts of nuclear generating stations: An analysis of the Rancho Seco and peach bottom facilities

This paper presents the results of a comparative analysis of the socioeconomic impacts of the construction and operation of two nuclear generating stations. Although the construction of nuclear power plants is typically a multiyear process utilizing large numbers of workers and requiring large expenditures for equipment and materials, the socioeconomic changes in the areas in which the plants are located were small and temporary. The extent and size of the changes were found to be related to the size of the work force residing in the local area, the magnitude of local utility and contractor purchases, the amount of tax payments resulting from the plant, and the level of involvement of area groups over plant-related issues.     

Isotopic evidence on the origin of the Shamrocke copper mine,Rhodesia

The Shamrocke ore body is a stratiform deposit of disseminated copper-iron sulphides found within lenses of calcareous meta-arkose occurring in Lomagundi Group graphitic schist (Late Precambrian) at Karoi, Rhodesia. Both sulphides and sediments were subjected to high grades of regional metamorphism. Argon/argon isochron ages indicate a major metamorphic event at 550 m. y. (Damaran orogeny) with later minor argon losses. Ore sulphide ³⁴S values range from +3.0 to +14.8 CDT and a general decrease from footwall to hanging-wall reflects an original sedimentary environment where sulphides formed about the sea bed from hydrothermal fluids progressively mixing with sea water. Isotopically lighter sulphides formed syngenetically in the host rocks from bacterial reduction of sulphate. The pyrrhotite was probably formed from pyrite during metamorphism, and owing to reducing conditions maintained similar ³⁴S values to the original pyrite. Oxygen and carbon isotopic analyses of mineralised and unmineralised carbonate lenses are consistent with deposition as marine limestones in an evaporitic environment and/or near hot spring vents.I. N. S. Contribution No 734