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221.
The link between stress and hydrologic properties was examined at two sites that are distinguished by different rock types
and different stress states. This investigation is based upon the analysis and interpretation of geophysical logs obtained
in water wells at the two locations. At the northeast site (Newark Basin), the hydrologic characteristics of sedimentary rocks
are dependent upon the relationship to the current regional stress field of two primary types of orthogonal features that
serve as preferential pathways for fluid flow. Subhorizontal bedding-plane partings are highly transmissive near the surface
and delineate transversely isotropic fluid flow at shallow depths. With increasing depth, the subhorizontal planes become
less dominant and steeply dipping fractures become more influential hydrologically. These high-angle features define anisotropic
flow pathways that are preferentially oriented along strike. At the southwest site (west Texas), extrusive rocks are subjected
to topographically modified tectonic and gravitational stresses that vary spatially within a valley setting. The attendant
changes in stress invariants cause fracture connectivity within the rock mass to systematically increase with depth along
the valley flanks, but to remain relatively low in the central valley. The degree of fracture connectivity predicted within
this valley configuration is consistent with variations in transmissivity determined at several well locations. In each of
these cases, the idealized understanding of the hydrologic system is enhanced by considering the effects of regional and local
stresses that act upon the fractured-rock aquifer.
Electronic Publication 相似文献
222.
Daniel N. Leavell 《Environmental Geology》2006,50(8):1141-1149
Granville Solvents Inc. (GSI) operated for years on the Raccoon Creek flood plain in the central Ohio town of Granville, handling bulk quantities of fuels and solvents. The site is adjacent to Granville’s public water supply. The water wells of the village draw from a substantial buried-valley aquifer, which is vulnerable to surface influences and contaminants. GSI received a 3-year Resource Conservation and Recovery Act (RCRA) Part A permit without any siting evaluation because of its grandfather status, and was able to expand its operations greatly. During the expansion, continued negotiations involving permit applications and modifications were conducted; with continued disregard for site contamination. Although a Part B permit was never granted, GSI operated for 6 years under RCRA. Today the site is a Superfund operation and more than five million dollars have been spent on cleanup and maintenance of contaminated soil and groundwater. After 15 years of cleanup, the active remediation has been suspended for the present; however, management of the site for the protection of the adjacent well field is likely to continue well into the future. 相似文献
223.
A. J. Rabideau L. S. Matott I. Jankovic J. R. Craig M. W. Becker 《Environmental Geology》2005,48(1):57-67
Groundwater modelers have embraced the use of automated calibration tools based on classical nonlinear regression techniques. While clearly an improvement over trial-and-error calibration, it is not clear to what extent these popular inverse modeling tools yield accurate parameter sets for groundwater flow models. The impact of model configuration and precision upon automated parameter estimation is also unclear. An extensive set of numerical experiments was performed to explore the influence of model configuration on the calibration of a regional groundwater flow model developed using the analytic element method. The results provided insight into the manner in which the specified level of model precision and the location of observation points influence the results of inverse modeling based on nonlinear regression. While the importance of these issues is application-specific, obtaining an accurate model calibration for the case study required both a careful placement of test observations and a greater-than-anticipated level of model precision. The required level of model precision for calibration was more than necessary to produce an acceptable flow solution. 相似文献
224.
Reliable estimates for how much water can be safely withdrawn from aquifers without harming the environment is crucial for
identifying new water supply sources and fostering sustainable growth. Methodologies to estimate groundwater availability
that are rooted in science and yet accomplishable with minimal data are particularly useful for effectual aquifer management.
Also, as groundwater management is increasingly becoming a participatory process, these methodologies must be transparent
and easily understood by a wide range of audiences. In addition, proposed approaches must also reconcile imprecision and uncertainties
arising from lack of data, differences in stakeholders’ perceptions and limitations associated with incomplete aquifer characterization.
In this study, the fundamental concept of water balance is coupled with fuzzy regression to develop a scheme for assessing
regional-scale groundwater availability. Using the mass-balance approach, anthropogenic water demands (municipal, industrial
and agricultural) and ecological demands (baseflows to rivers) can be incorporated into the availability estimation process.
The use of fuzzy regression enables the specification of decision makers’ preferences to the adopted procedure and renders
the parameter estimation to be more robust in the presence of extreme values. The methodology is illustrated by using it to
estimate groundwater availability in the Gulf coast aquifer, underlying Refugio County, TX, USA. 相似文献
225.
Lakes are common in glaciated mountain regions and geomorphic principles suggest that lake modifications to water and sediment fluxes should affect downstream channels. Lakes in the Sawtooth Mountains, Idaho, USA, were created during glaciation and we sought to understand how and to what extent glacial morphology and lake disruption of fluxes control stream physical form and functions. First, we described downstream patterns in channel form including analyses of sediment entrainment and hydraulic geometry in one catchment with a lake. To expand on these observations and understand the role of glacial legacy, we collected data from 33 stream reaches throughout the region to compare channel form and functions among catchments with lakes, meadows (filled lakes), and no past or present lakes. Downstream hydraulic geometry relationships were weak for both the single catchment and regionally. Our data show that downstream patterns in sediment size, channel shape, sediment entrainment and channel hydraulic adjustment are explained by locations of sediment sources (hillslopes and tributaries) and sediment sinks (lakes). Stream reaches throughout the region are best differentiated by landscape position relative to lakes and meadows according to channel shape and sediment size, where outlets are wide and shallow with coarse sediment, and inlets are narrow and deep with finer sediment. Meadow outlets and lake outlets show similarities in the coarse‐sediment fraction and channel capacity, but meadow outlets have a smaller fine‐sediment fraction and nearly mobile sediment. Estimates of downstream recovery from lake effects on streams suggest 50 per cent recovery within 2–4 km downstream, but full recovery may not be reached within 20 km downstream. These results suggest that sediment sinks, such as lakes, in addition to sources, such as tributaries, are important local controls on mountain drainage networks. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
226.
Data from time series of transects made over a tidal period across a section of the upper Chesapeake Bay, USA, reveal the influence of lateral dynamics on sediment transport in an area with a deep channel and broad extents of shallower flanks. Contributions to lateral momentum by rotation (Coriolis plus channel curvature), cross channel density gradients and cross channel surface slope were estimated, and the friction and acceleration terms needed to complete the balance were compared to patterns of observed lateral circulation. During ebb, net rotation effects were larger because of river velocity and reinforcement of Coriolis by curvature. During flood, stratification was greater because of landward advection of strong vertical density gradients. Together, the ebb intensified lateral circulation and flood intensified stratification focused sediment and sediment transport along the left side of the estuary (looking seaward). The tendency for greater stratification on flood and net sediment flux toward the left-hand shoal are contrary to more common models which, in the northern hemisphere, predict greater resuspension on flood and move sediment toward the right-hand shoal. These tidal asymmetries interact with the lateral circulation to focus net sediment flux on the left side of the estuary, and to produce net ebb directed sediment transport at the surface of the same order of magnitude as net flood directed sediment transport at the bottom. 相似文献
227.
Bayesian analysis of groundwater quality in a semi-arid coastal county of south Texas 总被引:1,自引:0,他引:1
V. Uddameri 《Environmental Geology》2007,51(6):941-951
Bayesian frameworks for comparing water quality information to a pre-specified standard or goal and comparing water quality characteristics among two different entities are presented and illustrated using chloride and total dissolved solids (TDS) measurements obtained in the shallower Chicot and the deeper Evangeline formations of the Gulf coast aquifer underlying Refugio County, TX. The Bayesian approach seeks to present evidence in favor of the competing hypotheses which are weighed equally and unlike classical statistics do not make a decision in favor of one hypothesis. When comparing water quality information to a specified goal, the Bayesian approach addresses the more practical question—given all the information, what is the probability of meeting the goal? Similarly, when comparing the water quality between two entities, the approach simply emphasizes the nature and extent of differences and as such is better suited for evaluative studies. Bayesian analysis indicated that average chloride concentrations in the Evangeline formation was 1.65 times the concentrations in the Chicot formation while the corresponding TDS concentration ratio was close to unity. The probability of identifying water with TDS ≤1,000 g/m3 was extremely low, especially in the more prolific Evangeline formation. The probability of groundwater supplies with mean chloride concentrations ≤500 g/m3 was relatively high in the Chicot formation but very low in the Evangeline formation indicating the possible need for blending groundwater with other sources to meet municipal water quality goals. 相似文献
228.
THOMPSON R. N.; OTTLEY C. J.; SMITH P. M.; PEARSON D. G.; DICKIN A. P.; MORRISON M. A.; LEAT P. T.; GIBSON S. A. 《Journal of Petrology》2005,46(8):1603-1643
The <80 ka basaltsbasanites of the Potrillo VolcanicField (PVF) form scattered scoria cones, lava flows and maarsadjacent to the New MexicoMexico border. MgO ranges upto 12·5%; lavas with MgO < 10·7% have fractionatedboth olivine and clinopyroxene. Cumulate fragments are commonin the lavas, as are subhedral megacrysts of aluminous clinopyroxene(with pleonaste inclusions) and kaersutitic amphibole. REE modellingindicates that these megacrysts could be in equilibrium withthe PVF melts at 1·61·7 GPa pressure. Thelavas fall into two geochemical groups: the Main Series (85%of lavas) have major- and trace-element abundances and ratiosclosely resembling those of worldwide ocean-island alkali basaltsand basanites (OIB); the Low-K Series (15%) differ principallyby having relatively low K2O and Rb contents. Otherwise, theyare chemically indistinguishable from the Main Series lavas.Sr- and Nd-isotopic ratios in the two series are identical andvary by scarcely more than analytical error, averaging 87Sr/86Sr= 0·70308 (SD = 0·00004) and 143Nd/144Nd = 0·512952(SD=0·000025). Such compositions would be expected ifboth series originated from the same mantle source, with Low-Kmelts generated when amphibole remained in the residuum. ThreePVF lavas have very low Os contents (<14 ppt) and appearto have become contaminated by crustal Os. One Main Series picritehas 209 ppt Os and has a Os value of +13·6, typical forOIB. This contrasts with published 187Os/188Os ratios for KilbourneHole peridotite mantle xenoliths, which give mostly negativeOs values and show that Proterozoic lithospheric mantle formsa thick Mechanical Boundary Layer (MBL) that extends to 70 kmdepth beneath the PVF area. The calculated mean primary magma,in equilibrium with Fo89, has Na2O and FeO contents that givea lherzolite decompression melting trajectory from 2·8GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modellingof REE abundances in Main Series Mg-rich lavas is successfulfor a model invoking decompression melting of convecting sub-lithosphericlherzolite mantle (Nd = 6·4; Tp 1400°C) between90 and 70 km. Nevertheless, such a one-stage model cannot accountfor the genesis of the Low-K Series because amphibole wouldnot be stable within convecting mantle at Tf 1400°C. Thesemagmas can only be accommodated by a three-stage model thatenvisages a Thermal Boundary Layer (TBL) freezing conductivelyonto the 70 km base of the Proterozoic MBL during the 20 Myrtectonomagmatic quiescence before PVF eruptions. As it grew,this was veined by hydrous small-fraction melts from below.The geologically recent arrival of hotter-than-ambient (Tp 1400°C) convecting mantle beneath the Potrillo area re-meltedthe TBL and caused the magmatism. KEY WORDS: western USA; picrites; SrNdOs isotopes; petrogenetic modelling; thermal boundary layer 相似文献
229.
J. C. Archer G. T. Murauskas F. M. Shelley P. J. Taylor E. R. White 《The Professional geographer》1985,37(3):279-287
County scale cartographic analysis of Ronald Reagan's reelection landslide of 1984 shows the geographical core of his electoral strength to be in the Interior West, non-ethnic areas of the South, and suburbs across the nation. Examination of spatio-temporal variations using factor analysis at a state scale shows this to be a continuation and reinforcement of recent electoral-geographical cleavages. T-mode factor analysis traces the emergence of a contemporary Conservative Normal Vote during the past several presidential elections and the manifestation of this cleavage in the election of 1984. 相似文献
230.