Mass Discharge in a Tracer Plume: Evaluation of the Theissen Polygon Method     

Douglas M. Mackay  Murray D. Einarson  Phil M. Kaiser  Mamie Nozawa‐Inoue  Sham Goyal  Irina Chakraborty  Ehsan Rasa  Kate M. Scow 《Ground water》2012,50(6):895-907

A tracer plume was created within a thin aquifer by injection for 299 d of two adjacent “sub‐plumes” to represent one type of plume heterogeneity encountered in practice. The plume was monitored by snapshot sampling of transects of fully screened wells. The mass injection rate and total mass injected were known. Using all wells in each transect (0.77 m well spacing, 1.4 points/m² sampling density), the Theissen Polygon Method (TPM) yielded apparently accurate mass discharge (M_d) estimates at three transects for 12 snapshots. When applied to hypothetical sparser transects using subsets of the wells with average spacing and sampling density from 1.55 to 5.39 m and 0.70 to 0.20 points/m², respectively, the TPM accuracy depended on well spacing and location of the wells in the hypothesized transect with respect to the sub‐plumes. Potential error was relatively low when the well spacing was less than the widths of the sub‐plumes (>0.35 points/m²). Potential error increased for well spacing similar to or greater than the sub‐plume widths, or when less than 1% of the plume area was sampled. For low density sampling of laterally heterogeneous plumes, small changes in groundwater flow direction can lead to wide fluctuations in M_d estimates by the TPM. However, sampling conducted when flow is known or likely to be in a preferred direction can potentially allow more useful comparisons of M_d over multiyear time frames, such as required for performance evaluation of natural attenuation or engineered remediation systems.  相似文献   

The role of peri‐glacial active layer development in determining soil‐regolith thickness across a Triassic sandstone outcrop in the UK     

Andrew M. Tye  Simon J. Kemp  R. Murray Lark  Antoni E. Milodowski 《地球表面变化过程与地形》2012,37(9):971-983

This paper examines the weathering processes that have combined to produce the distribution of soil‐regolith (SR) thickness across the Triassic Sherwood Sandstone Group outcrop (750 km²) in Nottinghamshire, UK. Archive borehole logs (n = 282) taken across the outcrop showed that SR thickness had mean and median depths of ~1·8 and 1·5 m, respectively. Cores were taken from a forested site to depths ~3 m for geochemical analysis. At this site the SR thickness was ~1·7 m. Analysis of the loss of elements, compared to bedrock using mass balance calculations (τ) showed that all the calcite and gypsum cement had been removed to depths of >3 m. Thus the major difference between the SR and the underlying saprolite was that the former exists as loose sand as opposed to a semi‐durable rock. Scanning electron microscopy (SEM) analysis of core samples suggested that the non‐durable rock or saprolite had greater cementation of clay particles. We propose that the mechanism through which the clay cement (and other interlocking grain bonds) was eased apart was through freeze–thaw processes associated with the summer ‘active layer development (ALD)’ during the last glacial activity in the UK. We tested this theory by developing a Monte Carlo simulation based on a simplified version of the Stefan equation. Current Arctic datasets of air and ground temperatures were obtained to provide reasonable starting conditions for input variables. These were combined with known data for thermal conductivity, bulk density and moisture content of the Sherwood Sandstone regolith. Model predictions (n = 1000) of the distribution of SR thickness accurately reflect the observed distribution thickness from the borehole logs. This is strong evidence that freeze–thaw and ‘ALD’ processes are major factors in determining the thickness of SR across this outcrop. British Geological Survey © NERC 2012  相似文献   

Sampling transects for affordable, high-resolution plume characterization and monitoring     

Einarson MD  Mackay DM  Bennett PJ 《Ground water》2010,48(6):805-808

  相似文献   

News & Views     

Vicki Kretsinger Grabert  Dawn Samara Kaback  Beth L. Parker  Steven W. Chapman  John A. Cherry  Francis H. Chapelle  Michael A. Singletary  Murray D. Einarson  Doug M. Mackay  Peter J. Bennett 《Ground water》2010,48(6):799-808

  相似文献   

Spatial analysis using clustering methods: Evaluating central point and median approaches     

Alan T. Murray 《Journal of Geographical Systems》1999,1(4):367-383

Spatial analysis is an important area of research which continues to make major contributions to the exploratory capabilities of geographical information systems. The use and application of classic clustering methods is being pursued as an exploratory approach for the analysis of spatially referenced data. Numerous potential clustering approaches exist, so research assessing the relative differences of these approaches is important. This paper evaluates the median and central points optimization based clustering approaches for use in the context of exploratory spatial data analysis. Functional and visual comparisons using three spatial applications across a range of cluster values are carried out. The empirical results suggest that these two clustering approaches identify very similar groupings. The significance of this finding is that the development of clustering tools for exploratory analysis may be limited to the median based approach given relative computational and solvability considerations. Received: 28 September 1998/Accepted: 9 August 1999  相似文献   

The evolution of Holocene coastal dunefields, Jutland, Denmark: A record of climate change over the past 5000 years     

Lars B. Clemmensen  Andrew Murray  Jan Heinemeier  Rixt de Jong 《Geomorphology》2009,105(3-4):303-313

Coastal dunefields have developed on the west coast of Jutland in Denmark over the past 5000 years. The dunefields are situated in a temperate climate zone with frequent high energy wind events. Dunefield development was characterized by repeated periods of transgressive dune formation punctuated by periods of dune stabilization and soil formation. The chronology of dunefield evolution is based on Accelerator Mass Spectrometry (AMS) radiocarbon dating of peaty palaeosols (24 samples) and Optically Stimulated Luminescence (OSL) dating of aeolian sand deposits (19 samples). These dates indicate that the completeness of the stratigraphic record varies considerably, but that the timing of aeolian activity events was identical in the dunefields examined here. Initiation of aeolian activity occurred around 2200 BC, 800 BC, AD 100, AD 1050–1200, and between AD 1550 and 1650. Proxy-climate data from bogs in southwestern Sweden suggest that these periods of dunefield activity were initiated during wet/cool summers. Most likely these climatic situations were associated with a more frequent passage of cyclones across Denmark in the summer seasons (increased storminess) causing aeolian sand movement and dune formation. The continued removal of vegetation in Jutland between 4000 BC and AD 1850 allowed the aeolian sand, when first activated, to move more and more easily across the coastal plains; the final phase of aeolian sand movement between AD 1550–1650 and 1900 had catastrophic implications for many coastal inhabitants in the region.  相似文献   

Key features of artificial aquifers for use in modeling contaminant transport     

Close M  Bright J  Wang F  Pang L  Manning M 《Ground water》2008,46(6):814-828

Two large-scale (9.5 m long, 4.7 m wide, 2.6 m deep), three-dimensional artificial aquifers were constructed to investigate the influence of spatial variations in aquifer properties on contaminant transport. One aquifer was uniformly filled with coarse sand media (0.6 to 2.0 mm) and the other was constructed as a heterogeneous aquifer using blocks of fine, medium, and coarse sands. The key features of these artificial aquifers are described. An innovative deaeration tower was constructed to overcome a problem of the aquifers becoming blocked with excess air from the ground water source. A series of tracer injection experiments were conducted to test the homogeneity of the first aquifer that was purposely built as a homogeneous aquifer and to calculate values of aquifer parameters. Experimental data show that the aquifer is slightly heterogeneous, and hydraulic conductivity values are significantly higher down one side of the aquifer compared to the mean value. There was very good agreement in estimated dispersivity values between the plume area ratio methods and the curve fitting of tracer breakthrough curves. Dispersivity estimates from a full areal source injection (12.2 m2) experiment using a 1D analytical model were higher than estimates from a limited source injection (0.2 m2) experiment using a 3D model, possibly because the 1D model does not take account of the heterogeneity of hydraulic conductivity in the aquifer, thus overestimating dispersivity. Transverse and vertical dispersivity values were about five times less than the longitudinal dispersivity. There was slight sorption of Rhodamine WT onto the aquifer media.  相似文献   

The shape of patterns to come: from initial formation to long‐term evolution          下载免费PDF全文

A. Brad Murray  Evan B. Goldstein  Giovanni Coco 《地球表面变化过程与地形》2014,39(1):62-70

Many studies focus on the emergence and development of rhythmic landscape patterns. In this contribution we explore the different behaviors found as patterns evolve; the trajectories that patterns exhibit as they transit from infinitesimal‐amplitude perturbation to a statistically steady state (or in some cases to continued statistical evolution). The variety of behaviors observed, either through field and laboratory experiments or numerical modeling, can be reduced to four classes: (a) simple stabilization where predictions based on the initial growth of small perturbations corresponds with the characteristics of patterns observed in nature; (b) significant pattern coarsening en route to saturated wavelength, where non‐linear interactions between finite‐amplitude pattern elements change the geometric properties of a pattern as it approaches steady‐state; (c) perpetual coarsening where the wavelength associated with the emerging pattern continues to grow over time and is only limited by physical boundaries or external constrains; (d) slow evolution toward a different attractor, a novel behavior observed in numerical modeling that involves profound temporal changes in pattern characteristics. Within these classes we also observe generalizable non‐linear behaviors: dependence on initial conditions, the emergence of pattern‐scale variables such as pattern defects, and the presence of multiple stable states. Predicting the shape of patterns to come remains a challenge – one that we suggest requires a range of modeling approaches to address both initial instabilities and the emergent properties of evolving patterns, which involve disparate forms of non‐linear interactions. Consideration of generic system behaviors at the pattern scale could enhance future pattern formation studies, facilitating appropriate pairings of analysis approaches and pattern‐evolution modes. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Delineation of a landfill leachate plume and flow channels in coastal sands near Christchurch, New Zealand, using a shallow electromagnetic survey method     

David C. Nobes  Mark J. Armstrong  Murray E. Close 《Hydrogeology Journal》2000,8(3):328-336

The Burwood landfill, which serves the city of Christchurch, New Zealand, is situated on coastal sands underlain by a sequence of aquifers and aquitards. Groundwater flow is toward the coast, located approximately 700 m from the landfill boundary. Shortly after completion of the first phase of the landfill, an array of wells was installed to detect any contaminant from the landfill. Leachate was detected in the wells closest to the landfill. A shallow electromagnetic (EM31) survey was carried out between the landfill and the coast, in order to delineate any leachate plume that may be present. On the basis of the geophysical results, a contaminant plume and buried channels connected to the coast were identified. Leachate flow initially occurs in what is probably a channel or pair of channels. Downgradient, the plume spreads out to the north and south as it moves eastward toward the coast. Using the geophysical results as a guide, a new set of wells was installed to confirm the presence of high leachate concentrations. Pore-water sampling confirms the presence of a leachate plume. Received, June 1998 /Revised, March 1999, January 2000/Accepted, January 2000  相似文献   

Monitoring Growth in Rapidly Urbanizing Areas Using Remotely Sensed Data     

Douglas Ward  Stuart R. Phinn  Alan T. Murray 《The Professional geographer》2000,52(3):371-386

Urbanization and the ability to manage for a sustainable future present numerous challenges for geographers and planners in metropolitan regions. Remotely sensed data are inherently suited to provide information on urban land cover characteristics, and their change over time, at various spatial and temporal scales. Data models for establishing the range of urban land cover types and their biophysical composition (vegetation, soil, and impervious surfaces) are integrated to provide a hierarchical approach to classifying land cover within urban environments. These data also provide an essential component for current simulation models of urban growth patterns, as both calibration and validation data. The first stages of the approach have been applied to examine urban growth between 1988 and 1995 for a rapidly developing area in southeast Queensland, Australia. Landsat Thematic Mapper image data provided accurate (83% adjusted overall accuracy) classification of broad land cover types and their change over time. The combination of commonly available remotely sensed data, image processing methods, and emerging urban growth models highlights an important application for current and next generation moderate spatial resolution image data in studies of urban environments.  相似文献