A Practical Modeling Framework for Non‐Fickian Transport and Multi‐Species Sequential First‐Order Reaction          下载免费PDF全文

Daniel K. Burnell  Jie Xu  Scott K. Hansen  Lawrence S. Sims  Charles R. Faust 《Ground water》2018,56(4):524-540

Many studies indicate that small‐scale heterogeneity and/or mobile–immobile mass exchange produce transient non‐Fickian plume behavior that is not well captured by the use of the standard, deterministic advection‐dispersion equation (ADE). An extended ADE modeling framework is presented here that is based on continuous time random walk theory. It can be used to characterize non‐Fickian transport coupled with simultaneous sequential first‐order reactions (e.g., biodegradation or radioactive decay) for multiple degrading contaminants such as chlorinated solvents, royal demolition explosive, pesticides, and radionuclides. To demonstrate this modeling framework, new transient analytical solutions are derived and are inverted in Laplace space. Closed‐form, steady‐state, multi‐species analytical solutions are also derived for non‐Fickian transport in highly heterogeneous aquifers with linear sorption–desorption and matrix diffusion for use in spreadsheets. The solutions are general enough to allow different degradation rates for the mobile and immobile zones. The transient solutions for multi‐species transport are applied to examine the effects of source remediation on the natural attenuation of downgradient plumes of both parent and degradation products in highly heterogeneous aquifers. Results for representative settings show that the use of the standard, deterministic ADE can over‐estimate cleanup rates and under‐predict the cleanup timeframe in comparison to the extended ADE analytical model. The modeling framework and calculations introduced here are also applied for a 30 year groundwater cleanup program at a site in Palm Bay, Florida. The simulated plume concentrations using the extended ADE exhibited agreement with observed long concentration tails of trichloroethene, cis 1,2 DCE, and VC that remained above cleanup goals.  相似文献   

Quantifying River‐Groundwater Interactions of New Zealand's Gravel‐Bed Rivers: The Wairau Plain          下载免费PDF全文

Thomas Wöhling  Moritz J. Gosses  Scott R. Wilson  Peter Davidson 《Ground water》2018,56(4):647-666

New Zealand's gravel‐bed rivers have deposited coarse, highly conductive gravel aquifers that are predominantly fed by river water. Managing their groundwater resources is challenging because the recharge mechanisms in these rivers are poorly understood and recharge rates are difficult to predict, particularly under a more variable future climate. To understand the river‐groundwater exchange processes in gravel‐bed rivers, we investigate the Wairau Plain Aquifer using a three‐dimensional groundwater flow model which was calibrated using targeted field observations, “soft” information from experts of the local water authority, parameter regularization techniques, and the model‐independent parameter estimation software PEST. The uncertainty of simulated river‐aquifer exchange flows, groundwater heads, spring flows, and mean transit times were evaluated using Null‐space Monte‐Carlo methods. Our analysis suggests that the river is hydraulically perched (losing) above the regional water table in its upper reaches and is gaining downstream where marine sediments overlay unconfined gravels. River recharge rates are on average 7.3 m³/s, but are highly dynamic in time and variable in space. Although the river discharge regularly hits 1000 m³/s, the net exchange flow rarely exceeds 12 m³/s and seems to be limited by the physical constraints of unit‐gradient flux under disconnected rivers. An important finding for the management of the aquifer is that changes in aquifer storage are mainly affected by the frequency and duration of low‐flow periods in the river. We hypothesize that the new insights into the river‐groundwater exchange mechanisms of the presented case study are transferable to other rivers with similar characteristics.  相似文献   

Heterogeneous water storage and thermal regime of supraglacial ponds on debris‐covered glaciers          下载免费PDF全文

Cameron Scott Watson  Duncan J. Quincey  Jonathan L. Carrivick  Mark W. Smith  Ann V. Rowan  Robert Richardson 《地球表面变化过程与地形》2018,43(1):229-241

The water storage and energy transfer roles of supraglacial ponds are poorly constrained, yet they are thought to be important components of debris‐covered glacier ablation budgets. We used an unmanned surface vessel (USV) to collect sonar depth measurements for 24 ponds to derive the first empirical relationship between their area and volume applicable to the size distribution of ponds commonly encountered on debris‐covered glaciers. Additionally, we instrumented nine ponds with thermistors and three with pressure transducers, characterizing their thermal regime and capturing three pond drainage events. The deepest and most irregularly‐shaped ponds were those associated with ice cliffs, which were connected to the surface or englacial hydrology network (maximum depth = 45.6 m), whereas hydrologically‐isolated ponds without ice cliffs were both more circular and shallower (maximum depth = 9.9 m). The englacial drainage of three ponds had the potential to melt ~100 ± 20 × 10³ kg to ~470 ± 90 × 10³ kg of glacier ice owing to the large volumes of stored water. Our observations of seasonal pond growth and drainage with their associated calculations of stored thermal energy have implications for glacier ice flow, the progressive enlargement and sudden collapse of englacial conduits, and the location of glacier ablation hot‐spots where ponds and ice cliffs interact. Additionally, the evolutionary trajectory of these ponds controls large proglacial lake formation in deglaciating environments. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Variability of the useful life of reservoirs in tropical locations: A case study from the Burdekin Falls Dam,Australia     

Michelle Cooper  Stephen E. Lewis  Thomas C. Stieglitz  Scott G. Smithers 《国际泥沙研究》2018,33(2):93-106

Large dam construction for irrigation, hydropower, water supply, and flood control in tropical to sub-tropical areas increased markedly after 1950. Many of the dams built during this period have filled with sediment and no longer perform to their original design specifications; in many cases, forecast dam life was greatly overestimated. This study investigates the useful life of Burdekin Falls Dam (BFD), northeastern Australia and compares the findings with other tropical reservoirs. Using two independent methods it is estimated that between 61 and 65 million m³ of sediment has been deposited in the reservoir over the 24 years of operation through 2011. This sediment volume equates to an average of 0.15% of capacity lost per year since construction was completed. If current sediment loads/climate regimes persist, reservoir capacity will be reduced by 50% after 345 years. However, the useful life of the BFD reduces to just 276 years when drawdown data are considered; these data show reservoir use would be affected once 40% of storage was filled with sediment, with a 60% drawdown return period of 1 in 15 years. When compared to similar large tropical to sub-tropical reservoirs, the BFD has a slightly longer reservoir useful life than dams in India and a much longer half-life than for both similar-sized and larger dams in China, Brazil, and Iran. Properties of the BFD that promote a longer useful life include a lower trap efficiency, relatively low annual sediment load delivered to the reservoir, limited sediment deposition behind the dam wall (and uniform distribution of deposited sediment), and the export of highly turbid annual floodwaters before settling and deposition of any remaining sediment within the reservoir.  相似文献   

Advancing Contaminant Mass Flux Analysis to Focus Remediation: The Three‐Compartment Model          下载免费PDF全文

John Horst  Scott Potter  Matthew Schnobrich  Nicklaus Welty  Ankit Gupta  Joseph Quinnan 《Ground Water Monitoring & Remediation》2017,37(4):15-22

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An individual-level network model for a hypothetical outbreak of Japanese encephalitis in the USA     

Mahbubul?H.?RiadEmail authorView author&#;s OrcID profile  Caterina M.?Scoglio  D.?Scott?McVey  Lee?W.?Cohnstaedt 《Stochastic Environmental Research and Risk Assessment (SERRA)》2017,31(2):353-367

Japanese encephalitis (JE) is a vector-borne disease transmitted by mosquitoes and maintained in birds and pigs. To examine the possible epidemiology of JE in the United States, we use an individual-level network model that explicitly considers the feral pig population and implicitly considers mosquitoes and birds in specific areas of Florida, North Carolina, and South Carolina. To model the virus transmission among feral pigs within a small geographic area (<60 sq mi areas), two network topologies are considered: Fully connected and Erdos–Renyi networks. Long-distance connections (interstate) are created with limited probability and based on fall and spring bird migration patterns. Patterns of simulated outbreaks support the use of the Erdos–Renyi network because maximum incidence occurs during the fall migration period which is similar to the peak incidence of the closely related West Nile virus, another virus in the Japanese encephalitis group (Flaviviridae) that is transmitted by both birds and mosquitoes. Simulation analysis suggested two important mitigation strategies: for low mosquito vectorial capacity, insecticidal spraying of infected areas reduces transmission and limits the outbreak to a single geographic area. Alternatively, in high mosquito vectorial capacity areas, birds rather than mosquitoes need to be removed/controlled.  相似文献   

Assessing the footprint and volume of oil deposited in deep-sea sediments following the Deepwater Horizon oil spill     

Scott A. Stout  Shahrokh Rouhani  Bo Liu  Jacob Oehrig  Robert W. Ricker  Gregory Baker  Christopher Lewis 《Marine pollution bulletin》2017,114(1):327-342

The lateral and vertical extents of Macondo oil in deep-sea sediments resulting from the 2010 Deepwater Horizon oil spill were determined using chemical forensics and geostatistical kriging of data from 2397 sediment samples from 875 cores collected in 2010/2011 and 2014. The total mass of Macondo-derived hopane on the seafloor in 2010/2011 was conservatively estimated between 2.00 and 2.26 metric tons, derived from 219,000 to 247,000 barrels of oil; or 6.9 to 7.7% of the 3.19 million barrels spilled. Macondo-derived hopane was deposited over 1030 to 1910 km² of the seafloor, mostly (> 97%) in surface (0–1 cm) and near-surface (1–3 cm) sediments, which is consistent with short-term oil deposition. Although Macondo oil was still present in surface sediments in 2014, the total mass of Macondo-derived hopane was significantly lower (~ 80 to 90%) than in 2010/2011, affirming an acute impact from the spill and not long-term deposition from natural seeps.  相似文献   

Uncertainty in quantitative analyses of topographic change: error propagation and the role of thresholding     

Scott W. Anderson 《地球表面变化过程与地形》2019,44(5):1015-1033

Topographic surveys inevitably contain error, introducing uncertainty into estimates of volumetric or mean change based on the differencing of repeated surveys. In the geomorphic community, uncertainty has often been framed as a problem of separating out real change from apparent change due purely to error, and addressed by removing measured change considered indistinguishable from random noise from analyses (thresholding). Thresholding is important when quantifying gross changes (i.e. total erosion or total deposition), which are systematically biased by random errors in stable parts of a landscape. However, net change estimates are not substantially influenced by those same random errors, and the use of thresholds results in inherently biased, and potentially misleading, estimates of net change and uncertainty. More generally, thresholding is unrelated to the important process of propagating uncertainty in order to place uncertainty bounds around final estimates. Error propagation methods for uncorrelated, correlated, and systematic errors are presented. Those equations demonstrate that uncertainties in modern net change analyses, as well as in gross change analyses using reasonable thresholds, are likely to be dominated by low-magnitude but highly correlated or systematic errors, even after careful attempts to reduce those errors. In contrast, random errors with little to no correlation largely cancel to negligible levels when averaged or summed. Propagated uncertainty is then typically insensitive to the precision of individual measurements, and is instead defined by the relative mean error (accuracy) over the area of interest. Given that real-world mean elevation changes in many landscape settings are often similar in magnitude to potential mean errors in repeat topographic analyses, reducing highly correlated or systematic errors will be central to obtaining accurate change estimates, while placing uncertainty bounds around those results provides essential context for their interpretation. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.  相似文献   

Are trends in SeaWiFS chlorophyll time-series unusual relative to historic variability   总被引：1，自引：0，他引：1  

James A Yoder  Maureen A Kennelly  Scott C Doney  Ivan D Lima 《海洋学报(英文版)》2010,29(2):1-4

For selected locations in the Atlantic and Pacific Ocean, we compared surface ocean chlorophyll time series extracted from SeaWiFS imagery from 1997–2004 with the results of an ocean coupled circulation and biogeochemical model covering the period 1958–2004. During the 1997–2004 time period, linear trends in model and satellite time series were significantly correlated at most of the 44 sites we studied. Eleven sites were selected for further study, and we used the longer time series of the model to assess whether trends observed during the SeaWiFS period at these 11 sites were unusual in relation to those observed over the longer historical period covered by the model. The results show that the trends observed during the SeaWiFS period were not unusual and fell well within the range in magnitude of linear trends observed in other 8-year periods of model output. This result implies that the SeaWiFS satellite ocean color time series is not yet sufficiently long, on its own, to directly observe any long term changes in phytoplankton chlorophyll that may be occurring in the surface waters of the open ocean as a result of increased ocean stratification linked to global climate changed.  相似文献   

Lithostratigraphy of the Windermere Supergroup,Northern England     

B. C. Kneller  R. W. Scott  N. J. Soper  E. W. Johnson  P. M. Allen 《Geological Journal》1994,29(3):219-240

A formal lithostratigraphy is erected for the Late Ordovician and Silurian rocks of northern England. The scheme presented herein emphasizes natural depositional sequences within the supergroup and defines new type sections or cites existing definitions where appropriate. It is intended to be sufficiently flexible and comprehensive to apply to all the inliers, and to bear some rational relationship to lithostratigraphical usage in other British Lower Palaeozoic inliers.  相似文献