Time vs. Money: A Quantitative Evaluation of Monitoring Frequency vs. Monitoring Duration          下载免费PDF全文

Thomas E. McHugh  Poonam R. Kulkarni  Charles J. Newell 《Ground water》2016,54(5):692-698

The National Research Council has estimated that over 126,000 contaminated groundwater sites are unlikely to achieve low ug/L clean‐up goals in the foreseeable future. At these sites, cost‐effective, long‐term monitoring schemes are needed in order to understand the long‐term changes in contaminant concentrations. Current monitoring optimization schemes rely on site‐specific evaluations to optimize groundwater monitoring frequency. However, when using linear regression to estimate the long‐term zero‐order or first‐order contaminant attenuation rate, the effect of monitoring frequency and monitoring duration on the accuracy and confidence for the estimated attenuation rate is not site‐specific. For a fixed number of monitoring events, doubling the time between monitoring events (e.g., changing from quarterly monitoring to semi‐annual monitoring) will double the accuracy of estimated attenuation rate. For a fixed monitoring frequency (e.g., semi‐annual monitoring), increasing the number of monitoring events by 60% will double the accuracy of the estimated attenuation rate. Combining these two factors, doubling the time between monitoring events (e.g., quarterly monitoring to semi‐annual monitoring) while decreasing the total number of monitoring events by 38% will result in no change in the accuracy of the estimated attenuation rate. However, the time required to collect this dataset will increase by 25%. Understanding that the trade‐off between monitoring frequency and monitoring duration is not site‐specific should simplify the process of optimizing groundwater monitoring frequency at contaminated groundwater sites.  相似文献   

Characterization and Source History Modeling Using Low‐k Zone Profiles at Two Source Areas          下载免费PDF全文

David T. Adamson  Steven W. Chapman  Shahla K. Farhat  Beth L. Parker  Phillip deBlanc  Charles J. Newell 《Ground Water Monitoring & Remediation》2015,35(2):52-69

Like tree rings, high‐resolution soil sampling of low‐permeability (low‐k) zones can be used to evaluate the style of source history at contaminated sites (i.e., historical pattern of concentration and composition vs. time since releases occurred at the interface with the low‐k zone). This is valuable for the development of conceptual site model (CSM) and can serve as an important line of evidence supporting monitored natural attenuation (MNA) as a long‐term remedy. Source histories were successfully reconstructed at two sites at Naval Air Station Jacksonville using a simple one‐dimensional (1D) model. The plume arrival time and historical composition were reconstructed from the time initial releases that were suspected to occur decades earlier. At the first site (Building 106), the source reconstructions showed relatively constant source concentrations, but significant attenuation over time in the downgradient plume in the transmissive zone, suggesting MNA may not be an appropriate remedy if source control is a requirement, but attenuation processes are clearly helping to maintain plume stability and reduce risk. At the second site (Building 780), source concentrations in the transmissive zone showed an approximately a one order of magnitude over time, but apparently less attenuation in the downgradient plume. The source reconstruction method appeared to reflect site remediation efforts (excavation, soil vapor extraction) implemented in the 1990s. Finally, a detailed analysis using molecular biological tools, carbon isotopes, and by‐products suggests that most degradation activity is associated with high‐k zones but not with low‐k zones at these source areas. Overall, the source reconstruction methodology provided insight into historical concentration trends not obtainable otherwise given the limited long‐term monitoring data.  相似文献   

Transport and retention in an upwelling region: The role of across-shelf structure   总被引：2，自引：0，他引：2  

Moninya Roughan  Newell Garfield  John Largier  Edward Dever  Clive Dorman  Dwight Peterson  Jeff Dorman 《Deep Sea Research Part II: Topical Studies in Oceanography》2006,53(25-26):2931

The paradox of upwelling is the relationship between strong wind forcing, nutrient enrichment, and shelf productivity. Here we investigate how across-shelf structure in velocity and hydrography plays a role in the retention (inshore) and export (offshore) of particles such as nutrients, plankton and larvae. We examine the spatial structure of the coastal currents during wind-driven upwelling and relaxation on the northern Californian Shelf. The field work was conducted as part of the Wind Events and Shelf Transport (WEST) project, a 5-year NSF/CoOP-funded study of the role of wind-driven transport in shelf productivity off Bodega Bay (northern California) from 2000 to 2003. We combine shipboard velocity profiles (ADCP) and water properties from hydrographic surveys during the upwelling season to examine the mean across-shelf structure of the hydrography and velocity fields during three contrasting upwelling seasons, and throughout the upwelling-relaxation cycle. We also present results from two winter seasons that serve as contrast to the upwelling seasons.During all three upwelling seasons clear spatial structure is evident in velocity and hydrography across the shelf, exemplified by current reversals inshore and the presence of a persistent upwelling jet at the shelf break. This jet feature changes in structure and distance from the coast under different wind forcing regimes. The jet also changes from the north of our region, where it is a single narrow jet, adjacent to the coast, and to the south of our region, where it broadens and at times two jets become evident. We present observations of the California Under Current, which was observed at the outer edge of our domain during all three upwelling seasons. The observed across-shelf structure could aid both in the retention of plankton inshore during periods of upwelling followed by relaxation and in the export of plankton offshore in the upwelling jet.  相似文献   

Intensifying aeolian activity following the end-Permian mass extinction: Evidence from the Late Permian–Early Triassic terrestrial sedimentary record of the Ordos Basin,North China     

Zhicai Zhu  Hongwei Kuang  Yongqing Liu  Michael J. Benton  Andrew J. Newell  Huan Xu  Wei An  Shu'an Ji  Shichao Xu  Nan Peng  Qingguo Zhai 《Sedimentology》2020,67(5):2691-2720

Sedimentary successions provide direct evidence of climate and tectonics, and these give clues about the causes of the mass extinction around the Permian–Triassic boundary. Terrestrial Permian–Triassic boundary strata in the eastern Ordos Basin, North China, include the Late Permian Sunjiagou, Early Triassic Liujiagou and late Early Triassic Heshanggou formations in ascending order. The Sunjiagou Formation comprises cross-bedded sandstones overlaid by mudstones, indicating meandering rivers with channel, point bar and floodplain deposits. The Liujiagou Formation was formed in braided rivers of arid sand bars interacting with some aeolian dune deposits, distinguished by abundant sandstones where diverse trough and planar cross-bedding and aeolian structures (for example, inverse climbing-ripple, translatent-ripple lamination, grainfall and grainflow laminations) interchange vertically and laterally. The Heshanggou Formation is a rhythmic succession of mudstones interbedded with thin medium-grained sandstones mainly deposited in a shallow lacustrine environment. Overall, the sharp meandering to braided to shallow lake sedimentary transition documents palaeoenvironmental changes from semi-arid to arid and then to semi-humid conditions across the Permian–Triassic boundary. The die-off of tetrapods and plants, decreased bioturbation levels in the uppermost Sunjiagou Formation, and the bloom of microbially-induced sedimentary structures in the Liujiagou Formation marks the mass extinction around the Permian–Triassic boundary. The disappearance of microbially-induced sedimentary structures, increasingly intense bioturbation from bottom to top and the reoccurrence of reptile footprints in the Heshanggou Formation reveal gradual recovery of the ecosystem after the Permian–Triassic boundary extinction. This study is the first to identify the intensification of aeolian activity following the end-Permian mass extinction in North China. Moreover, while northern North China continued to be uplifted tectonically from the Late Palaeozoic to Late Mesozoic, the switch of sedimentary patterns across the Permian–Triassic boundary in Shanxi is largely linked to the development of an arid and subsequently semi-humid climate condition, which probably directly affected the collapse and delayed recovery in palaeoecosystems.  相似文献   

Vertical Discretization Impact in Numerical Modeling of Matrix Diffusion in Contaminated Groundwater     

Shahla K. Farhat  David T. Adamson  Arun R. Gavaskar  Sophia A. Lee  Ronald W. Falta  Charles J. Newell 《Ground Water Monitoring & Remediation》2020,40(2):52-64

Understanding the effects of contaminants that can diffuse into low-permeability (“low-k”) zones is crucial for effective groundwater remedial decision-making. Because low-k zones can serve as low-level sources of contamination to more transmissive zones over time, an accurate evaluation of the impacts of matrix diffusion at contaminated sites is vital. This study compared numerical groundwater flow and transport simulations using MODFLOW/RT3D at a hypothetical site using three cases, each with increasing discretization of the vertical 10-m thick domain: (1) a coarse multilayer heterogeneous grid based on one layer for each of four different hydrogeological units, (2) a “low-resolution” discretization approach where the low-k units were divided into several sublayers giving the model 10 layers, and (3) a “high-resolution” numerical model with 199 layers that are a few centimeters thick. When comparing the results of each case, significant differences were observed between the discretizations used, even though all other model input data were identical. The conventional grid models (Cases 1 and 2) appeared to underestimate groundwater plume concentrations by a factor ranging from 1.1 to 36 when compared to the high-resolution grid model (Case 3), and underestimated predicted cleanup times by more than a factor of 10 for some of the hypothetical sampling points in the modeling domain. These results validate the implication of Chapman et al. (2012), that conventional vertical discretization of numerical groundwater flow and transport models at contaminated sites (with layers that are greater than 1 m thick) can lead to significant errors when compared to more accurate high-resolution vertical discretization schemes (layers that are centimeters thick).  相似文献   

Hydrological resilience to forest fire in the subarctic Canadian shield     

Christopher Spence  Newell Hedstrom  Suzanne E. Tank  William L. Quinton  David Olefeldt  Stefan Goodman  Nicole Dion 《水文研究》2020,34(25):4940-4958

Understanding the role of forest fires on water budgets of subarctic Precambrian Shield catchments is important because of growing evidence that fire activity is increasing. Most research has focused on assessing impacts on individual landscape units, so it is unclear how changes manifest at the catchment scale enough to alter water budgets. The objective of this study was to determine the water budget impact of a forest fire that partially burned a ~450 km² subarctic Precambrian Shield basin. Water budget components were measured in a pair of catchments: one burnt and another unburnt. Burnt and unburnt areas had comparable net radiation, but thaw was deeper in burned areas. There were deeper snow packs in burns. Differences in streamflow between the catchments were within measurement uncertainty. Enhanced winter streamflow from the burned watershed was evident by icing growth at the streamflow gauge location, which was not observed in the unburned catchment. Wintertime water chemistry was also clearly elevated in dissolved organics, and organic-associated nutrients. Application of a framework to assess hydrological resilience of watersheds to wildfire reveal that watersheds with both high bedrock and open water fractions are more resilient to hydrological change after fire in the subarctic shield, and resilience decreases with increasingly climatically wet conditions. This suggests significant changes in runoff magnitude, timing and water chemistry of many Shield catchments following wildfire depend on pre-fire land cover distribution, the extent of the wildfire and climatic conditions that follow the fire.  相似文献   

Urban resilience for whom,what, when,where, and why?     

Sara Meerow  Joshua P. Newell 《Urban geography》2019,40(3):309-329

ABSTRACT

In academic and policy discourse, the concept of urban resilience is proliferating. Social theorists, especially human geographers, have rightfully criticized that the underlying politics of resilience have been ignored and stress the importance of asking “resilience of what, to what, and for whom?” This paper calls for careful consideration of not just resilience for whom and what, but also where, when, and why. A three-phase process is introduced to enable these “five Ws” to be negotiated collectively and to engender critical reflection on the politics of urban resilience as plans, initiatives, and projects are conceived, discussed, and implemented. Deployed through the hypothetical case of green infrastructure in Los Angeles, the paper concludes by illustrating how resilience planning trade-offs and decisions affect outcomes over space and time, often with significant implications for equity.  相似文献   

Plankton dynamics due to rainfall,eutrophication, dilution,grazing and assimilation in an urbanized coastal lagoon     

David Rissik  Edward Ho Shon  Brooke Newell  Mark E. Baird  Iain M. Suthers 《Estuarine, Coastal and Shelf Science》2009

After a prolonged summer dry period, the effects of a distinctive and continuing rainfall on the nutrients and plankton of an urban coastal lagoon were investigated over 2 months. The lagoon filled up over 5 weeks from <10% of its maximum volume until it broke open to the sea. Nutrients (ammonia and oxidised nitrogen) significantly increased the day after initial rainfall, before returning to pre-rainfall conditions within 5 days. Phytoplankton biomass grew 10 fold within a week after initial rainfall in the 25–30 °C water and declined to near initial levels 2 weeks later. The assemblage of phytoplankton and zooplankton changed dramatically after 1 day and again by 6 days later, gradually returning to the original community by 2 weeks after the initial rainfall. Zooplankton responded within a day with a two fold increase in the adult stages of the calanoid copepod Oithona sp., followed a week later by nauplii and adult Acartia bispinosa. The influx of adult Oithona indicates resting populations that were previously under sampled by our plankton net. The plankton community returned to the initial state by 2 weeks, to being dominated by a centric diatom and A. bispinosa after 5 weeks. Dilution of the lagoon reached a maximum of 0.25 d⁻¹, while growth rates of the phytoplankton population reached a maximum of 1 d⁻¹, and A. bispinosa nauplii growth of 2.5 d⁻¹. Declines in chlorophyll biomass from the maximum 10 μg l⁻¹, at a rate of approximately 10% d⁻¹ are consistent with the modelled uptake by zooplankton. The nutrients from runoff, growth and the influx of new zooplankton into the water column, resulted in a depleted δ¹³C and δ¹⁵N stable isotope signature of A. bispinosa by 2–4 ppt within 1–2 weeks, consistent with diatom growth and the terrestrial supply of depleted nutrients. δ³⁴S of A. bispinosa was enriched by 2 ppt for 1–2 weeks after rainfall, but unlike C and N, returned to pre-rainfall levels by the end of the study period. We suggest that plankton studies in coastal lakes with variable water levels that are not tidally driven, should account for the influence of changes in water levels to help explain data variability.  相似文献   

Discrete wetland groundwater discharges revealed with a three-dimensional temperature model and botanical indicators (Boxford,UK)     

Andrew R. House  James P. R. Sorensen  Daren C. Gooddy  Andrew J. Newell  Ben Marchant  J. Owen Mountford  Peter Scarlett  Peter J. Williams  Gareth H. Old 《Hydrogeology Journal》2015,23(4):775-787

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Some relationships between lithology,basin form and hydrology: a case study from the Thames basin,UK   总被引：1，自引：0，他引：1  

J. P. Bloomfield  S. H. Bricker  A. J. Newell 《水文研究》2011,25(16):2518-2530

The role of lithology in influencing basin form and function is explored empirically by investigating correlations between a range of catchment variables, where the spatial unit of analysis is not surface catchments but lithologically coherent groundwater units. Using the Thames basin, UK, as a case study, nine groundwater units have been identified. Values for 11 hydrological and geomorphological variables, including rainfall, drainage density, Baseflow Index, aquifer porosity, storage coefficient and log‐hydraulic conductivity, aquifer and drainage elevation, river incision, and hypsometric integral have been estimated for each of the groundwater units in the basin, and Pearson correlation coefficients calculated for all pairs of variables. Seven of the correlation coefficients are found to be significant at a confidence level of > 99%. Negative correlations between drainage density and log aquifer hydraulic conductivity, and between drainage density and river incision, and positive correlations between log‐hydraulic conductivity and river incision, log‐hydraulic conductivity and Baseflow Index, and between Baseflow Index and river incision are inferred to have consistent causal explanations. For example, incision of rivers into aquifers leads to relative increases in hydraulic gradients in the vicinity of rivers which, in turn, promotes the development of secondary porosity increasing both aquifer hydraulic conductivity and, hence, Baseflow Index. The implication of this interpretation is that the geomorphological evolution of basins is intimately linked to the evolution of hydraulic conductivity of the underlying aquifers. This is consistent with, and supports the notion of a coupled complexly evolving surface water‐groundwater system. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献