Effects of photoirradiation on the adsorption of dissolved organic matter to goethite     

Michael J. Pullin  Christina A. Progess 《Geochimica et cosmochimica acta》2004,68(18):3643-3656

The effects of photoirradiation of dissolved organic matter (DOM) on its subsequent adsorption to the Fe(III)oxyhydroxide mineral goethite were investigated at 22°C in 0.10 mol L⁻¹ NaClO₄ solutions at pH 3.5 and 5.5, Photoirradiation of DOM decreased the abundance of high molecular-weight components and formed new lower molecular-weight components, including low molecular weight carboxylic acids (i.e., formic, malonic, and acetic acids). Adsorption of non-irradiated DOM decreased from pH 3.5 to 5.5 and was dominated by the intermediate molecular weight (1251-3750 Da) fraction, although the 451-1250 and 3751-11350 Da fractions also contributed to adsorption at pH 3.5. Irradiation resulted in a substantial decrease in DOM adsorption affinity at pH 3.5, primarily due to loss of components in the 1251-3750 and 3751-11350 Da fractions. Irradiation resulted in only a small decrease in DOM adsorption affinity at pH 5.5; the loss of components in the 3751-11350 Da fraction upon irradiation had little effect on adsorption because they played little or no role in the non-irradiated sample at this pH. Irradiation of DOM also affected its interactions with Fe in solution and the solution iron(II)/iron(III) speciation. The combined effects of irradiation followed by adsorption produced DOM that was lower in molecular weight and had a decreased UV-Vis absorptivity than either process, alone. Together, these two processes are likely to have important environmental consequences in terms of UV penetration of surface waters, contaminant mobility, and DOM bioavailability.  相似文献   

Tephra-fall deposits from the 1992 eruption of Crater Peak, Alaska: implications of clast textures for eruptive processes     

Cynthia A. Gardner  Katharine V. Cashman  Christina A. Neal 《Bulletin of Volcanology》1998,59(8):537-555

 The 1992 eruption of Crater Peak, Mount Spurr, Alaska, involved three subplinian tephra-producing events of similar volume and duration. The tephra consists of two dense juvenile clast types that are identified by color, one tan and one gray, of similar chemistry, mineral assemblage, and glass composition. In two of the eruptive events, the clast types are strongly stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third. Tan clasts have average densities between 1.5 and 1.7 g/cc and vesicularities (phenocryst free) of approximately 42%. Gray clasts have average densities between 2.1 and 2.3 g/cc, and vesicularities of approximately 20%; both contain abundant microlites. Average maximum plagioclase microlite lengths (13–15 μm) in gray clasts in the upper layer are similar regardless of eruptive event (and therefore the repose time between them) and are larger than average maximum plagioclase microlite lengths (9–11 μm) in the tan clasts in the lower layer. This suggests that microlite growth is a response to eruptive processes and not to magma reservoir heterogeneity or dynamics. Furthermore, we suggest that the low vesicularities of the clasts are due to syneruptive magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater. Received: 5 September 1997 / Accepted: 1 February 1998  相似文献   

Reducing monitoring gaps at the aquifer–river interface by modelling groundwater–surface water exchange flow patterns     

Matthias Munz  Stefan Krause  Christina Tecklenburg  Andrew Binley 《水文研究》2011,25(23):3547-3562

This study investigates spatial patterns and temporal dynamics of aquifer–river exchange flow at a reach of the River Leith, UK. Observations of sub‐channel vertical hydraulic gradients at the field site indicate the dominance of groundwater up‐welling into the river and the absence of groundwater recharge from surface water. However, observed hydraulic heads do not provide information on potential surface water infiltration into the top 0–15 cm of the streambed as these depths are not covered by the existing experimental infrastructure. In order to evaluate whether surface water infiltration is likely to occur outside the ‘window of detection’, i.e. the shallow streambed, a numerical groundwater model is used to simulate hydrological exchanges between the aquifer and the river. Transient simulations of the successfully validated model (Nash and Sutcliff efficiency of 0·91) suggest that surface water infiltration is marginal and that the possibility of significant volumes of surface water infiltrating into non‐monitored shallow streambed sediments can be excluded for the simulation period. Furthermore, the simulation results show that with increasing head differences between river and aquifer towards the end of the simulation period, the impact of streambed topography and hydraulic conductivity on spatial patterns of exchange flow rates decreases. A set of peak flow scenarios with altered groundwater‐surface water head gradients is simulated in order to quantify the potential for surface water infiltration during characteristic winter flow conditions following the observation period. The results indicate that, particularly at the beginning of peak flow conditions, head gradients are likely to cause substantial increase in surface water infiltration into the streambed. The study highlights the potential for the improvement of process understanding of hyporheic exchange flow patterns at the stream reach scale by simulating aquifer‐river exchange fluxes with a standard numerical groundwater model and a simple but robust model structure and parameterization. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Hydrogeologic controls on streamflow sensitivity to climate variation     

Anne Jefferson  Anne Nolin  Sarah Lewis  Christina Tague 《水文研究》2008,22(22):4371-4385

Climate models project warmer temperatures for the north‐west USA, which will result in reduced snowpacks and decreased summer streamflow. This paper examines how groundwater, snowmelt, and regional climate patterns control discharge at multiple time scales, using historical records from two watersheds with contrasting geological properties and drainage efficiencies. In the groundwater‐dominated watershed, aquifer storage and the associated slow summer recession are responsible for sustaining discharge even when the seasonal or annual water balance is negative, while in the runoff‐dominated watershed subsurface storage is exhausted every summer. There is a significant 1 year cross‐correlation between precipitation and discharge in the groundwater‐dominated watershed (r = 0·52), but climatic factors override geology in controlling the inter‐annual variability of streamflow. Warmer winters and earlier snowmelt over the past 60 years have shifted the hydrograph, resulting in summer recessions lasting 17 days longer, August discharges declining 15%, and autumn minimum discharges declining 11%. The slow recession of groundwater‐dominated streams makes them more sensitive than runoff‐dominated streams to changes in snowmelt amount and timing. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Russian eruption warning systems for aviation     

Christina Neal  Olga Girina  Sergey Senyukov  Alexander Rybin  Jeffrey Osiensky  Pavel Izbekov  Gail Ferguson 《Natural Hazards》2009,51(2):245-262

More than 65 potentially active volcanoes on the Kamchatka Peninsula and the Kurile Islands pose a substantial threat to aircraft on the Northern Pacific (NOPAC), Russian Trans-East (RTE), and Pacific Organized Track System (PACOTS) air routes. The Kamchatka Volcanic Eruption Response Team (KVERT) monitors and reports on volcanic hazards to aviation for Kamchatka and the north Kuriles. KVERT scientists utilize real-time seismic data, daily satellite views of the region, real-time video, and pilot and field reports of activity to track and alert the aviation industry of hazardous activity. Most Kurile Island volcanoes are monitored by the Sakhalin Volcanic Eruption Response Team (SVERT) based in Yuzhno-Sakhalinsk. SVERT uses daily moderate resolution imaging spectroradiometer (MODIS) satellite images to look for volcanic activity along this 1,250-km chain of islands. Neither operation is staffed 24 h per day. In addition, the vast majority of Russian volcanoes are not monitored seismically in real-time. Other challenges include multiple time-zones and language differences that hamper communication among volcanologists and meteorologists in the US, Japan, and Russia who share the responsibility to issue official warnings. Rapid, consistent verification of explosive eruptions and determination of cloud heights remain significant technical challenges. Despite these difficulties, in more than a decade of frequent eruptive activity in Kamchatka and the northern Kuriles, no damaging encounters with volcanic ash from Russian eruptions have been recorded.  相似文献   

Scaling Issues in Forest Succession Modelling   总被引：5，自引：0，他引：5  

Harald Bugmann  Marcus Lindner  Petra Lasch  Michael Flechsig  Beatrix Ebert  Wolfgang Cramer 《Climatic change》2000,44(3):265-289

This paper reviews scaling issues in forest succession modelling, focusing on forest gap models. Two modes of scaling are distinguished: (1) implicit scaling, i.e. taking scale-dependent features into account while developing model equations, and (2) explicit scaling, i.e. using procedures that typically involve numerical simulation to scale up the response of a local model in space and/or time. Special attention is paid to spatial upscaling methods, and downscaling is covered with respect to deriving scenarios of climatic change to drive gap models in impact assessments. When examining the equations used to represent ecological processes in forest gap models, it becomes evident that implicit scaling is relevant, but has not always been fully taken into consideration. A categorization from the literature is used to distinguish four methods for explicit upscaling of ecological models in space: (1) Lumping, (2) Direct extrapolation, (3) Extrapolation by expected value, and (4) Explicit integration. Examples from gap model studies are used to elaborate the potential and limitations of these methods, showing that upscaling to areas as large as 3000 km² is possible, given that there are no significant disturbances such as fires or insect outbreaks at the landscape scale. Regarding temporal upscaling, we find that it is important to consider migrational lags, i.e. limited availability of propagules, if one wants to assess the transient behaviour of forests in a changing climate, specifically with respect to carbon storage and the associated feedbacks to the atmospheric CO₂ content. Regarding downscaling, the ecological effects of different climate scenarios for the year 2100 were compared at a range of sites in central Europe. The derivation of the scenarios is based on (1) imposing GCM grid-cell average changes of temperature and precipitation on the local weather records; (2) a qualitative downscaling technique applied by the IPCC for central and southern Europe; and (3) statistical downscaling relating large-scale circulation patterns to local weather records. Widely different forest compositions may be obtained depending on the local climate scenario, suggesting that the downscaling issue is quite important for assessments of the ecological impacts of climatic change on forests.  相似文献   

Regional ash fall hazard I: a probabilistic assessment methodology   总被引：1，自引：0，他引：1  

Susanna Jenkins  Christina Magill  John McAneney  Russell Blong 《Bulletin of Volcanology》2012,74(7):1699-1712

Volcanic ash is one of the farthest-reaching volcanic hazards and ash produced by large magnitude explosive eruptions has the potential to affect communities over thousands of kilometres. Quantifying the hazard from ash fall is problematic, in part because of data limitations that make eruption characteristics uncertain but also because, given an eruption, the distribution of ash is then controlled by time and altitude-varying wind conditions. Any one location may potentially be affected by ash falls from one, or a number of, volcanoes so that volcano-specific studies may not fully capture the ash fall hazard for communities in volcanically active areas. In an attempt to deal with these uncertainties, this paper outlines a probabilistic framework for assessing ash fall hazard on a regional scale. The methodology employs stochastic simulation techniques and is based upon generic principles that could be applied to any area, but is here applied to the Asia-Pacific region. Average recurrence intervals for eruptions greater than or equal to Volcanic Explosivity Index 4 were established for 190 volcanoes in the region, based upon the eruption history of each volcano and, where data were lacking, the averaged eruptive behaviour of global analogous volcanoes. Eruption histories are drawn from the Smithsonian Institution’s Global Volcanism Program catalogue of Holocene events and unpublished data, with global analogues taken from volcanoes of the same type category: Caldera, Large Cone, Shield, Lava dome or Small Cone. Simulated are 190,000 plausible eruption scenarios, with ash dispersal for each determined using an advection–diffusion model and local wind conditions. Key uncertainties are described by probability distributions. Modelled results include the annual probability of exceeding given ash thicknesses, summed over all eruption scenarios and volcanoes. A companion paper describes the results obtained for the Asia-Pacific region  相似文献   

Exposure of dissolved organic matter to UV-radiation increases bacterial growth efficiency in a clear-water Alpine stream and its adjacent groundwater     

Christina?Fasching  Tom?J.?BattinEmail author 《Aquatic Sciences - Research Across Boundaries》2012,74(1):143-153

In many streams, microbial growth largely relies on terrigenous organic carbon that has been initially stored in soils and that is generally believed to be recalcitrant to microbial metabolism. The various mechanisms that underlie the availability of terrigenous organic carbon as it enters streams remain poorly understood. One possible mechanism can be photodegradation of terrigenous dissolved organic carbon (DOC) upon exposure to sunlight in streamwater. To explore this, we experimentally exposed streamwater, shallow groundwater and soil water from a clear-water Alpine headwater stream, and both soil and algal extracts, to UV-radiation and studied the effects on DOC optical properties and implications for microbial growth. Our results on the apparent quantum yield suggest that DOC from groundwater and soil water is more resistant to photodegradation than DOC in the streamwater itself. This would highlight the relevance of the exposure history of DOC to sunlight. Overall, UV-radiation decreased the aromaticity (as SUVA254) of the DOC and reduced its molecular weight as indicated by the slope ratio, S _R (S _275–295/S _350–400). UV-treatment significantly increased bacterial growth rate and bacterial growth efficiency in the streamwater, soil water, groundwater and soil extract but not in algal extract. Our findings suggest photodegradation as one mechanism that contributes to the microbial utilisation of terrigenous DOC even in clear-water streams.  相似文献   

Integration of Local Observations into the One Dimensional Fog Model PAFOG   总被引：1，自引：0，他引：1  

Christina Thoma  Werner Schneider  Matthieu Masbou  Andreas Bott 《Pure and Applied Geophysics》2012,169(5-6):881-893

The numerical prediction of fog requires a very high vertical resolution of the atmosphere. Owing to a prohibitive computational effort of high resolution three dimensional models, operational fog forecast is usually done by means of one dimensional fog models. An important condition for a successful fog forecast with one dimensional models consists of the proper integration of observational data into the numerical simulations. The goal of the present study is to introduce new methods for the consideration of these data in the one dimensional radiation fog model PAFOG. First, it will be shown how PAFOG may be initialized with observed visibilities. Second, a nudging scheme will be presented for the inclusion of measured temperature and humidity profiles in the PAFOG simulations. The new features of PAFOG have been tested by comparing the model results with observations of the German Meteorological Service. A case study will be presented that reveals the importance of including local observations in the model calculations. Numerical results obtained with the modified PAFOG model show a distinct improvement of fog forecasts regarding the times of fog formation, dissipation as well as the vertical extent of the investigated fog events. However, model results also reveal that a further improvement of PAFOG might be possible if several empirical model parameters are optimized. This tuning can only be realized by comprehensive comparisons of model simulations with corresponding fog observations.  相似文献   

Impact of Heterogeneity on Oxygen Transfer in a Fluctuating Capillary Fringe          下载免费PDF全文

Christina M. Haberer  Massimo Rolle  Olaf A. Cirpka  Peter Grathwohl 《Ground water》2015,53(1):57-70

We performed quasi‐two‐dimensional flow through laboratory experiments to study the effect of a coarse‐material inclusion, located in the proximity of the water table, on flow and oxygen transfer in the capillary fringe. The experiments investigate different phases of mass transfer from the unsaturated zone to anoxic groundwater under both steady‐state and transient flow conditions, the latter obtained by fluctuating the water table. Monitoring of flow and transport in the different experimental phases was performed by visual inspection of the complex flow field using a dye tracer solution, measurement of oxygen profiles across the capillary fringe, and determination of oxygen fluxes in the effluent of the flow‐through chamber. Our results show significant effects of the coarse‐material inclusion on oxygen transfer during the different phases of the experiments. At steady state, the oxygen flux across the unsaturated/saturated interface was considerably enhanced due to flow focusing in the fully water‐saturated coarse‐material inclusion. During drainage, a zone of higher water saturation formed in the fine material overlying the coarse lens. The entrapped oxygen‐rich aqueous phase contributed to the total amount of oxygen supplied to the system when the water table was raised back to its initial level. In case of imbibition, pronounced air entrapment occurred in the coarse lens, causing oxygen to partition between the aqueous and gaseous phases. The oxygen mass supplied to the anoxic groundwater following the imbibition event was found to be remarkably higher (approximately seven times) in the heterogeneous system compared with a similar experiment performed in a homogeneous porous medium.  相似文献