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- PublicationAccès libre
- PublicationMétadonnées seulementEvaluation and field-scale application of an analytical method to quantify groundwater discharge using mapped streambed temperatures(2007)
;Schmidt, Christian ;Conant, Brewster ;Bayer-Raich, MartiA method for calculating groundwater discharge through a streambed on a sub-reach to a reach scale has been developed using data from plan-view mapping of streambed temperatures at a uniform depth along a reach of a river or stream. An analytical solution of the one-dimensional steady-state heat-diffusion-advection equation was used to determine fluxes from observed temperature data. The method was applied to point measurements of streambed temperatures used to map a 60 m tong reach of a river by Conant Jr. [Conant Jr. B., 2004. Delineating and quantifying ground water discharge zones using streambed temperatures. Ground Water 42(2), 243-257] and relies on the underlying assumption that streambed temperatures are in a quasi-steady-state during the period of mapping. The analytical method was able to match the values and pattern of flux previously obtained using an empirical, relationship that related streambed temperatures to fluxes obtained from piezometers and using Darcy's taw. A second independent test of the analytical method using temperature mapping and seepage meter fluxes along a first-order stream confirmed the validity of the approach. The USGS numerical. heat transport model VS2DH was also used to evaluate the thermal. response of the streambed sediments to transient variations in surface water temperatures and showed that quasi-steady-state conditions occurred for most, but not all, conditions. During mapping events in the winter, quasi-steady-state conditions were typically observed for both high and tow groundwater discharge conditions, but during summer mapping events quasi-steady-state conditions were typically not achieved at low flux areas or where measurements were made at shallow depths. Major advantages of using this analytical method include: it can be implemented using a spreadsheet; it does not require the installation or testing of piezometers or seepage meters (although they would help to confirm the results); and it needs only a minimal amount of input data related to water temperatures and the thermal properties of water and the sediments. The field results showed the analytical solution tends to underestimate high fluxes. However, a sensitivity analysis of possible model inputs shows the solution is relatively robust and not particularly sensitive to small uncertainties in input data and can produce reasonable flux estimates without the need for calibration. (c) 2007 Elsevier B.V. All rights reserved.
- PublicationMétadonnées seulementA study of long-term MTBE attenuation in the borden aquifer, Ontario, Canada(1998)
;Barker, James FIn 1988 and 1989, a natural gradient tracer test was performed in the shallow aerobic sand aquifer at Canadian Forces Base (CFB) Borden. A mixture of ground water containing dissolved oxygenated gasoline was injected below the water table along with chloride (Cl-) as a conservative tracer. The migration of BTEX, MTBE, and Cl- was monitored in detail for 16 months. The mass of BTEX compounds in the plume diminished significantly with time due to intrinsic aerobic biodegradation, while MTBE showed only a small decrease in mass over the 16-month period. In 1995/96, a comprehensive ground water sampling program was undertaken to define the mass of MTBE still present in the aquifer. Since the plume had migrated into an unmonitored section of the Borden Aquifer, numerical modeling and geostatistical methods were applied to define an optimal sampling grid and to improve the level of confidence in the results. A drive point profiling system was used to obtain ground water samples. Numerical modeling with no consideration of degradation predicted maximum concentrations in excess of 3000 mu g/L field sampling found maximum concentrations of less than 200 mu g/L. A mass balance for the remaining MTBE mass in the aquifer eight years after injection showed that only 3% of the original mass remained. Sorption, volatilization, abiotic degradation, and plant uptake are not considered significant attenuation processes for the field conditions. Therefore, we suggest that biodegradation may have played a major role in the attenuation of MTBE within the Borden Aquifer.
- PublicationMétadonnées seulementModeling the impact of ethanol on the persistence of benzene in gasoline-contaminated groundwater(2002)
;Molson, John W ;Barker, James F ;Frind, Emil O The effect of ethanol on the persistence of benzene in gasoline-contaminated aquifers is simulated using a multicomponent reactive transport model. The conceptual model includes a residual gasoline source which is dissolving at the water table into an aquifer containing a limited amount of dissolved oxygen. The coupled processes include nonaqueous phase liquid (NAPL) source dissolution, transport of the dissolved components, and competitive aerobic biodegradation. Comparisons are made between dissolved benzene plumes from a gasoline spill and those from an otherwise equivalent spill containing 10% ethanol (gasohol). Simulations have shown that under some conditions a 10% ethanol component in gasoline can extend the travel distance of a benzene plume by up to 150% relative to that from an equivalent ethanol-free gasoline spill. The increase occurs because ethanol preferentially consumes oxygen, which reduces the biodegradation rate of benzene. The impact is limited, however, because sufficient oxygen disperses behind the ethanol plume into the slightly retarded benzene plume. A sensitivity analysis for two common spill scenarios showed that background oxygen concentrations and benzene retardation have the most significant influence on ethanol-induced benzene persistence. The results are highly relevant in light of the increasing use of ethanol-enhanced fuels throughout the world and the forthcoming ban of methyl tertiary-butyl-ether (MTBE) in California and its probable replacement by ethanol by the end of 2002.
- PublicationMétadonnées seulementTransport behaviour and natural attenuation of organic contaminants at spill sites(2004)
;Butler, Barbara JOrganic contaminants pose a significant threat to groundwater resources. These contaminants are often released as nonaqueous phase liquids (NAPLs) during spills of, for example, gasoline, crude oil, creosote, coal tar or chlorinated solvents. Once released, the liquids seep downward and dissolve into the groundwater. In many cases, the impacted groundwater contains a mixture of contaminants, either due to the complexity of the NAPL (e.g., gasoline) or due to co-disposal/co-spillage (e.g., landfill leachates). Many organic contaminants are hazardous to human health and the environment and therefore threaten our potable water resources and natural ecosystems. Active remediation of contaminated groundwater is often very expensive so that cost-effective alternatives have to be found. If natural attenuation is intended to be used as a means of achieving specific remedial objectives at a contaminated site, it will require a sound understanding of the ongoing processes as well as careful control and monitoring ("monitored natural attenuation" (MNA)). Therefore, a major goal of remediation research today is to develop methods to predict the mass fate of multiple organic compounds in heterogeneous aquifers under natural conditions. (C) 2004 Elsevier Ireland Ltd. All rights reserved.
- PublicationMétadonnées seulementInfluence of transient flow on contaminant biodegradation(2001)
; ;Durrant, Graham C ;Molson, John WFrind, Emil OThe rate of biodegradation in contaminated aquifers depends to a large extent on dispersive mixing processes that are now generally accepted to result from spatial variations in the velocity field. It has been shown, however, that transient flow fields can also contribute to dispersive mixing. The influence of transient flow on biodegrading contaminants is particularly important since it can enhance mixing with electron acceptors, further promoting the reactive process. Using numerical simulations, the effect of transient flow on the behavior of a biodegradable contaminant is evaluated here both with respect to the development of apparently large horizontal transverse dispersion and also,vith respect to enhanced mixing between the substrate (electron donor) and electron acceptor. The numerical model BIO3D, which solves for advective-dispersive transport coupled with Monod-type biodegradation of substrates in the presence of an electron acceptor, was used for the simulations. The model was applied in a two-dimensional plan view mode considering a single substrate. Transient flow fields were found to yield larger apparent transverse dispersion because the longitudinal dispersivity also acts transverse to the mean flow direction. In the reactive case, the transient now field increases substrate-oxygen mixing, which in turn enhances the overall rate of biodegradation. The results suggest that in the case of moderate changes of flow directions, a steady-state flow field can be justified, thereby avoiding the higher computational costs of a fully transient simulation. The use of a higher transverse horizontal dispersivity in a steady flow field can, under these conditions, adequately forecast plume development.
- PublicationMétadonnées seulementLaboratory evidence of MTBE biodegradation in Borden aquifer material(2003)
; ;Butler, Barbara J ;Church, Clinton D ;Barker, James FNadarajah, NalinaMainly due to intrinsic biodegradation, monitored natural attenuation can be an effective and inexpensive remediation strategy at petroleum release sites. However, gasoline additives such as methyl tert-butyl ether (MTBE) can jeopardize this strategy because these compounds often degrade, if at all, at a slower rate than the collectively benzene, toluene, ethylbenzene and the xylene (BTEX) compounds. Investigation of whether a compound degrades under certain conditions, and at what rate, is therefore important to the assessment of the intrinsic remediation potential of aquifers. A natural gradient experiment with dissolved MTBE-containing gasoline in the shallow, aerobic sand aquifer at Canadian Forces Base (CFB) Borden (Ontario, Canada) from 1988 to 1996 suggested that biodegradation was the main cause of attenuation for MTBE within the aquifer. This laboratory study demonstrates biologically catalyzed MTBE degradation in Borden aquifer-like environments, and so supports the idea that attenuation due to biodegradation may have occurred in the natural gradient experiment. In an experiment with batch microcosms of aquifer material, three of the microcosms ultimately degraded MTBE to below detection, although this required more than 189 days (or >300 days in one case). Failure to detect the daughter product tert-butyl alcohol (TBA) in the field and the batch experiments could be because TBA was more readily degradable than MTBE under Borden conditions. (C) 2002 Elsevier Science B.V. All rights reserved.
- PublicationMétadonnées seulementPulsed gas injection: A minimum effort approach for enhanced natural attenuation of chlorobenzene in contaminated groundwater(2009)
;Balcke, Gerd Ulrich ;Paschke, Heidrun ;Vogt, CarstenChlorobenzene-contaminated groundwater was used to assess pulsed gas sparging as a minimum effort aeration strategy to enhance intrinsic natural attenuation. In contrast to existing biosparging operations, oxygen was supplied at minimum rate by reducing the gas injection frequency to 0.33 day(-1). Field tests in a model aquifer were conducted in a 12 m long reactor, filled with indigenous aquifer material and continuously recharged with polluted groundwater over 3 years. The closed arrangement allowed yield balances, cost accounting as well as the investigation of spatial distributions of parameters which are sensitive to the biodegradation process. Depending on the injection frequency and on the gas chosen for injection (pure oxygen or air) oxygen-deficient conditions prevailed in the aquifer. Despite the limiting availability of dissolved oxygen in the groundwater, chlorobenzene degradation under oxygen-deficient conditions proved to be more effective than under conditions with dissolved oxygen being available in high concentrations. (C) 2009 Elsevier Ltd. All rights reserved.
- PublicationMétadonnées seulementBiodegradation modelling of a dissolved gasoline plume applying independent laboratory and field parameters(2000)
; ;Molson, John W ;Frind, Emil OBarker, James FBiodegradation of organic contaminants in groundwater is a microscale process which is often observed on scales of 100s of metres or larger. Unfortunately, there are no known equivalent parameters for characterizing the biodegradation process at the macroscale as there are, for example, in the case of hydrodynamic dispersion. Zero- and first-order degradation rates estimated at the laboratory scale by model fitting generally overpredict the rate of biodegradation when applied to the field scale because limited electron acceptor availability and microbial growth are not considered. On the other hand, Field-estimated zero- and first-order rates are often not suitable for predicting plume development because they may oversimplify or neglect several key field scale processes. phenomena and characteristics. This study uses the numerical model BIO3D to link the laboratory and field scales by applying laboratory-derived Monod kinetic degradation parameters to simulate a dissolved gasoline field experiment at the Canadian Forces Base (CFB) Borden. All input parameters were derived from independent laboratory and field measurements or taken from the literature a priori to the simulations. The simulated results match the experimental results reasonably well without model calibration. A sensitivity analysis on the most uncertain input parameters showed only a minor influence on the simulation results. Furthermore, it is shown that the flow field, the amount of electron acceptor (oxygen) available, and the Monod kinetic parameters have a significant influence on the simulated results, it is concluded that laboratory-derived Monod kinetic parameters can adequately describe field scale degradation, provided all controlling factors are incorporated in the field scale model. These factors include advective-dispersive transport of multiple contaminants and electron accepters and large-scale spatial heterogeneities. (C) 2000 Elsevier Science B.V. All rights reserved.
- PublicationMétadonnées seulementNatural attenuation research at the contaminated megasite Zeitz(: Elsevier Science Bv, 2002)
; ;Dahmke, Andreas ;Dietrich, Peter ;Dietze, Michael ;Godeke, Stefan ;Richnow, Hans Hermann ;Schirmer, Kristin ;Weiss, HolgerTeutsch, GeorgMonitored natural attenuation (MNA) as a possible remediation strategy has been intensively discussed over the last several years. However, in order to apply natural attenuation (NA) as a rational remediation option, suitable methods for the assessment and evaluation of the efficiency of these processes must be developed, the sustainability of the NA processes has to be demonstrated and regulatory and public acceptance has to be established. The RETZ-INA (Reference test site Zeitz for the implementation of the "Natural Attenuation" approach) Project deals with the implementation of the MNA approach as a remediation concept at the heavily contaminated former industrial field site Zeitz (Germany). Due to the long history of industrial production, the multiple contaminant sources present and the large spatial extent.