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  • 11
    In: Water Resources Research, July 2010, Vol.46(7), pp.n/a-n/a
    Description: Biodegradation of continuously emitted compounds that need a dissolved reaction partner, which is not jointly introduced with the contaminant into the subsurface, is mainly controlled by transverse dispersive mixing. Previous analytical approaches of evaluating mixing‐controlled bioreactive transport in steady state have been based on the assumption that the bulk aqueous‐phase concentration of the reactants is directly available to the specific biomass catalyzing the reaction. These models predict a very narrow stripe of active biomass with high specific biomass concentration. Experimental studies have indicated that such behavior may be unrealistic, particularly for anaerobic biodegradation. I extend the previous analysis to include kinetic solute uptake by the biomass, expressed as a first‐order mass‐transfer process coupled to dual Monod kinetics in the bio‐available domain. The approach is based on the evaluation of conservative components undergoing advective‐dispersive transport, the solution of a quadratic speciation problem within the immobile bio domain, and iterative simulation of linear transport of a single reactive constituent in steady state. Convergence is typically achieved within less than ten iterations. The comparison with simulations assuming instantaneous solute uptake by the biomass indicate that mass‐transfer kinetics may explain larger overlap of reactive constituents and a wider spatial distribution of specific biomass observed in experiments. Depending on the rate coefficient of mass transfer, the overall transformation of the contaminant may be significantly reduced or only slightly shifted to a region farther downstream.
    Keywords: Biodegradation ; Kinetic Mass Transfer ; Reactive Transport ; Monod Kinetics
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 12
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  • 13
    Language: English
    In: Procedia Environmental Sciences, 2015, Vol.25, pp.66-73
    Description: Transverse mixing by local-scale dispersion plays a pivotal role in mixing-controlled reactions of continuously injected compounds and controls the length of steady-state plumes. In two-dimensional heterogeneous media, high-permeability inclusions cause a net enhancement of transverse mixing because the effect of reducing the transverse diffusion lengths in such inclusions prevails over the effect of reduced diffusion time. A simple scaling law for effective transverse mixing in two-dimensional domains could be derived by transferring the steady-state transport equation into streamline-coordinates. The net enhancement factor does not depend on scale. Closed-form expressions for effective mixing and its uncertainty could be derived for uniform-in-the-mean velocity in a multi-Gaussian log-conductivity field with isotropic, exponential covariance function. These expressions could be used to estimate the uncertainty of steady-state concentrations and reactive-plume lengths. The picture is more complex in three-dimensional domains where spatially variable orientation of anisotropy can lead to twisting streamlines. Macroscopically helical flow can lead to an enhancement of transverse mixing by about an order of magnitude more than the flow-focusing effects in two-dimensional flows.
    Keywords: Transverse Dispersion ; Solute Mixing ; Reactive Plume Length ; Heterogeneity ; Anisotropy ; Helicity ; Flow Topology ; Environmental Sciences
    ISSN: 1878-0296
    E-ISSN: 1878-0296
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  • 14
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  • 15
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  • 16
    Language: English
    In: Journal of Contaminant Hydrology, June, 2013, Vol.149, p.13(14)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.jconhyd.2013.02.009 Byline: Stephane K. Ngueleu, Peter Grathwohl, Olaf A. Cirpka Abstract: Colloidal particles can act as carriers for adsorbing pollutants, such as hydrophobic organic pollutants, and enhance their mobility in the subsurface. In this study, we investigate the influence of colloidal particles on the transport of pesticides through saturated porous media by column experiments. We also investigate the effect of particle size on this transport. The model pesticide is lindane (gamma-hexachlorocyclohexane), a representative hydrophobic insecticide which has been banned in 2009 but is still used in many developing countries. The breakthrough curves are analyzed with the help of numerical modeling, in which we examine the minimum model complexity needed to simulate such transport. The transport of lindane without particles can be described by advective-dispersive transport coupled to linear three-site sorption, one site being in local equilibrium and the others undergoing first-order kinetic sorption. In the presence of mobile particles, the total concentration of mobile lindane is increased, that is, lindane is transported not only in aqueous solution but also sorbed onto the smallest, mobile particles. The models developed to simulate separate and associated transport of lindane and the particles reproduced the measurements very well and showed that the adsorption/desorption of lindane to the particles could be expressed by a common first-order rate law, regardless whether the particles are mobile, attached, or strained. Article History: Received 11 August 2012; Revised 17 February 2013; Accepted 25 February 2013
    ISSN: 0169-7722
    Source: Cengage Learning, Inc.
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  • 17
    Language: English
    In: Water Resources Management, Sept, 2013, Vol.27(12), p.4349(20)
    Description: Byline: Ashutosh Singh (1), Claudius M. Burger (1), Olaf A. Cirpka (1) Keywords: Urban hydrogeology; Groundwater management; MODFLOW; Numerical modeling; Response matrix; Optimization Abstract: In rapidly developing urban areas of emerging countries, increased water demand has led to enormous groundwater withdrawal, calling out for sustainable groundwater management. We suggest implementing a sustainable pumping rate concept based on numerical modeling of the managed aquifer. Sustainability is achieved by constraints regarding (1) a minimum groundwater discharge rate to gaining rivers (ecological constraint) and (2) a maximum drawdown along the city boundaries (social constraints) to prevent excessive groundwater depletion in the neighboring peri-urban and rural areas. The total groundwater extraction is maximized subject to these constraints, leading to specific extraction patterns throughout the city, depending upon the values set for the constraints. The optimization is performed by linear programming. For a given extraction rate, the two constraints can be traded off by the groundwater manager, causing different wells to be activated or deactivated. We demonstrate the applicability of the methodology by the example of the city of Lucknow, India, but it can be transferred to other cities facing conflicts of managing groundwater resources. Author Affiliation: (1) Center for Applied Geoscience (ZAG), University of Tubingen, Holderlinstr. 12, 72074, Tubingen, Germany Article History: Registration Date: 05/08/2013 Received Date: 15/03/2013 Accepted Date: 05/08/2013 Online Date: 18/08/2013
    Keywords: Hydrogeology -- Models ; Water Resource Management -- Models ; Aquifers -- Models ; Groundwater -- Models ; Developing Countries -- Models
    ISSN: 0920-4741
    E-ISSN: 15731650
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  • 18
    Language: English
    In: Quaternary Research, July 2012, Vol.78(1), pp.139-148
    Description: The Qaidam Basin is the largest intermontane basin of the northeastern Tibetan Plateau and contains a continuous Cenozoic sequence of lacustrine sediments. A ~ 1000-m-deep drilling (SG-1) with an average core recovery of ~ 95% was carried out in the depocenter of the Chahansilatu playa (sub-depression) in the western Qaidam Basin, aimed to obtain a high-resolution record of the paleoenvironmental evolution and the erosion history. Stepwise alternating field and thermal demagnetization, together with rock magnetic results, revealed a stable remanent magnetization for most samples, carried by magnetite. The polarity sequence consisted of 16 normal and 15 reverse zones which can be correlated with chrons 1n to 2An of the global geomagnetic polarity time scale. Magnetostratigraphic results date the entire core SG-1 at ~ 2.77 Ma to ~ 0.1 Ma and yielded sediment accumulation rate (SAR) ranging from 26.1 cm/ka to 51.5 cm/ka. Maximum SARs occurred within the intervals of ~ 2.6–2.2 Ma and after ~ 0.8 Ma, indicating two episodes of erosion, which we relate to pulse tectonic uplift of the NE Tibetan Plateau with subsequent global cooling.
    Keywords: Magnetostratigraphy ; Tectonic Evolution ; Qaidam Basin ; Tibetan Plateau ; Geology
    ISSN: 0033-5894
    E-ISSN: 1096-0287
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  • 19
    Language: English
    In: Journal of Contaminant Hydrology, Sept, 2012, Vol.138-139, p.22(18)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.jconhyd.2012.05.007 Byline: Olaf A. Cirpka, Massimo Rolle, Gabriele Chiogna, Felipe P.J. de Barros, Wolfgang Nowak Keywords: Transverse dispersion; Groundwater transport; Mixing-controlled reactions; Stochastic subsurface hydrology Abstract: We study plumes originating from continuous sources that require a dissolved reaction partner for their degradation. The length of such plumes is typically controlled by transverse mixing. While analytical expressions have been derived for homogeneous flow fields, incomplete characterization of the hydraulic conductivity field causes uncertainty in predicting plume lengths in heterogeneous domains. In this context, we analyze the effects of three sources of uncertainty: (i) The uncertainty of the effective mixing rate along the plume fringes due to spatially varying flow focusing, (ii) the uncertainty of the volumetric discharge through (and thus total mass flux leaving) the source area, and (iii) different parameterizations of the Darcy-scale transverse dispersion coefficient. The first two are directly related to heterogeneity of hydraulic conductivity. In this paper, we derive semi-analytical expressions for the probability distribution of plume lengths at different levels of complexity. The results are compared to numerical Monte Carlo simulations. Uncertainties in mixing and in the source strength result in a statistical distribution of possible plume lengths. For unconditional random hydraulic conductivity fields, plume lengths may vary by more than one order of magnitude even for moderate degrees of heterogeneity. Our results show that the uncertainty of volumetric flux through the source is the most relevant contribution to the variance of the plume length. The choice of different parameterizations for the local dispersion coefficient leads to differences in the mean estimated plume length. Article History: Received 14 January 2012; Revised 2 April 2012; Accepted 23 May 2012
    Keywords: Hydrogeology -- Analysis ; Monte Carlo Methods -- Analysis ; Groundwater -- Analysis
    ISSN: 0169-7722
    Source: Cengage Learning, Inc.
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  • 20
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