Geyer, Tobias

[["dc.bibliographiccitation.firstpage","452"],["dc.bibliographiccitation.issue","3-4"],["dc.bibliographiccitation.journal","Journal of Hydrology"],["dc.bibliographiccitation.lastpage","463"],["dc.bibliographiccitation.volume","348"],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Birk, Steffen"],["dc.contributor.author","Liedl, Rudolf"],["dc.contributor.author","Sauter, Martin"],["dc.date.accessioned","2018-11-07T11:19:07Z"],["dc.date.available","2018-11-07T11:19:07Z"],["dc.date.issued","2008"],["dc.description.abstract","The estimation of the temporal distribution of recharge in karst aquifers is a challenge due to Large heterogeneities in geometric and hydraulic parameters of the vadose and phreatic zone. This article provides a time-continuous approach for the estimation of inflow into the conduit system of a karst aquifer which consists of the sum of direct recharge and flow from the fissured matrix blocks into the conduit system. The approach employs the first time derivative of the spring hydrograph and the recession coefficient of the conduit system for the determination of this function. The first time derivative of the hydrograph describes the rate of change in spring discharge. It reflects the ratio of inflow to and outflow from the conduit system. The recession coefficient depends on the hydraulic diffusivity, which controls the velocity of the signal transmission through the system. As shown in parameter studies with a simplified serial two-reservoir model, direct recharge into the conduit system clearly dominates the early hydrograph response during recharge events even if the fraction of direct recharge represents just a few percent of total recharge. This behaviour is caused by a large contrast in recession coefficients between conduit system and fissured matrix blocks. The direct recharge component can therefore be separated from the estimated total inflow into the conduit system. Estimation of inflow into the conduit system of the Gallusquelle spring (Swabian Alb, Germany) after a storm event yields similar results as those obtained from parameter studies. The separation of the direct recharge component is in agreement with information from an independent isotope study. The methodology has been further applied to a recharge event initiated by snowmelt. As a result of daily variations of the air temperature, a clearly diurnal cycle of inflow into the conduit system is estimated that is not easily recognizable from the spring hydrograph. The applicability of the proposed methodology requires knowledge about the magnitude of the recession coefficient of the conduit system. The characteristics of the conduit system become also apparent in the rapid transport of tracers. The recession coefficient of the conduit system can, for example, directly be estimated from the reciprocal of the mean tracer travel time. However, for this type of analysis, only tracer experiments, covering the extent of the catchment should be taken into account. For the Gallusquelle catchment the recession coefficient obtained from the tracer experiment corresponds to that obtained from hydrograph recession analysis. The estimated inflow into the conduit system and interpretation of temporal distribution of recharge is therefore based on a plausible and physically based parameter. (c) 2007 Elsevier B.V. All rights reserved."],["dc.identifier.doi","10.1016/j.jhydrol.2007.10.015"],["dc.identifier.isi","000252749000018"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/55196"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Elsevier Science Bv"],["dc.relation.issn","0022-1694"],["dc.title","Quantification of temporal distribution of recharge in karst systems from spring hydrographs"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","W11503"],["dc.bibliographiccitation.journal","Water Resources Research"],["dc.bibliographiccitation.volume","47"],["dc.contributor.author","Reimann, Thomas"],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Shoemaker, W. Barclay"],["dc.contributor.author","Liedl, Rudolf"],["dc.contributor.author","Sauter, Martin"],["dc.date.accessioned","2018-11-07T08:49:58Z"],["dc.date.available","2018-11-07T08:49:58Z"],["dc.date.issued","2011"],["dc.description.abstract","Well-developed karst aquifers consist of highly conductive conduits and a relatively low permeability fractured and/or porous rock matrix and therefore behave as a dual-hydraulic system. Groundwater flow within highly permeable strata is rapid and transient and depends on local flow conditions, i.e., pressurized or nonpressurized flow. The characterization of karst aquifers is a necessary and challenging task because information about hydraulic and spatial conduit properties is poorly defined or unknown. To investigate karst aquifers, hydraulic stresses such as large recharge events can be simulated with hybrid (coupled discrete continuum) models. Since existing hybrid models are simplifications of the system dynamics, a new karst model (ModBraC) is presented that accounts for unsteady and nonuniform discrete flow in variably saturated conduits employing the Saint-Venant equations. Model performance tests indicate that ModBraC is able to simulate (1) unsteady and nonuniform flow in variably filled conduits, (2) draining and refilling of conduits with stable transition between free-surface and pressurized flow and correct storage representation, (3) water exchange between matrix and variably filled conduits, and (4) discharge routing through branched and intermeshed conduit networks. Subsequently, ModBraC is applied to an idealized catchment to investigate the significance of free-surface flow representation. A parameter study is conducted with two different initial conditions: (1) pressurized flow and (2) free-surface flow. If free-surface flow prevails, the systems is characterized by (1) a time lag for signal transmission, (2) a typical spring discharge pattern representing the transition from pressurized to free-surface flow, and (3) a reduced conduit-matrix interaction during free-surface flow."],["dc.description.sponsorship","Deutsche Forschungsgemeinschaft (DFG) [LI 727/11-1, SA 501/24-1]"],["dc.identifier.doi","10.1029/2011WR010446"],["dc.identifier.isi","000296622300002"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/21580"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Amer Geophysical Union"],["dc.relation.issn","0043-1397"],["dc.title","Effects of dynamically variable saturation and matrix-conduit coupling of flow in karst aquifers"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","36"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","GROUND WATER"],["dc.bibliographiccitation.lastpage","45"],["dc.bibliographiccitation.volume","45"],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Birk, Steffen"],["dc.contributor.author","Licha, Tobias"],["dc.contributor.author","Liedl, Rudolf"],["dc.contributor.author","Sauter, Martin"],["dc.date.accessioned","2018-11-07T11:06:37Z"],["dc.date.available","2018-11-07T11:06:37Z"],["dc.date.issued","2007"],["dc.description.abstract","A method to estimate reactive transport parameters as well as geometric conduit parameters from a multitracer test in a karst aquifer is provided. For this purpose, a calibration strategy was developed applying the two-region nonequilibrium model CXTFIT. The ambiguity of the model calibration was reduced by first calibrating the model with respect to conservative tracer breakthrough and later transferring conservative transport parameters to the reactive model calibration. The reactive transport parameters were only allowed to be within a defined sensible range to get reasonable calibration values. This calibration strategy was applied to breakthrough curves obtained from a large-scale multitracer test, which was performed in a karst aquifer of the Swabian Alb, Germany. The multitracer test was conducted by the simultaneous injection of uranine, sulforhodamine G, and tinopal CBS-X. The model succeeds to represent the tracer breakthrough curves (TBCs) of uranine and sulforhodamine G and verifies that tracer-rock interactions preferably occur in the immobile fluid region, although the fraction of this region amounts to only 3.5% of the total water. However, the model failed to account for the long tailing observed in the TBC of tinopal CBS-X. Sensitivity analyses reveal that model results for the conservative tracer transport are most sensitive to average velocity and volume fraction of the mobile fluid region, while dispersion and mass transfer coefficients are least influential. Consequently, reactive tracer calibration allows the determination of sorption sites in the mobile and immobile fluid region at small retardation coefficients."],["dc.identifier.doi","10.1111/j.1745-6584.2006.00261.x"],["dc.identifier.isi","000243474000011"],["dc.identifier.pmid","17257337"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/52358"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Blackwell Publishing"],["dc.relation.issn","0017-467X"],["dc.title","Multitracer test approach to characterize reactive transport in karst aquifers"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","797"],["dc.bibliographiccitation.issue","4"],["dc.bibliographiccitation.journal","ENVIRONMENTAL GEOLOGY"],["dc.bibliographiccitation.lastpage","808"],["dc.bibliographiccitation.volume","57"],["dc.contributor.author","Heinz, B."],["dc.contributor.author","Birk, S."],["dc.contributor.author","Liedl, Rudolf"],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Straub, K. L."],["dc.contributor.author","Andresen, J."],["dc.contributor.author","Bester, Kai"],["dc.contributor.author","Kappler, A."],["dc.date.accessioned","2018-11-07T08:31:00Z"],["dc.date.available","2018-11-07T08:31:00Z"],["dc.date.issued","2009"],["dc.description.abstract","The concurrent use of karst aquifers as drinking water resources and receptors of combined sewer overflow lacking appropriate pre-treatment may cause conflicts between drinking water supply and storm water management. A storm water tank (SWT) for combined wastewater is identified as the source of sporadic contamination of a karst spring (Gallusquelle, \"Schwabische Alb\", SW Germany) used for public water supply. Spring water quality was examined by routine and event sampling and by evaluating physicochemical and microbiological parameters. The total number of microbial colonies growing at 20A degrees C and the number of Escherichia coli colonies rose to values up to four orders of magnitude higher than background, 2-5 days after overflow of the SWT. High concentrations of chloride, sodium, and total organic carbon (TOC) and high values of turbidity coincide with this increase. However, high bacterial contamination is also observed while turbidity and TOC are low. Several wastewater-related organic micro-pollutants such as chlorinated and non-chlorinated organophosphates were detected in the SWT and, depending on their K (ow) values and their biodegradability, in lower concentrations at the spring."],["dc.description.sponsorship","German Research Foundation (DFG) [LI 727/10]; Emmy Noether fellowship"],["dc.identifier.doi","10.1007/s00254-008-1359-0"],["dc.identifier.isi","000264174100007"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/17021"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Springer"],["dc.relation.issn","0943-0105"],["dc.title","Water quality deterioration at a karst spring (Gallusquelle, Germany) due to combined sewer overflow: evidence of bacterial and micro-pollutant contamination"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","381"],["dc.bibliographiccitation.issue","3"],["dc.bibliographiccitation.journal","GROUND WATER"],["dc.bibliographiccitation.lastpage","388"],["dc.bibliographiccitation.volume","43"],["dc.contributor.author","Birk, S."],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Liedl, Rudolf"],["dc.contributor.author","Sauter, M."],["dc.date.accessioned","2018-11-07T11:03:45Z"],["dc.date.available","2018-11-07T11:03:45Z"],["dc.date.issued","2005"],["dc.description.abstract","A tracer test in a carbonate aquifer is analyzed using the method of moments and two analytical advection-dispersion models (ADMs) as well as a numerical model. The numerical model is a coupled continuum-pipe flow and transport model that accounts for two different flow components in karstified carbonate aquifers, i.e., rapid and often turbulent conduit flow and Darcian flow in the fissured porous rock. All techniques employed provide reasonable fits to the tracer breakthrough curve (TBC) measured at a spring. The resulting parameter estimates are compared to investigate how each conceptual model of flow and transport processes that forms the basis of the analyses affects the interpretation of the tracer test. Numerical modeling results suggest that the method of moments and the analytical ADMs tend to overestimate the conduit volume because part of the water discharged at the spring is wrongly attributed to the conduit system if flow in the fissured porous rock is ignored. In addition, numerical modeling suggests that mixing of the two flow components accounts for part of the dispersion apparent in the measured TBC, while the remaining part can be attributed to Taylor dispersion. These processes, however, cannot reasonably explain the tail of the TBC. Instead, retention in immobile-fluid regions as included in a nonequilibrium ADM provides a possible explanation."],["dc.identifier.doi","10.1111/j.1745-6584.2005.0033.x"],["dc.identifier.isi","000229051400012"],["dc.identifier.pmid","15882329"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/51682"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Blackwell Publishing Inc"],["dc.relation.issn","0017-467X"],["dc.title","Process-based interpretation of tracer tests in carbonate aquifers"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","227"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","HYDROLOGY AND EARTH SYSTEM SCIENCES"],["dc.bibliographiccitation.lastpage","241"],["dc.bibliographiccitation.volume","18"],["dc.contributor.author","Reimann, Thomas"],["dc.contributor.author","Giese, Marcus"],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Liedl, Rudolf"],["dc.contributor.author","Marechal, J. C."],["dc.contributor.author","Shoemaker, W. Barclay"],["dc.date.accessioned","2018-11-07T09:46:33Z"],["dc.date.available","2018-11-07T09:46:33Z"],["dc.date.issued","2014"],["dc.description.abstract","Karst aquifers are characterized by highly conductive conduit flow paths embedded in a less conductive fissured and fractured matrix, resulting in strong permeability contrasts with structured heterogeneity and anisotropy. Groundwater storage occurs predominantly in the fissured matrix. Hence, most mathematical karst models assume quasi-steady-state flow in conduits neglecting conduit-associated drainable storage (CADS). The concept of CADS considers storage volumes, where karst water is not part of the active flow system but hydraulically connected to conduits (for example karstic voids and large fractures). The disregard of conduit storage can be inappropriate when direct water abstraction from karst conduits occurs, e. g., large-scale pumping. In such cases, CADS may be relevant. Furthermore, the typical fixed-head boundary condition at the karst outlet can be inadequate for water abstraction scenarios because unhampered water inflow is possible. The objective of this work is to analyze the significance of CADS and flow-limited boundary conditions on the hydraulic behavior of karst aquifers in water abstraction scenarios. To this end, the numerical discrete-continuum model MODFLOW-2005 Conduit Flow Process Mode 1 (CFPM1) is enhanced to account for CADS. Additionally, a fixed-head limited-flow (FHLQ) boundary condition is added that limits inflow from constant head boundaries to a user-defined threshold. The effects and the proper functioning of these modifications are demonstrated by simplified model studies. Both enhancements, CADS and FHLQ boundary, are shown to be useful for water abstraction scenarios within karst aquifers. An idealized representation of a large-scale pumping test in a karst conduit is used to demonstrate that the enhanced CFPM1 is able to adequately represent water abstraction processes in both the conduits and the matrix of real karst systems, as illustrated by its application to the Cent Fonts karst system."],["dc.identifier.doi","10.5194/hess-18-227-2014"],["dc.identifier.isi","000331123300014"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/11690"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/34895"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Copernicus Gesellschaft Mbh"],["dc.relation.issn","1607-7938"],["dc.relation.issn","1027-5606"],["dc.rights","CC BY 3.0"],["dc.rights.uri","https://creativecommons.org/licenses/by/3.0/"],["dc.title","Representation of water abstraction from a karst conduit with numerical discrete-continuum models"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]