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An observed error in PMPATH particle tracking algorithm for MODFLOW in case of varying porosity and a proposed correction
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
2012 (English)In: Computers & Geosciences, ISSN 0098-3004, E-ISSN 1873-7803, Vol. 45, 1-3 p.Article in journal (Refereed) Published
Abstract [en]

MODFLOW (Harbaugh and McDonald, 1996) is a code widely used for groundwater modeling. It has been used for numerous projects and thousands of articles have been published with results of MODFLOW, since the code was first released in 1983. Numerous codes have been developed around MODFLOW, the pre- and post processing code Processing MODFLOW (Chiang, 2005) being one example. This code includes the advective particle tracking code PMPATH (Chiang, 2005), especially convenient and commonly used for stochastic groundwater modeling analyses as it includes an easy-to-use random field generator. The particle tracking code has been regularly used to simulate the advective transport of contaminants and tracers (e.g. Stauffer, 2005; Hefting et al., 2006; Dhiman and Keshari, 2006).

A suspected error in the PMPATH code was found by us, when studying flow and particle transport in a strongly heterogeneous hydraulic conductivity fields with co-varying porosity. It emerged in the form of calculated particle flow paths converging into narrower and narrower channels. After many attempts to understand the results turned out to be unsuccessful, we decided to check into details of the code and discovered a significant but non-obvious error. This error is discussed in this paper and a procedure to obtain the correct flow paths and travel times is proposed. 

Place, publisher, year, edition, pages
Elsevier, 2012. Vol. 45, 1-3 p.
Keyword [en]
PMPATH, MODFLOW, Particle tracking, Groundwater, Solute transport, Channeling
National Category
Oceanography, Hydrology, Water Resources
Research subject
Hydrology
Identifiers
URN: urn:nbn:se:uu:diva-180406DOI: 10.1016/j.cageo.2012.03.001ISI: 000306771100001OAI: oai:DiVA.org:uu-180406DiVA: diva2:550042
Funder
Formas, 245-2006-1152
Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Heterogeneity-Induced Channelling, Flow-Wetted Surface, and Modelling of Transport in Fractured Rock
Open this publication in new window or tab >>Heterogeneity-Induced Channelling, Flow-Wetted Surface, and Modelling of Transport in Fractured Rock
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Heterogeneities in fractured rock are found at all scales; from the scale of individual fractures, to the scale of fracture networks, and to the largest regional scales. These heterogeneities cause challenges for modelling and parameter estimation of flow and solute transport. The heterogeneities in fracture aperture, characterization of the flow channelling they are causing, and implementation of this information into numerical simulation models of the solute transport in fractured media are the subjects of this thesis.

Aperture variability within a fracture causes the flow channelling, where the water flow is focused in a few channels and other areas of the fracture have practically stagnant water. The flow-wetted surface is the area where the flowing water is in contact to the fracture area. Contaminants are transported with the flowing water and therefore the flow-wetted surface is an important parameter that influences the diffusion into the rock matrix and sorption to the fracture rock surface. The specific flow-wetted surface (sFWS) is the flow-wetted surface divided by the total fracture area. The sFWS is systematically analyzed for different fracture aperture distribution characteristics. The local aperture is linked to the local hydraulic conductivity K. Increasing standard deviation of the hydraulic conductivity K field (σln K) leads to decreased sFWS. The sFWS is found to be independent of the correlation length (λ) of the field. An empirical relationship is developed, which describes the sFWS as a function of the σln K. A method is also introduced to determine this key parameter by analysis of the breakthrough curve from a single-well injection-withdrawal (SWIW) test. Further, an approach is presented to incorporate the effect of fracture level heterogeneity into fracture network models and to analyze the effect on sorption and matrix diffusion, by including the sFWS parameter into the transport calculations. The results show that the median transport time is proportional to the square of the sFWS-value. The results also suggest that there are an averaging behaviour in the fracture network, the sFWS-value of each individual fracture is not important for the transport over the domain, but a mean-value can be utilized in the numerical model.

Abstract [sv]

Heterogeniteter i sprickigt berg finns i alla skalor, från millimeterskala till en skala på hundratals kilometer. Dessa heterogeniteter orsakar problem vid beräkning av vattenflöde och ämnestransport. Aperturen i en spricka är öppningen mellan de två omslutande bergsidorna, den varierar både inom och mellan olika sprickor. Ämnet för denna avhandling är heterogeniteter i aperturerna inom enskilda sprickor, karaktärisering av den flödeskanalisering som uppstår på grund av dessa heterogeniteter och hur man kan använda denna information till en numerisk modell.Variabilitet av aperturen i en enskild spricka gör att vattenflödet blir fokuserat i ett fåtal kanaler, medan andra områden av sprickan kan ha praktiskt taget stillastående vatten. Den flödesvätta ytan är det område där det strömmande vattnet kommer i kontakt med sprickytan. Den flödesvätta ytan som påverkar diffusionen in i bergmatrisen och sorptionen till sprickytan är en viktig parameter eftersom föroreningar transporteras med det strömmande vattnet. Den specifika flödesvätta ytan (sFWS) är den flödesvätta ytan dividerad med den totala sprickarean. I avhandlingen analyserades sFWS systematiskt för olika statistik över sprickaperturen. Den lokala aperturen är kopplad till den lokala hydrauliska konduktiviteten K. En ökad standardavvikelse för det hydrauliska konduktivitetsfältet (σln K) ledde till minskad sFWS. sFWS visades vara oberoende av konduktivitetsfältets korrelationslängd (λ). En empirisk relation utvecklades som beskriver sFWS som en funktion av σln K. Ett SWIW-test är en typ av spårämnesförsök, där ett spårämne injiceras i en brunn följt av vatten i en bestämd tidsperiod, innan flödet vänds och en genombrottskurva registreras. Testet används traditionellt för att bestämma bergets diffusions- och sorptionsegenskaper. En metod presenterades för att bestämma den specifika flödesvätta ytan genom analys av genombrottskurvan för ett SWIW-test. Ett tillvägagångssätt introducerades för att analysera effekterna av sorption och matrisdiffusion i heterogena sprickor i en spricknätverksmodell genom att inkludera sFWS-parametern i transportberäkningar. Resultaten visade att medianvärdet för transporttiden är proportionell mot kvadraten på sFWS-värdet. Resultaten visade också att transporten genom spricknätverket inte är beroende av sFWS-värdet i de individuella sprickorna, utan att medelvärdet kan användas för modellering.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 76 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 971
Keyword
channelling, flow-wetted surface, fractured rock, solute transport, groundwater modelling, groundwater
National Category
Oceanography, Hydrology, Water Resources
Research subject
Hydrology
Identifiers
urn:nbn:se:uu:diva-180409 (URN)978-91-554-8468-2 (ISBN)
Public defence
2012-10-19, Hambergsalen, Geocentrum, Uppsala Universitet, Villavägen 16, 75236 Uppsala, Uppsala, 13:00 (English)
Opponent
Supervisors
Funder
Formas, 245-2006-1152
Available from: 2012-09-28 Created: 2012-09-06 Last updated: 2013-01-23

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Larsson, MartinNiemi, AuliTsang, Chin-Fu

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