Applied numerical modeling of saturated / unsaturated flow and ...

Uncertainty assessment of contaminant plume length
estimates in heterogeneous aquifers
Abstract
Journal of Contaminant Hydrology 87 (2006) 73–95
Christof Beyer ⁎ , Sebastian Bauer, Olaf Kolditz
www.elsevier.com/locate/jconhyd
Center for Applied Geoscience, University of Tübingen, Sigwartstraße 10, D 72076 Tübingen, Germany
Received 4 November 2005; received in revised form 14 April 2006; accepted 25 April 2006
Available online 16 June 2006
The Virtual Aquifer approach is used in this study to assess the uncertainty involved in the estimation of
contaminant plume lengths in heterogeneous aquifers. Contaminant plumes in heterogeneous twodimensional
conductivity fields and subject to first order and Michaelis–Menten (MM) degradation kinetics
are investigated by the center line method. First order degradation rates and plume lengths are estimated
from point information obtained along the plume center line. Results from a Monte-Carlo investigation
show that the estimated rate constant is highly uncertain and biased towards overly high values. Uncertainty
and bias amplify with increasing heterogeneity up to maximum values of one order of magnitude.
Calculated plume lengths reflect this uncertainty and bias. On average, plume lengths are estimated to about
50% of the true plume length. When plumes subject to MM degradation kinetics are investigated by using a
first order rate law, an additional error is introduced and uncertainty as well as bias increase, causing plume
length estimates to be less than 40% of the true length. For plumes with MM degradation kinetics,
therefore, a regression approach is used which allows the determination of the MM parameters from center
line data. Rate parameters are overestimated by a factor of two on average, while plume length estimates are
about 80% of the true length. Plume lengths calculated using the MM parameters are thus closer to the
correct length, as compared to the first order approximation. This approach is therefore recommended if
field data collected along the center line of a plume give evidence of MM kinetics.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Natural Attenuation; Heterogeneity; Plume length; Center line method; Uncertainty analysis; Monte-Carlo;
Numerical modelling
⁎ Corresponding author. Tel.: +49 7071 29 73176; fax: +49 7071 5059.
E-mail address: christof.beyer@uni-tuebingen.de (C. Beyer).
0169-7722/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jconhyd.2006.04.006