Topic
Period
Biogeochemical transport modelling of natural attenuation processes at the
Banisveld landfill site
2004-2005
Description
Landfill sites have been recognised to contribute significantly to the pollution of
groundwater. Even a small country like the Netherlands harbors about 3800 old municipal
household-refuse landfill sites, while over 100,000 landfill sites are spread across the USA
(Suflita et al., 1992). Landfills were constructed without taking too much care of the
surrounding environment before the 1980s. Landfills were generally not capped after closure
to prevent the formation of leachate by infiltration of rainwater, nor were precautions taken
to prohibit spreading of leachate to underlying aquifers. Therefore, release of hazardous
chemicals to the environment continues, although many landfills are closed now. As a result,
leachate plumes have been formed downstream from many landfill sites with a length up to a
few kilometers (Christensen et al., 2001).
Monitored natural attenuation (MNA) is a potential remediation option for many of those
landfill sites. In order to improve the understanding of the biogeochemical processes that
occur during MNA, a comprehensive field study has been carried out over the past few years
for the Banisveld landfill site near Boxtel, Netherlands (van Breukelen, 2003,
http://www.geo.vu.nl/users/breb/thesis/thesis.html). The study resulted in a detailed
characterisation of the geochemistry, hydrochemistry, isotope geochemistry and
microbiology of this site. Various techniques were applied to determine which redox
processes were coupled to degradation of DOC inside the plume. Both inverse geochemical
modelling and one-dimensional forward reactive transport modelling with PHREEQC-2
(Parkhurst and Appelo, 1999) were performed to deduce and quantify the network of
biogeochemical processes changing the leachate composition downstream of the source zone
(van Breukelen, 2003). One of the forward reactive transport modelling studies focussed on
the simulation of processes along a flow-path in the core of the leachate plume, whereas a
second study simulated the processes at the plume fringe during temporal (advective) upward
movement of the leachate plume.
Processes occurring at the fringe of a pollution plume are assumed to be of considerable
importance for natural attenuation (NA) of the entire plume. Diffusive and dispersive mixing
of anoxic polluted groundwater with (sub)oxic pristine groundwater at the plume fringe
results in dilution and may enhance degradation: degradation potential for electron-donors is
absent or limited inside plumes, but excellent in the plume fringe, due to mixing with the
soluble electron-acceptors oxygen, nitrate and sulfate. Transversal mixing between oxic and
anoxic groundwater therefore enables faster biodegradation for many compounds.
The proposed thesis project is aimed at integrating the existing one-dimensional modelling
studies into a single reactive transport model under consideration of a more realistic, multidimensional flow field. Particular attention will subsequently be given to the analyses of the
role that vertical transverse dispersion plays for the evolution of the leachate plume and its
(natural) attenuation.
Objectives
(i)
Construction of a two-dimensional, cross-sectional flow model using MODFLOW
(ii)
Formulation of a suitable reaction module for the reactive multi-component reactive
transport model PHT3D (Prommer et al., 2003, http://www.pht3d.org)
(iii)
Construction of a transport model with PHT3D
(iv)
(Re-)calibration of model parameters with the parameter estimation tool PEST
(http://www.sspa.com/pest/) using the measured vertical concentration profiles from
multilevel sampling devises (MLS) for both, organic and inorganic chemicals.
(v)
Analysis of the modelling results – (a) determination of parameter sensitivity (b)
identification and quantification of major attenuation processes and (c) determination
of the role of transversal dispersion on rates of natural attenuation.
(vi)
Comparison of results with similar ongoing studies of the EU project CORONA
(http://www.shef.ac.uk/corona)
Candidates
For this MSc – project we are looking for a motivated student with a background in hydrology
or geochemistry.
Contact & information
The project will be supervised by Phil Ham (ham@geo.uu.nl), Henning Prommer (UU/visiting
scientist at CSIRO Australia; Henning.Prommer@csiro.au), and Dr. Boris van Breukelen (Vrije
Universiteit Amsterdam).