Agriculture Reference
In-Depth Information
Traditional monitoring techniques may neither capture the contaminant distribution nor
their removal rates sufficiently (Aagaard et al., 2004). Subsurface characterization of
contaminant distribution over large scales is challenging, since the contaminants may have
moved erratically through the profile as illustrated in Figure 3, and point samples as
provided with conventional sampling techniques (as discussed above) may not provide a
representative measurement of the situation. The most common measurement technique for
monitoring contaminant transport in the unsaturated zone is sampling of soil water through
suction cups. These consist of a porous material such as ceramics or an inert material e.g.
Teflon and quartz with a pore size small enough to ensure contact between the filter and the
soil. An example of such a system is the experimental lysimeter trench at Moreppen near
Oslo airport, Gardermoen, Norway (French et al., 1994). This experimental site has more
than 100 Prenart suctions cups and various other soil physical measurements. It was
constructed to conduct controlled experiments of transport of Propylene glycol and
Potassium Acetate during snowmelt. Several studies were carried out at the same site in
order to examine the hydrogeological properties in the unsaturated and saturated zones and
the transport processes.
Contaminants may, depending on their chemical properties, affect the geophysical signature
of the soil. Salts will increase the electrical conductivity (EC) of the pore fluid, while
hydrocarbons will have the opposite effect. The organic and inorganic de-icing salts will
reduce the electrical resistivity of the soils, while Propylene glycol will not affect the
electrical conductivity of the pore fluid. Electrical and electromagnetic methods are widely
applied for soil mapping and detecting of contaminated plume. Over the last decade new
geophysical methods such as induced polarisation (e.g. Godio and Naldi, 2003; Slater &
Mansoor, 2006), electromagnetics, GPR, micro-sesimics and self potential (Naudet et al.,
2003; Arora et al, 2007) have been explored as methods for exploring contaminated sites.
Low frequency electromagnetic (EM) methods are usually adopted for fast mapping and
preliminary assessment of the aerial extent of the potentially contaminated land. A
qualitative image of the soil mineralization, due to degradation of hydrocarbons, could be
inferred by integration of resistivity and induced polarisation data (e.g. Godio and Naldi,
2003, Slater et al. 2006). Electrical Resistivity Tomography (ERT) is a powerful tool for
investigating pore fluid properties (Olsen at al., 1999; Kemna et al., 2000; Depountis et al.,
2001; Damanesco and Fratta, 2006; ) as shown in laboratory experiments (Comina et al.,
2005) and for solute transport in undisturbed soil columns (Binley et al., 1996) and field sites
(Slater et al., 2000; French et al., 2002, Binley et al. 2005). How to estimate hydrogeophysical
parameter is still one of the major challenges, state-of the art knowledge is described by
Linde et al., (2006). Another challenge for combined interpretation of geophysical and point
measurements is that the support scale of different methods varies; hence a statistical
framework is required for joint interpretation.
4. Modelling implications
As evidence shows, the subsoil is in general heterogeneous (or spatially variable), and often
this heterogeneity is partly irregular. This irregular variation has been the motivation to
consider soil as an intrinsically random material, i.e., as a material that can only be described
statistically. This assumption has resulted in a large body of literature (Bellin et al., 1993;
Dagan, 1997; Keijzer et al., 1999; Janssen et al., 2006; Cirpka, O.A., P.K. Kitanidis, 2000; Fiori
et al., 2002), that is still actively being developed and is quite mathematically inclined:
