Agriculture Reference
In-Depth Information
stochastic groundwater hydrology and contaminant hydrology. For the present purpose, it
is not useful to emphasize the mathematics, although some of it cannot be avoided. Rather,
emphasis is given to illustrate the effects of heterogeneity and to give an impression of what
that means for decision making.
4.1 Stochastic hydrology
The mentioned stochastic approach to contaminant hydrology (that focuses on the transport
behaviour and fate of soil and groundwater contaminants) implies that particular properties
are considered to be random space functions (RSF): they vary irregularly in the horizontal
plane and with depth. However, they do not do so completely randomly: due to the genesis
of layering, and soil horizons, patterns of large and small values can be observed. These
patterns are described by autocovariance functions. Examples of strongly variable
properties are the hydraulic conductivity, and for the water unsaturated zone also the soil
water retention function (Van der Zee & Boesten, 1991). Much research has established that
the hydraulic conductivity, and the scaling parameter in this function that also is used to
describe water retention, are well described with a lognormal distribution. Also for several
important properties that control the adsorption behaviour of contaminants, the lognormal
distribution appeared to be reasonable (Van der Zee & Van Riemsdijk, 1987, 1988; Boekhold
& Van der Zee, 1991).
Randomness can have a large effect on flow and transport (Rode et al., 2010). Basically, this
effect is (i) a more irregular transport behaviour of the contaminants, and (ii) a larger
uncertainty about this behaviour. To give an impression of how heterogeneity affects the
transport pattern, Figure 3 shows the leaching behaviour of both inert chloride and
degrading de-icing chemical at Oslo Airport, Gardermoen (French et al., 2001). We observe a
very irregular 'plume' of contaminant. A problem with such complex patterns is that they
are difficult to communicate, other than by sending a picture.
Fig. 3 The erratic pattern of chloride and propylene glycol transport in a heterogeneous soil
representative of the Moreppen site at Gardermoen (French et al., 2001).
