Environmental Engineering Reference
In-Depth Information
is a key process in groundwater Risk Management. For this reason, no decision-
maker or stakeholder doubts the significance of the abilities of the groundwater
ecosystem today.
From this perspective it is interesting to note that appreciation for the soil and
groundwater ecosystems has undergone a dramatic evolution among 'problem own-
ers'. A few decades ago these 'problem owners' were confronted with the need to
protect the soil and groundwater ecosystem whose intrinsic value from their point of
view hardly seemed to counterbalance the high remediation costs. But today these
stakeholders make excellent use of these ecosystems by using them to degrade the
contaminants that they must be protected against.
17.3 Groundwater as Contaminant Pathway
17.3.1 Source-Pathway-Receptor Approach
With regard to the source-pathway-receptor approach , groundwater provides an
important pathway for contaminants, connecting a source to one or more recep-
tors. The transport velocity of water and contaminants is strongly dependent on soil
type, varying from high speeds in gravel and coarse sand to much slower in fine
sand and clay soils. Moreover, since contaminants undergo an intensive exchange
with soil materials (adsorption), the transport velocity of contaminants strongly
depends on the contaminant characteristics and adsorption properties of the soil and
aquifer material. Generally, contaminants show a high adsorption affinity towards
humic acids, organic clays and oxides covered with organic coatings (McCullough
et al. 1999 ). The principles of transport significantly differ for the water-unsaturated
upper soil versus the water-saturated groundwater zone. Generally speaking, the
transport of water and contaminants is much faster in the groundwater zone than
in the water-unsaturated upper soil layer. It should be noted, however, that surface
water in rivers generally moves much faster than the groundwater.
Obviously, the type of receptors (protection targets) that are threatened depends
on the direction of groundwater flow. Human health may be affected when con-
taminants migrate downwards and reach drinking water extraction points. Less
frequently, human health may be threatened when humans come in contact with
contaminants after an upwards flow of groundwater. A major pathway for human
exposure is through inhalation of contaminated indoor air, and to a lesser extent
outdoor air, after vapour intrusion of contaminants from groundwater into buildings
in residential, industrial, or recreational areas. This pathway is intensively described
in McAlary et al. ( Chapter 10 of this topic).
Ecological protection targets are reached after the leaching of pore water into
the groundwater, since the aquifer ecosystem will then experience a subsequent
exposure to contaminants. Indirectly, contaminated groundwater may impact terres-
trial ecosystems in nature reserves and agricultural products after an upwards flow.
Eventually, contaminated groundwater will threaten the surface water ecosystem,
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