Groundwater-Related Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 752

With regard to Risk Assessment, the conclusion that can be derived from what
has been outlined above is that groundwater needs to be approached from two dif-
ferent perspectives. Firstly, groundwater is an important protection target as it may
be threatened by contamination and, therefore, needs to be protected. Secondly,
groundwater is an important pathway , that is, a means of transport (carrier) for
contaminants.
17.1.2 Terminology
As described in the Introduction chapter (see Section 1.2.1 ), soil can be divided into
two different entities, that is, a water-unsaturated upper soil layer (upper soil) and
a water-saturated groundwater zone . These two entities are separated by a ground-
water table. Most definitions describe groundwater as all water present beneath the
soil's surface. In fact, according to this definition three types of groundwater can be
distinguished, covering both entities described above. The first is the pore water in
the water-unsaturated upper soil layer. This water shares the soil pores with soil gas
and is partially unavailable, since it is attached to soil particles. The water volume
(and, hence, gas volume) in the unsaturated zone changes according to relatively
short time frames due to rain and snow events, evapotranspiraton and water flow.
The second is the water usually referred to as groundwater , that is, the water in
the water-saturated zone. These water volumes are extremely important, since they
constitute the major source for drinking water in many countries. The major ground-
water reservoirs are present in aquifers , that is, in layers that carry huge volumes
of renewable or fossil water. In phreatic aquifers, a free water table exists at atmo-
spheric pressure. In confined and artesian aquifers, the water is held in aquitards, that
is, layers with poor water permeability. Therefore, the water pressure can rise above
the top of the aquifer, even higher than the soil surface, creating free-flowing wells,
under so-called artesian conditions. The third origin of groundwater is in so-called
secondary porosity and permeability as found, for example, in fractures or karstic
phenomenon in consolidated rocks, often at great depths beneath the soil surface.
Although this groundwater occurrence is not often included in Risk Assessment, it
can be an important resource at a local or even regional scale that needs to be pro-
tected, and can also play an often underestimated role as a pathway for contaminants
(B. Harris 2009 'personal communication'). The reason for this is that, in regions
with karstic or pseudo-karstic features, contamination can spread at much higher
rates than would normally be expected for groundwater.
Remarkably, in the great majority of studies that focus on Risk Assessment
related to contaminated aquifers, it is referred to as 'contaminated groundwater',
while actually groundwater is only one of the two constituents of an aquifer. It often
seems that the solid phase of the water-saturated soil has been forgotten in Risk
Assessments. There are several reasons, however, to put this much emphasis on
the groundwater compartment. Firstly, analogous to the solid phase compartment
in the upper soil layer (for example, soil used as building material), groundwater
has been directly used as a vital human resource since early mankind. Secondly,