Groundwater-Related Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 779

sewage flooding occurs when sewage systems are not capable of evacuating the great
volume of water during storms. As stated by the European Environment Agency,
extreme floods are the most common natural disaster in Europe (Estrela et al. 2001 )
and probably in other parts of the world as well. Because of the increase of intense
precipitation episodes, flooding is expected to happen more often in the future. It
must be noted that extreme rainfall is indeed the main cause of flooding, but this
factor is not the only one. The presence of vegetation and the soil capacity for water
retention are other key factors with regard to flooding, and these factors are highly
impacted by humans.
During flooding, the contaminants transported, mainly bound to particulate mat-
ter in the flood wave, can infiltrate and contaminate soil and, indirectly, groundwater.
The properties of the vadouse zone are then essential in order to estimate the
potential impact of flooding on the groundwater quality.
A special case of flooding happens when, due to occasional high rainfall
events, the groundwater table rises above normal levels, and inundates roads and
properties during weeks or even months (UK Groundwater Forum 2009 ). This
phenomenon is known as static groundwater flooding , to differentiate it from tran-
sient groundwater flooding (US Geological Survey 2009 ) which happens when
wet conditions persist for more than a year in terrains shaped from continental
glaciers.
In some areas of the world, flood irrigation is practised. Especially in arid areas,
groundwater is used for that purpose by means of the construction of irrigation
wells at different depths. Such a process implies a disturbance of the natural flow
regime and produces alterations in the recharge zone. As a consequence, a number
of contaminants may be incorporated into the downward flow.
17.7.2 Mingling Groundwater Plumes
In many intensively used urban areas of the world, different groundwater plumes
may intermingle in the subsurface. This phenomenon is important both in terms of
groundwater as a protection target as well as in terms of groundwater as a means
of transport (carrier) of contaminants with regard to transport to other protection
targets. Intermingling of groundwater bodies is sometimes recognized but rarely
incorporated in Groundwater-related Risk Assessment. There are several reasons
for this. A political reason is that different groundwater plumes usually are related
to another source and therefore to another 'problem owner', sometimes located in
different jurisdictions. A technical reason is that the Risk Assessment with regard
to a single groundwater plume is already a relatively unreliable process and assess-
ing the risks from several mingling plumes is yet more difficult and, hence, may
give rise to extreme uncertainties. Often the most reliable way to assess the risks
from intermingling groundwater plumes, therefore, is by monitoring the contami-
nant transport pattern at strategic points or planes of compliance. However, after
doing so, it is almost impossible to distinguish between the different sources of
groundwater contamination and hence between the different polluters.