Groundwater-Related Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 766

recharge can occur from storm drainage systems. Large amounts of water are
imported into most cities for their water supply, distributed through underground
pipes, and then collected again in sewers or septic tanks.
17.3.2.2 Impact of Heterogeneous Soils or Aquifer
The subsurface of the soil is generally structured in layers. However, these layers
do not follow a complete geometrically symmetrical pattern, but show significant
irregularities. Moreover, in urban areas the soil layers are pierced with many civil
constructions. And some soils show a fissured structure. Consequently, groundwater
flow and contaminant transport show an irregular pattern.
17.3.2.3 Impact of Surface Water Bodies and Anthropogenic Subsurface
Processes and Structures
The presence of surface water bodies such as canals, rivers, lakes, ditches, etc.,
does impact the groundwater flow and contaminant transport, since the flow lines
tend to move towards these groundwater bodies. Generally speaking, the bigger
the groundwater body, the higher the impact. Additionally, many anthropogenic
subsurface processes and structures influence groundwater flow and contaminant
transport. A typical example of an anthropogenic impact on the groundwater flow is
the presence of public groundwater extraction wells , often at depths of several hun-
dred meters, or of private wells . Obviously, these wells impact the groundwater flow
and contaminant transport pattern. Many civil structures intrude into the subsurface,
mainly in urban areas, and so impact the groundwater flow and contaminant trans-
port. Examples of such structures are subsurface car parks, tunnels, groundwater
wells, water supply nets, and sewage pipelines.
Other examples of anthropogenic impact are hydro modification projects , which
have a substantial effect on the natural regime of surface water and, indirectly,
groundwater. The construction and operation of dams, for example, may magnify
the transport of contaminants into the groundwater body due to the higher hydraulic
pressure differences. The construction of a dam or a reservoir can modify the
relationship between surface water and groundwater, and therefore may result in
increased recharge and discharge. River regulation using reservoirs, channel imper-
meabilisation of natural streams, canal and irrigation channels, etc., are some other
factors that impact water flow and contaminant transport.
Underground or open cast mines, quarries and mineral extraction activities also
strongly impact the flow pattern, since large withdrawals of groundwater take place
when dewatering a mine in preparation for mining activities. Moreover, if a deep
mine intersects an aquifer, variations in the natural flow regime and changes in
recharge or discharge may result.
A specific case of anthropogenic impact on the groundwater flow, and hence
of contaminant transport, is found in polder areas , that is, isolated areas with an
artificial groundwater table deeper then the groundwater table in the surroundings,
and often below sea level. Characteristically, the groundwater level in these polders