Groundwater-Related Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 778

the groundwater flow and contaminant transport, the most common being lysime-
ter and column experiments. Lysimeters are reservoirs which were originally used
for the assessment of water fluxes through soil, for example, by measuring evap-
otranspiraton rates. In a similar way contaminant fluxes can be measured within
lysimeters, either in the field or in the laboratory. As a measurement for contami-
nant transport, it is possible to monitor the leachate that drains from the lysimeter
either as concentration over time or as the accumulated amount of leached liquid
and contaminants. An example of a lysimeter study is found in Kale et al. ( 2001 ),
who studied the distribution of PCBs (polychlorobiphenyls) (congeners 28 and 52)
and PAHs (fluoranthene and benzo[ a ]pyrene) in German soils.
Column experiments are performed in the laboratory. For this purpose, a metal or
plastic column is filled with soil material. The soil material then preferably mimics
the natural setting of the soil as much as possible. That means that for the most opti-
mal column experiments a column of soil is cut out from the soil. An example of a
column experiment is given in Mayes et al. ( 2000 ), who investigated the hydrolog-
ical and geochemical processes that govern the fate and transport of, among others,
metals in undisturbed saturated columns of weathered, fractured shale saprolite.
Column experiments have the major advantage of enabling full control of the
transport conditions such as the water and contaminant input on the surface of
the soil material in the column. The major disadvantage is that it is often hard
to maintain the layered soil structure of natural soils in a column. Moreover, it is
often inevitable that preferential flow will occur along the side of the column, since
usually there are open spaces between the wall of the column and the soil material.
As with lysimeters, it is possible to monitor the leachate that drains from the col-
umn. It is also possible, however, to measure the concentrations at different depths
in the column by placing sensors in the column.
17.7 A Closer look into Groundwater-Related Risk Assessment
17.7.1 Impact of Climate Change
A highly current environmental political issue is climate change caused by the
increased emission of greenhouse gasses to the atmosphere. In addition to a sig-
nificant worldwide increase in temperature, climate models are predicting serious
changes in rainfall volume and distribution in the future. Although there is a lot of
uncertainty in the model predictions, most climatic models point to an increase in
frequency of intense precipitation episodes. As a consequence, dramatic changes in
groundwater recharge and discharge regimes and in interactions between groundwa-
ter and surface water systems are expected as aquifers are replenished by effective
rainfall. This phenomenon may impact the groundwater quality both in positive and
in negative terms.
Another factor that influences groundwater flow and contaminant transport is
flooding . Fluvial flooding occurs when rivers overflow and burst their banks, and