Groundwater-Related Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 765

and eventually in some cases the oceans, after drainage into drain tubes, ditches,
canals and rivers.
When an intrinsic value is attributed to groundwater (see Section 17.2.3 ), 'clean
groundwater adjoining a contaminated groundwater body' also should be consid-
ered as a protection target. In that case, the two different meanings of groundwater
in Risk Assessment, that is, as a protection target and as a means of transport (car-
rier) for contaminants, come together: migrating contaminated groundwater (the
pathway) contaminates clean groundwater (the protection target).
Several types of measurement for risks from groundwater transport exist. With
regard to a threat to identified protection targets, often some specific measurements
are used such as:

• the estimated concentration in the layers where groundwater is extracted for

drinking water consumption (protection target: human health);

• the estimated concentration in surface water (protection target: aquatic ecological

effects);

• the estimated concentration in the root zone after an upwards flow of groundwater

(protection target: Food Safety).

With regard to the intrinsic value of groundwater, the volume of clean ground-
water in which a specific risk level threatens to be exceeded within a specified
timeframe is often used as measure for risks. The more general measurements used
are contaminant profiles with depth at several specified moments or contaminant
concentration profiles over time at a specific depth ( breakthrough curves ).

17.3.2 Transport Characteristics

17.3.2.1 General Transport Pattern

The transport in the water-unsaturated upper soil is predominantly vertical. The
velocity of water flow and, hence, contaminant leaching into the groundwater is
strongly dependent on the water content of the upper soil. This velocity is min-
imal when soil is at its driest level, gradually increasing as soil becomes wetter.
The maximum water flow velocity in the water-unsaturated upper soil, under (tem-
porary) water-saturated conditions, equals the velocity of groundwater flow in the
saturated zone.
Groundwater flow in the water-saturated soil layer is the major contaminant
transport mechanism in soils, bringing contaminants from as far away as many
kilometres from the source. The transport direction in the water-saturated soil
layer is predominantly horizontal. As a consequence of this contaminant transport,
contaminants may threaten just about any protection target.
The sources of and pathways for groundwater recharge in urban areas are more
numerous and complex than in rural environments (Lerner 2002 ). Buildings, roads,
and other surface infrastructures, combined with man-made drainage networks,
change the pathways for precipitation. Some direct recharge is lost, but additional