Page 331 - Soil and water contamination, 2nd edition

318 Soil and Water Contamination

concentrations in groundwater within an area or grid cell can usually be directly related to
the land use class or soil type within that same area.
At the regional scale or below, groundwater flow does become important. Due to
the bending and the convergence of flow line s and stream tube s, the spatial differences
in contaminant inputs at the soil surface that occur over several hundreds of metres are
mirrored in downgradient vertical variation in groundwater composition to a depth of several
-
metres (see Figures 17.1 and 10.3). This effect can be illustrated by Cl concentration–depth
-
profiles in groundwater. Since Cl is conservative, the concentration–depth profile is not
-
interfered with by chemical reactions, but only by differences in Cl input.
-
Figure 17.8 shows the Cl concentration–depth profile in a single multilevel well (A1)

located in a sandy, unconfined aquifer at the bottom of an ice-pushed ridge in the Salland
area in the Netherlands (see Figure 17.3) (Vissers et al., 1999). The well is situated in a
patch of forest and the land cover in the upgradient groundwater recharge area consists of
alternating patches of forest and heather. The evapotranspiration rate for forest is greater than
-
for heather. Consequently, the Cl concentration in groundwater underneath the forest is also
greater, due to the concentration effect of evapotranspiration (see box 5.II). Compared with
-
-
the average Cl concentration in precipitation, the Cl concentration of infiltrating water is
about four to five times higher underneath forest and about two times higher underneath
-
-1
heather. In the Salland region, the average Cl concentration in rainwater is about 4 mg l ,
-1
-
so the Cl concentrations are 16–20 mg l in groundwater originating from forests and about
-1
8 mg l in groundwater originating from heather. Considering these different concentrations,
five different stream tube s originating from different patches of land can be distinguished
-
in the concentration–depth profile. In the top stream tube the Cl concentration is around
-1
5 mg l , which suggests a recent intensive recharge by rainwater that was not affected by the
concentration effect of evapotranspiration. The deeper stream tubes refer to recharge areas
farther away and, as the age of groundwater increases with depth, to land cover occurring
-
in the recharge areas further back in time. It should thus be emphasised that the Cl
concentration reflects the land cover at the time of infiltration , which does not necessarily
correspond with the present land cover. An important point about the Salland area is that in
th
the course of the 20 century, much of the heather was planted to trees. So, it is possible that
the recharge areas of the stream tubes linked to heather are now under forest or other land
cover types.

-
Figure 17.8 Cl concentration depth profile in a well (A1) in Salland, the Netherlands, with subdivision into stream
tubes originating from different patches of forest and heather Adapted from Vissers et al. (1999).

10/1/2013 6:47:04 PM
Soil and Water.indd 330 10/1/2013 6:47:04 PM
Soil and Water.indd 330