Page 325 - Soil and water contamination, 2nd edition

312 Soil and Water Contamination

17.3 HYDROCHEMICAL SYSTEMS ANALYSIS
From Figure 17.2 it can be seen that a groundwater system can be described as a set of stream
tube s. For groundwater quality studies it is useful to define the stream tubes on the basis of
a geochemically homogeneous groundwater recharge area. Among the factors that determine
chemical homogeneity of an area are, for example, soil type and land use (Vissers et al., 1999;
Broers, 2002; Vissers, 2006). Boundaries between soil types and fields with different land
use can thus be followed along the groundwater flow lines that form the boundaries of the
stream tubes underground (see Figure 10.4). In addition to spatial variation, the chemical
inputs at the soil surface may also vary with time: for example, seasonal fluctuations in the
mineralisation of organic matter , and seasonal and annual variations in net precipitation
causing differences in leaching rates; changes in land cover or crop type causing variations in
fertiliser rates; or long-term changes in atmospheric deposition rates. This temporal variation
of chemical inputs is subsequently propagated along the flow line s (see also Figure 10.4),
but the magnitude of the fluctuations may fade due to longitudinal mixing. Furthermore,
the position of stream tube s may shift in time as a result of changes in the groundwater
flow pattern induced by climate change or by human disturbances of groundwater flow: for
example, due to groundwater abstraction or artificial drainage. Such shifts may thus also
cause considerable temporal changes in groundwater composition near the displaced stream
tube boundaries.
During transport along the groundwater flow lines, the interactions between water and
sediment cause groundwater to be exposed to a sequence of chemical conditions that vary
in pH, redox potential, temperature, etc. (see also Section 3.3.3). These chemical conditions
are governed by the geochemical composition of the sediment or bedrock and are thus
geologically defined. The minerals (clay minerals, carbonate minerals, reactive iron, pyrite
in particular; see Van Gaans et al., 2011) and the organic matter in the sediment act as a
buffering agent. If the buffering agent is abundant (for example, organic matter in organic
sediments, or calcite in limestone rocks) the chemical conditions can be considered as fixed
in space and time, at least at the time scale of centuries. This results in steady transitions
in groundwater composition at the boundary between geological layers. If the reaction
rate is fast compared to the groundwater flow rate, for example in the case of most acid –
base reactions, the transition in groundwater is abrupt and coincides with the geological
boundary. In contrast, if the reaction rate is slow, as for most redox reactions , the transition
in groundwater composition is more gradual. On the other hand, if the buffering agent is
limited, for example calcite in sandy sediments or base cations adsorbed to exchange sites,
the transition in the chemical composition of groundwater migrates gradually along the
groundwater flow line s.
Stuyfzand (1999) has summarised the typical hydrochemical development of
groundwater in the direction of groundwater flow and has derived a sequence of
hydrochemical facies in stream tube s. He distinguished the following processes as illustrated
in Figure 17.4:
1. From strong fluctuations in groundwater composition, mainly caused by seasonal
atmospheric and biological variations, to a stable water composition, due to dispersion .
+
2–
-
2. From polluted groundwater containing, for instance, SO , NO , K , heavy metals ,
4 3
tritium , and organic pollutants, to unpolluted, due to a) elimination processes, such as
filtration, acid buffering , sorption , and decay; and b) the older age of the water exposed
to a smaller pollution load .
3. From acidic to basic water due to weathering and buffering reactions with the porous
-
medium, which consume acids like H CO , H SO , and HNO and produce HCO
2 3 2 4 3 3
(alkalinity ).

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