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346 Soil and Water Contamination
The areas that contribute most to increased runoff events are those that are hydrologically
active and connected to the streams via quick runoff pathways (for example, through
overland flow or tile drainage). They are generally located in relatively narrow zones along the
stream channel, but their surface areas vary between and during rainstorm events. Therefore,
these areas are usually referred to as variable source areas (Hewlitt and Hibbert, 1967; Beven
and Kirkby, 1979; McDonnell, 2003). If the topsoil in these areas is contaminated, these
variable source areas also act as critical source areas of diffuse pollution (see Section 18.2.1),
as the pollutants may be quickly transferred to the river network and thereby contribute to
river water contamination (e.g. Doppler et al., 2012). Note that critical source areas do not
necessarily coincide with variable source areas, as critical source areas may also refer to areas
that act as pollutant source via slow subsurface pathways.
Clearly, to be able to understand the response of river water composition to runoff events it
is essential to identify the hydrological pathways within the catchment and their associated
transit times. Conversely, the chemical or isotopic composition of the stream water can
be used to subdivide hydrographs into components of flow: a method called hydrograph
separation (Robson and Neal, 1990; Wels et al., 1991).
18.3.2 Hysteresis response of dissolved concentrations to changes in discharge
The timing of the arrival of water from various hydrological pathways, which brings about
the typical temporal patterns in chemical transport from catchments, is controlled by
topography, groundwater gradients and permeability of the aquifer , and surface resistance to
flow. Furthermore, spatial variations in the occurrence of the hydrological pathways due to
variations in rainfall, soil type, and land use across the catchment , can also significantly affect
the temporal patterns in streamwater chemistry (Walling and Webb, 1986). These temporal
variations become visible in characteristic cyclical shapes in the relationship between
instantaneous water discharge and instantaneous concentration at a measurement site during
a single storm event (Q–C relationship) (Evans and Davis, 1998). Such Q–C relationships
can be characterised by 1) the direction of the relationship (negative: the concentration
decreases in response to increased discharge; positive: the concentration increases with
discharge; zero: no response), 2) the timing of the peak or trough concentration relative to
peak discharge (before, at the same time, or after peak discharge), and 3) the difference in
concentrations occurring during the same discharge during the rising limb of the hydrograph
compared to the falling limb. These characteristics determine the nature of the hysteresis
of the Q–C relationship (clockwise or anticlockwise). Figure 18.7 shows some hypothetical
concentration responses to increased discharge. For example, if the concentration
increases with discharge, the peak concentration lags behind the peak discharge, and the
concentrations are higher during the falling limb of the hydrograph than during the rising
limb (Figure 18.7c), then an anti-clockwise hysteresis loop is obtained (Figure 18.7d).
Both the direction of the Q–C relationship and the direction (clockwise/anticlockwise)
of the hysteresis pattern can be used to infer the flow path ways dominating during individual
storm events (Evans and Davies, 1998). They reflect changes in water composition resulting
from changes in the contribution from groundwater, soil water, and surface event water
(overland flow ) during storms. For example, if the concentrations of dissolved substances
in overland flow are higher than in subsurface pathways, they will increase with increasing
volume of overland flow, so the concentration will peak early during storms with a positive,
clockwise Q–C response. Conversely, if it is the concentrations of chemicals in groundwater
discharge that are higher, then the overland flow during storms dilutes the in-stream
concentrations and produces negative, often anticlockwise Q–C responses. Figure 18.8 shows
an example of the response of different phosphorus forms to storm events in the Den Brook
catchment , Devon, UK (Haygarth et al., 2004). The temporal trend of reactive P (<0.45 μm)
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