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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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