Page 358 - Soil and water contamination, 2nd edition
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Patterns in surface water 345
including dilution by rainwater or meltwater, increased inputs from overland flow, increased
supply of groundwater discharge, mobilisation of particulate and dissolved material from
the river bed and terrestrial part of the catchment, and increased inputs from urban runoff
and, possibly, sewage overflows. This means that river water is a dynamic mixture of water
entering the streams through different hydrological pathways, each having their characteristic
transit time and chemical and isotopic signature (Kendall and McDonnell, 1998). The
statistical distribution of catchment water transit times (of both surface and subsurface
pathways) follows a power-law distribution characterised by a long tail of long transit times
(Kirchner et al., 2000; McGuire and McDonnell, 2006; Wörmann et al., 2007). Such power-
law distributions indicate self-similar fractal behaviour in the temporal scaling of surface–
subsurface interactions. This means that the majority of dissolved contaminants will initially
be flushed out rapidly, but the delivery of low-level contamination to streams continues for a
long time.
The contribution of the quick runoff pathways, such as overland flow, is larger in relatively
impermeable catchments with thin soils, or in catchments with wet soils and shallow
water tables. Overland flow originates mostly from the same small parts of the catchment,
and these comprise less than 10% (usually 1–3%) of the catchment area; on such areas,
only 10–30% of the rainfall produces overland flow (Freeze, 1974). Overland flow has a
short transit time and therefore is usually poor in major dissolved phase constituents, has
a low pH, but is rich in dissolved organic matter and may carry sediments and associated
substances. On the other hand, groundwater has a much longer transit time and is often
characterised by higher concentrations of weathering products, such as base cations and
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silica, Cl concentrations that are higher than in rainwater (due to evapotranspiration: see
Section 17.3), and high alkalinity (Jenkins et al., 1994). However, not all the water reaching
the surface water via subsurface pathways has a long transit time. For example, rainwater that
infiltrates into soils that are artificially drained by tile drainage, or groundwater in shallow
ephemeral perched aquifers that arise during the rainy season can be rapidly transferred to
the stream (e.g. Ocampo et al., 2003).
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Figure 18.6 gives an example of the typical response of the Ca concentration to
hydrological events as measured at 7-hourly intervals in the small Upper Hafren stream near
Plynlimon in Wales. This stream drains an area of relatively undisturbed moorland overlying
poorly permeable bedrock (Neal et al., 2012, 2013). As a result, the stream response to
rainfall is ‘flashy’, with the rising limb to peak flow typically lasting less than an hour, and the
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stream water is acidic with low solute concentrations. During baseflow conditions, the Ca
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concentrations are normally between 0.70 and 0.85 mg l , but decrease quickly in response
to the sudden increases in streamflow during runoff events, due to dilution by low-calcium
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rainwater. During the recession limb of the hydrograph, the stream water Ca concentration
slowly recovers to pre-event values.
2+
Figure 18.6 Response of the Ca concentration to hydrological events in the Uper Hafren stream near Plynlimon,
Wales (source: Neal et al., 2012, 2013).
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