Page 358 - Soil and water contamination, 2nd edition
P. 358

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
                          -
                   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).
                                                                           2+
                   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
                                                                                          2+
                   stream water is acidic with low solute concentrations. During  baseflow conditions, the Ca
                                                                -1
                   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
                                                                               2+
                   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).









                                                                                            10/1/2013   6:47:11 PM
        Soil and Water.indd   357
        Soil and Water.indd   357                                                           10/1/2013   6:47:11 PM
   353   354   355   356   357   358   359   360   361   362   363