Page 80 - Soil and water contamination, 2nd edition

Environmental compartments 67

of a deep lake is isolated from the atmosphere, it may also become depleted in oxygen if
sufficient organic matter is available for decomposition. Under these conditions, the redox
potential is considerably decreased.
Surface water is mostly less acidic than rainwater and groundwater. Primary producers
extract dissolved carbon dioxide (carbonic acid ) from solution for photosynthesis , which
causes the pH to increase. When water contains high levels of carbonates (e.g. water in
limestone catchments) and the growth of algae is not limited by nutrient shortage, the pH
can rise to well above 8. Therefore, most surface waters are neutral to basic (compare Figure
2.3), except water bodies in catchments with poorly buffered soils, for example catchments
draining upland moors or granite or sandstone bedrock.
River flow is highly dynamic. During periods of little precipitation, most rivers carry a
minimum amount of water through the river channel. This condition is called baseflow and
is in most cases controlled by groundwater discharge . Therefore, during baseflow, stream
waters generally reflect the composition of near-surface groundwater, which is, in turn,
determined by local geology. Considerable increases in discharge above the baseflow occur
during rainfall or snowmelt events. The increase in discharge in response to rainfall events
is a function of the rainfall intensity and duration, the surface area and permeability of the
upstream catchment , antecedent soil moisture conditions, groundwater levels, topography,
and surface resistance to flow. Surface water discharge may vary by one order of magnitude
in large drainage basins, as illustrated in Figure 3.7. In small catchments, discharge can be
rather flashy and may rapidly increase by even two or more orders of magnitude during
periods of prolonged heavy rain or snowmelt. During such events, the stream water becomes
diluted with rainwater and overland flow water, which usually contain less dissolved
substances.
Increased stream flow during rainfall events may also cause erosion of bed sediments ,
which may bring sediment and associated chemicals into suspension. Sediment is therefore
an important vector in contaminant transport in surface water. Suspended sediments
may come from erosion of hillslopes, stream banks, and stream beds. Their source exerts
an important influence on the features of the suspended sediment load (e.g. grain size
distribution, geochemical composition) (Walling and Webb, 1996). Fine sediments
transported in suspension can be transported across particularly long distances. Considerable
amounts of relatively insoluble contaminants contained in fine mineral particles or living or
dead organic matter can be transported in particulate form. However, particulate matter tend
to settle out and so the sediment-associated contaminants transported in surface water may
be stored temporarily in bed sediments or semi-permanently in floodplain or lake deposits
(Walling et al., 2003; Walling and Owens, 2003). Therefore, as noted above, interaction
between surface water and bed sediments through retention , release, erosion, and deposition
is a key process in controlling surface water composition.

3.5 FURTHER READING ON SOILS, GROUNDWATER, AND SURFACE WATER

Soil Science
• Gerrard, J., 2000, Fundamentals of Soils, (Oxford: Routledge).
• Miller, R.W. and Gardiner, D.T., 2008, Soils in Our Environment, 11th edition, (Upper
Saddle River NJ: Prentice-Hall).

Hydrology
• Ward, R.C. and Robinson, M., 2000, Principles of Hydrology, 4th edition, (London:
McGraw-Hill).

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