Page 76 - Soil and water contamination, 2nd edition

Environmental compartments 63

Stream
Stream
Stream
Bed sediments
ents
ents
Hyporheic zone

6642 6642 6642

Figure 3.5 Interaction between surface water and groundwater in the hyporheic zone.

gradients and, as a consequence, they are able to retain, store, or release significant amounts
of nutrients and heavy metals . Therefore, bed sediments may play a very important role in
controlling surface water quality. Interaction between surface water and bed sediment also
occurs through processes of sediment erosion and deposition. These processes will be further
discussed in Section 4.1 and Chapter 12.

3.4.3 Sources of surface water pollution

Because much of the surface water has previously passed through the soil (surface or profile)
or groundwater, many of the sources of soil pollution and groundwater pollution may be
sources of surface pollution as well. Urban runoff in particular contains increased levels of
contaminants including nitrogen , phosphorus , heavy metals , and organic compounds like
PAHs . If surface water pollution sources are derived from groundwater or runoff water from
upland soils, the pollutants will generally enter the surface water body over a large area. This
implies that sources of pollutants that reach surface water via the soil surface (overland flow )
or groundwater (upward seepage ) are diffuse source s. In navigable surface water bodies,
inland shipping may also be a significant diffuse source of pollutants as a result of wastewater
releases, oil spills, and leaching from anti-fouling paints. These paints are applied to ship
hulls to prevent organisms such as algae and molluscs from adhering to the hulls where they
slow down the vessels and increase their fuel consumption.
Direct point source releases of pollutants include drainage from mine areas and effluent
discharges of untreated or treated industrial and municipal wastewater. Sewage water is
particularly rich in pathogenic bacteria and viruses, organic matter, and nutrients, but may
also contain increased levels of heavy metals , and dissolved salts. As a result of oxidation of
the organic matter, the discharge of untreated wastewater drastically reduces the dissolved
oxygen concentration in the receiving water, causing massive fish kills and other health
hazards. Wastewater treatment involves several steps, which are further explained in Box 3.I.
Although it may improve the quality of the effluent water compared to the untreated water,
effluent discharges may still place a considerable burden on surface water quality downstream
from the outflow, especially if the wastewater has only undergone a limited treatment.

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