Page 75 - Soil and water contamination, 2nd edition

62 Soil and Water Contamination

3.4 SURFACE WATER
3.4.1 Definition of surface water

Surface water includes all water on the surface of the Earth found in rivers , streams, canals,
ditches, ponds, lakes , marshes, wetlands, coastal and marine waters, and as ice and snow
(EEA, 2006a). In this book, however, the definition of surface water is restricted to fresh
waters, so therefore excludes the water in seas and oceans. The water–atmosphere interface
constitutes the top boundary of the surface water compartment and, therefore, surface
waters are generally well aerated. The bottom boundary of a permanent surface water body
is formed by bed sediments , except where bare hard rock is present directly underneath the
surface water.
A notable difference between surface water and groundwater is that sunlight can
penetrate into surface water. This allows primary producers (green plants, including algae
and macrophytes) to grow in surface water, thereby producing both oxygen and organic
matter by means of photosynthesis . This process is crucial in the alteration of surface water
chemistry .
Surface water is fed by inflow of rising groundwater, and occasional overland flow or
throughflow (i.e. shallow, lateral flow of water through soil) from the surrounding uplands,
and precipitation that falls directly on the water surface. In streams, groundwater comprises
most of the base flow (i.e. dry weather flow). During and directly after rainfall events, a large
proportion of stream water may be derived from overland flow or throughflow. Water is
removed by discharge to downstream areas, infiltration (only at locations where the surface
water level is higher than the water table ), and evaporation . In addition, artificial surface
water sources and sinks may be present: for example, discharge of wastewater (effluent ), or
surface water extraction for industrial processes or the production of drinking water. The
flow velocity of surface water is positively related to water discharge and slope gradient of the
water level, and is inversely related to the cross-sectional area of the water body perpendicular
to the water flow and to roughness of the channel bed. The presence of ripples, dunes, or
aquatic vegetation increases the roughness and slows down water flow. Lakes have a large
cross-sectional area, so the flow velocity in lakes is generally low, varying between 0.001 and
-1
0.01 m s (near-surface values) (Meybeck and Helmer, 1996). Besides having a large cross-
sectional area, lakes also have a relatively large surface area exposed to wind. Wind blowing
across an open water surface generates waves and induces currents. This effect increases with
the unobstructed distance over which the wind can blow (wind fetch), and thus increases
with the size of an open water surface. Whereas the water flow in rivers is largely driven by
gravity, usually the principal means of water transport in lakes is wind-induced currents.
Transport rates in the surface water are usually much faster than in soil and groundwater:
whereas soil water or groundwater may move only tens of metres per year, surface waters may
cover this distance in a few seconds to hours.

3.4.2 Bed sediments

The submerged sediments present beneath or alongside rivers and lakes constitute a
dominant link between the subsurface environment and the surface water. They consist of
porous, hydraulically conductive, clastic (mineral) sediments and organic matter , which
is either allochthonous (i.e. the sediment material originates from upstream areas with a
possibly different geology) or autochthonous, (i.e. the material has been derived from local
rocks). The zone of bed sediments is also referred to as the hyporheic zone (Edwards, 1998)
(see Figure 3.5). Bed sediments are zones of intense biological activity and usually contain
substantial quantities of organic matter. They encompass steep, vertical physical and chemical

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