Page 350 - Soil and water contamination, 2nd edition

18

Patterns in surface water

18.1 INTRODUCTION

The patterns of surface water composition are controlled by a wide range of factors and
processes, including catchment geology, diffuse and point source inputs from anthropogenic

sources, catchment topography and hydrology , and in-stream processes (see e.g. Jarvie
et al., 2002). As we have seen in the previous chapters, catchment geology determines
the background concentrations in soils and groundwater, and thus also in surface water.
Catchment topography and hydrology play important roles in the transfer of contaminants
from the catchment surface to the drainage network. After contaminants have entered the
surface water via overland or underground pathways or a combination of both, in-stream
biogeochemical processes modify their concentrations. Thus, to understand, interpret, or
predict patterns in surface water composition, it is important to understand the processes and
patterns in soil and groundwater, and the physical and biogeochemical conditions affecting
the surface water (see e.g. Bouwman et al., 2013).
Surface water differs from groundwater by having a stronger interaction with the
atmosphere, a weaker interaction with sediments, and by being penetrated by sunlight. As
a result, the nature and rate of the various biogeochemical processes that act in surface water
differ from those in groundwater. Generally, the redox potential is higher in surface water
than in groundwater, because of the presence of relatively high concentrations of dissolved

oxygen . The oxygen originates from diffusion of atmospheric oxygen or photosynthesis
by primary producers (e.g. algae and submerged aquatic macrophytes) during daylight.
Photosynthesis simultaneously extracts dissolved CO (carbonic acid ) from the water, which
2
raises the pH . Therefore, surface waters are mostly neutral to basic (compare Figure 2.3),
except for water bodies that are susceptible to acidification in poorly buffered catchments.
Compared to groundwater, surface water flow s fast. Whereas groundwater travels at the
rate of centimetres per day or even less, surface water travels at a rate in the order of metres
to tens of metres per minute. Thanks to the fast transport rate of surface water, the reaction
kinetics usually become apparent in the spatial variation in surface water composition.
Accordingly, water pollutants that are discharged instantaneously or continuously into
surface waters are rapidly carried downstream and can assert their influence across distances
up to hundreds of kilometres, as unfortunately demonstrated by large accidental spill s into
rivers: for example, the Sandoz accident in the river Rhine in 1986 (see Figure 10.4) or the
Ajka alumina sludge spill near Kolontár, Hungary, in October 2010.
Whereas rivers have a pronounced downstream flow of water, lakes do not. Instead of
gravity, wind is the main force generating and driving currents in lakes. The downwind
current near the lake surface typically moves at a rate of 2 to 3 percent of the average wind
speed (Hemond and Fechner-Levy, 2000). Since the water flowing downwind cannot
accumulate indefinitely at the downwind lake shore, a return current arises, usually at a
greater depth. In large lakes the flow pattern may become very complex, as the water flow is

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