Page 197 - Soil and water contamination, 2nd edition

184 Soil and Water Contamination

The transport of chemicals associated with sediment is often referred to as particulate
transport and the transport in dissolved form as solute transport .
While travelling along the surface and subsurface flow path s, the transported chemicals
may be subjected to a wide range of physical and biogeochemical processes. Key processes
are transformation and retardation , which are commonly referred to as retention .
Transformation refers to processes that irreversibly alter a substance to a different substance,
such as denitrification , microbial degradation, or physical decay. Retardation means delay
due to, for example, sorption to sediments, chemical precipitation, diffusion into stagnant
water, and sediment deposition; it leads to a temporary or permanent storage of substances
within the catchment system. The residence time of pollutants in a given store or subsystem
is a critical factor in estimating environmental risk. Residence time is the average amount
of time a particular substance remains in a given store. For example, the residence time of
water in a lake is the amount of water in the lake divided by the rate at which water is lost
due to evaporation or runoff through a stream. Likewise, the residence time of nitrate in a
groundwater body is the amount of nitrate in that groundwater body divided by the rate of
mass loss due to groundwater flow and denitrification.
The rates of the physical and biogeochemical processes are rarely constant but
are controlled by the physico-chemical environmental conditions. For example, the
denitrification rate is controlled by temperature and redox conditions. The study of the rate
of the processes is called kinetics . As we saw above, the process rates are often also subject to
manifold feedback mechanisms, which thus means that the process rates are controlled by
the state variable itself. For example, the discharge from a lake is controlled by the lake level
(see Equation 10.1) and the denitrification rate is controlled by the nitrate concentration.
Studies of the environmental fate of substances usually start by identifying the different
natural and man-made sources in the system under study, their magnitude, and spatial and
temporal variability . As noted in chapter 1, pollutants can come from point source s or diffuse
source s. Pollution around a point source is mostly confined to a plume in the downstream
direction from the source with sharp concentration gradients at the fringe of the plume. In
contrast, the pollutant concentration gradients in soil and water originating from diffuse
sources are usually gradual. An exception is if the discharge rates from the diffuse sources
or the physical and biogeochemical processes that can alter the concentrations are spatially
or temporally discontinuous or regulated by thresholds. Sharp spatial boundaries occur in
groundwater due to discontinuities in diffuse sources: for example, when there are adjacent
plots of land with different land use (e.g. arable land and forest), causing a discontinuity
in nitrate loading from the soil surface. This is important because landscapes are often
patchy and discontinuous and the boundaries between the landscape units encompass
pronounced spatial heterogeneities. These sharp boundaries are propagated underground
along the groundwater flowpaths (Figure 10.3). In general, such sharp boundaries occur
more in groundwater than in surface water, since turbulence caused by currents and waves
mixes surface waters more thoroughly. Temporal variation of input from diffuse sources arises
due to seasonal changes in climate variables such as temperature and precipitation, and to
abrupt disturbance due to extreme weather events, the planting and harvesting of crops, the
application of fertilisers and pesticides , and the dumping of wastes.
Because the substances are retained and lost along their transport pathways, the loads
from the small subcatchments and the different sources often do not simply add up to
the total transport from a river basin. We have also seen that physical and biogeochemical
processes result in spatial and temporal patterns of concentrations of substances in soil,
groundwater, and surface water, not only because external and internal factors control the
process rates but also because the sources vary in space and time. The effects of the whole
of inputs and processes make the transport and fate of sediment and chemicals through
drainage basins a complex issue. The advantages of a systems approach arise from the fact

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