Page 192 - Soil and water contamination, 2nd edition

Part III

Transport processes of substances in

soil and water

The environmental fate of chemical constituents is determined not only by the reactivity
of the soil–water environment and of the chemical constituents themselves, but also by
physical movement. The flux of matter through river basins is an essential parameter in
biogeochemical cycles in riverine and coastal floodplains and wetlands. Various subdisciplines
of earth science, such as hydrology, soil science, and geochemistry, have elucidated the
mechanisms that govern the flux of dissolved and sediment-associated nutrients and
contaminants in soil, groundwater, and river networks. The conveyance of dissolved and
sediment-associated contaminants along the surface or underground flow paths connecting
each point within a catchment to the catchment outlet is governed by complex, often non-
linear, physico-chemical interactions between water, sediment, contaminants, and biological
components, which operate at different spatial and temporal scales. These interactions
include underground and/or river channel dispersion along and between these flowpaths,
adsorption and desorption processes between the dissolved phase and soil and suspended
sediment, and biochemical decay. Moreover, the interactions vary in magnitude due to spatial
and temporal variation in emission of nutrients and contaminants at the soil surface or river
channel, the spatio-temporal distribution of rainfall, and the length, tortuosity, and velocity
of the flow paths. The interactions can therefore be seen as a response to hydrological events
in the short term, and to climate change, land use change, and changes in nutrient and
contaminant emissions in the long-term. So, knowledge of the fundamentals of transport
and fate of chemicals in the environment allows us to identify and analyse environmental
issues at scales ranging from local to global. This part therefore gives an overview of the
relevant transport processes at the landscape or catchment scale, their driving mechanisms,
and the ways they can be formalised in mathematical models.

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