Page 64 - Soil and water contamination, 2nd edition

3

Environmental compartments

3.1 INTRODUCTION

The basic chemical turnover processes treated in the previous chapter play a key role in
the ultimate fate of pollutants in the environment. Since the nature and intensity of these
processes are very variable in space, pollution studies are traditionally performed in units or
environmental compartments that are more or less homogenous with respect to the prevailing
physico-chemical conditions. A common subdivision into environmental compartments is
based on the major phase present (gas, liquid, or solid phase) and distinguishes between soil,
water, and air. Water is usually further classified in surface waters that are in contact with the
free atmosphere and subsurface waters (groundwater). It is, however, important to realise that
the gas, liquid, and solid phases are often all present in these environmental compartments.
Although in principle the same wide range of chemical processes brings about the variation
in the overall composition of soil and water, their direction and equilibrium state are different
in the different compartments. The governing chemical processes include silicate weathering ,
carbonate dissolution and precipitation, redox processes , and sorption to solid surfaces.
The redox processes in particular are often biologically mediated. These processes control
amongst others the genesis of soil profile s, the total concentrations of substances in water, the
decomposition of organic substances, and the retention of chemicals in sediment .
It should be noted that many physical and chemical processes occur not only within the
compartments, but typically also at the interfaces between the compartments. Examples of
such processes are the exchange of oxygen between the atmosphere and surface water, soil
pollution due to deposition of atmospheric pollutants, the leaching of pollutants from
polluted soils to groundwater, and the exfiltration of polluted groundwater to soil or surface
water. In general, pollutants in the solid or adsorbed phase are rather immobile and are only

translocated by bioturbation or by suspended or bedload transport in flowing surface waters,
or when soil is displaced by people. On the other hand, pollutants in the gas or liquid phase
are generally much more mobile, so pollutant transport within and between the different
environmental compartments occurs mainly in these phases. The principal driving force
behind the transfer between the environmental compartments is the hydrological cycle
(Figure 3.1). Atmospheric water derived from evaporation of the oceans, and to a lesser
extent, from evapotranspiration at the land surface, returns to the oceans and land surface
through precipitation in the form of rain or snow. Part of the water that falls onto the land
infiltrates into the soil and another part runs off directly into rivers and lakes . In soil, the
water percolates towards the groundwater and, in the course of time, the groundwater
discharge s into rivers and lakes, which, in turn, discharge into the oceans. As water goes
through this hydrological cycle it collects many solutes and takes them along its pathways.
The different environmental compartments are thus closely interlinked and act as a
continuum. Nevertheless, the subdivision used in this book follows the general classification
mentioned above, but where necessary, special attention is given to the interaction between
the compartments. Because issues of atmospheric pollution are beyond the scope of this

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