Page 194 - Soil and water contamination, 2nd edition

10

Systems and models

10.1 A SYSTEMS APPROACH

Water participates as a reagent in a wide range of adsorption–desorption, dissolution–
precipitation, acid –base, and redox reactions . Moreover, water is the major medium
conveying dissolved ions, colloidal particles and particulate matter through soil, groundwater
and surface water. Given water’s role in the dispersal and fate of contaminants in the
environment, if we are to understand the direction and rate of dispersal and the spatial
and temporal variation of contaminant concentrations in soil and water, we need an
understanding of hydrology and hydrological pathways.
A useful way to study hydrological pathways is to consider the Earth or a part of it as
a system with clearly defined boundaries that exchanges energy and mass (water) with
its surroundings. In this manner, concepts and principles from systems theory , the
transdisciplinary study of the abstract organisation of phenomena proposed by the biologist
Ludwig von Bertalanffy in the 1940s (Von Bertalanffy, 1968), can be applied. Rather than
reducing an entity (e.g. soil) to the properties of its parts or elements (e.g. mineral grains or
organic matter), systems theory focuses on the quantitative description of the arrangement
of and relations between the parts and connects them into a whole. Many disciplines
(physics, chemistry , biology , geography , sociology, etc.) base their concepts and principles of
organisation on this theory.
Conceptual hydrological systems are based on the notion of stores of water or substance,
whose state depends on a variable amount of water or substance (volume or mass) in them.
These stores or subsystems may be defined in many ways: for example as functional units
(e.g. soil water, groundwater, surface water), morphological units (e.g. hill slope unit, river
channel, lake, estuary), or discrete spatial units constructed through regular or irregular
tessellation of space. Discrete spatial units are often applied in numerical modelling and
hydrological modelling using geographical information systems (GIS) (see Burrough and
McDonnell, 1998) (Figure 10.1). The relations between the stores are determined by the
fluxes between them, which have the potential to change the state of the store . The stores
at the system boundaries may also be influenced by the possible presence of inputs and
outputs across these boundaries. Because of the spatial organisation and ordering of the stores
distinguished, the hydrological system is often modelled by a cascade of stores (see Figure
10.2). The system should obey fundamental principles of thermodynamics and continuity;
in other words, it must obey the conservation laws of energy, momentum, and mass. This
implies that the change of mass and energy in each store and the whole system equals the
sum of input and output fluxes integrated over time. Section 11.1 goes into more detail on
the concepts of mass balance .
Water and substance transport is influenced by numerous feedback mechanisms. A

negative feedback mechanism is a controlling mechanism that tends to counteract some
kind of initial imbalance or perturbation. A good example is a simple system of a reservoir

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