Page 322 - Soil and water contamination, 2nd edition

17

Patterns in groundwater

17.1 INTRODUCTION

Contaminant patterns in groundwater, as in soil, arise from spatial and temporal variations in
the contaminant inputs, groundwater flow , and chemical interactions between the sediment
(or bedrock) and groundwater. Groundwater and its associated dissolved and colloidal
substances are transported along groundwater flow line s. Each groundwater flow line is a line
whose tangent at any point is parallel to the groundwater flow velocity. Groundwater flow
lines intersect perpendicularly with equipotential line s, i.e. lines along which the hydraulic
potential remains constant. The network of groundwater flow lines and equipotential lines is
called a flow net (Hubbert, 1940) and the volume bounded by arbitrarily selected flow lines
is called a stream tube . This implies that the amount of water flowing through a stream tube
is constant along the stream tube. Exchange of dissolved substances across the boundaries
of a stream tube is limited and, by definition, occurs only due to molecular diffusion and
transverse mechanical dispersion .
Figure 17.1 depicts the concept of a stream tube . Note that stream tubes do not
necessarily have to be regular in shape and smooth. Complex large-scale whirl patterns may
occur in groundwater flow lines, as a result of the occurrence of anisotropy and macroscale
permeability heterogeneities in the aquifer material: for example, due to sedimentary
structures or the presence of palaeosols (Hemker et al., 2004).

17.2 HYDROLOGICAL SYSTEMS ANALYSIS

An analysis of the spatial arrangement of flow line s or stream tube s in the subsurface gives
valuable insight into the movement of groundwater and related processes. This approach is
called hydrological systems analysis and was developed by Tóth (1963) and later refined by
Engelen and Kloosterman (1996). The definition of a groundwater system used by various

Water table
Flow line
Equipotential line
Stream tube

6642 6642 6642

Figure 17.1 Concept of flow nets and stream tubes (after Hubbert, 1940).

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