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Groundwater resources and environmental management 291

Fig. 8.6 Schematic representation of types of flow conditions at riverbank filtration sites. The majority of riverbank filtration schemes
are of Type 1. Groundwater flow beneath the river (Types 3, 4 and 6) is typically neglected at most sites. The formation of unsaturated
conditions beneath the river occurs if groundwater abstraction rates are not adapted to the hydraulic conductivity of the river bed or if the
hydraulic conductivity of the river bed material becomes clogged due to surface water pollution inputs (Type 4). At some sites, the river
bed cuts into the confining layer (Type 5). Collector wells are used with laterals at different depths, of different lengths and directions.
Type 6 gives only one example with a lateral towards the river. After Hiscock and Grischek (2002).

Compared with conventional surface water abstrac- Undesirable effects of riverbank filtration on water
tion, the natural attenuation processes of riverbank quality can include increases in hardness, ammonium
filtration can provide the following advantages: and dissolved iron and manganese concentrations and
elimination of suspended solids, particles, biodegrad- the formation of hydrogen sulphide and other malo-
able compounds, bacteria, viruses and parasites; dorous sulphur compounds as a result of changing
part elimination of adsorbable compounds; and the redox conditions (Fig. 8.7).
equilibration of temperature changes and concentra- The beneficial attenuation processes result mainly
tions of dissolved constituents in the bank filtrate. from mixing, biodegradation and sorption processes