RBF_Cover (for eps) - National Water Research Institute

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Riverbank Filtration Suitability
A RBF system is designed to infiltrate water from an adjacent surface-water source, using streambed
and riverbank deposits to naturally filter out suspended materials from source water. The first (and
obvious) requirement is that the facilities be placed in close proximity to a source of recharge, such
as a river. During the feasibility and siting stages of a project, a number of criteria must be
considered, including:
• Availability of water from a surface-water source that can recharge the aquifer.
• An efficient hydraulic interconnection between the river and aquifer.
• Suitable water quality in the surface-water source.
• Sustainable flow in the river to match anticipated withdrawal rates.
A detailed hydrogeologic investigation is required to verify that aquifer conditions are favorable
for considering a RBF facility to meet project water demands. The investigation must evaluate the
geology of the aquifer and the interconnection between the river and groundwater in the aquifer
to determine the feasibility of inducing infiltration from the river, evaluate possible well designs,
and determine likely well yields. This type of investigation typically includes exploratory test
drilling, aquifer (pumping) testing, and data analysis to develop the needed information for each
prospective project site. Based on the results of this investigation, well designs are developed and
alternatives are compared for feasibility, yield, and cost. These alternative designs are generally
discussed below.
Vertical Wells
Vertical wells represent the most conventional method for developing a groundwater supply in the
country. These wells consist of a vertical borehole that is usually completed with a screen and riser
casing to allow water to enter from the formation and be pumped from the borehole via a pumping
system installed within the riser casing. A diagram of a typical vertical well constructed in an
unconsolidated (e.g., sand and gravel) aquifer is shown in Figure 2. Vertical wells can be used
effectively when small to moderate yields are needed, or when a system is growing slowly over a
longer period of time, such that adding a well to the system every year or so meets growing water
demands. To meet larger demands, a series of vertical wells must be installed, spreading along
riverfront areas or grouped into well-field clusters.
Vertical wells can be constructed using a variety of drilling methods, including mud rotary, reverse
circulation, cable tool, bucket-auger, dual-rotary, and air rotary. The method selected for each site
will take into account a combination of well criteria, including well depths, diameters, water-table
elevations, screen requirements, and potential problems caused by geologic conditions
encountered. These wells are constructed by drilling a vertical borehole, and then installing the
desired well riser casing and screen in the borehole. In some cases, an artificial gravel-pack filter
is also added to help prevent the intrusion of sand and silt from the formation during pumping.
Collector Wells
A collector well, also called a horizontal collector or radial well, differs from a vertical well in that
the well screen is installed horizontally into the aquifer formation from a central reinforced
concrete caisson that serves as the wet well pumping station. These wells are constructed by
sinking sections of reinforced concrete (called lifts) into the aquifer adjacent to the river until the
lower portion of the caisson reaches the design elevation for installing the well screen. Individual