RBF_Cover (for eps) - National Water Research Institute

20
Laterals
Alternate Systems
In addition to vertical wells and radial collector wells, infiltration galleries can be used to develop
filtered surface-water supplies through RBF. These can include trenching parallel or beneath the
river and installing screened gallery pipes that can deliver filtered surface water to sumps and wet
wells. It is also possible to use horizontal directionally drilled wells to induce infiltration from an
adjacent river. These systems are often installed under low head conditions, such that lower per
foot yields are obtained, requiring longer gallery lengths to meet higher capacity needs. It is often
difficult to perform effective well-screen maintenance on these systems.
Well Maintenance
Pump Shaft
Pump
Pump House
Central
Collection
Caisson
Well Screens
(in Laterals)
Figure 3. Typical components of a radial collector well (Ray et al., 2002b).
Land Surface
It is normal for well screens to become plugged with chemical (mineral) precipitates and biological
growths (e.g., iron bacteria) over time in alluvial aquifers. The rate of plugging can be exacerbated
if the screen design creates excessive entrance velocities through the slot openings in the well
screens, so it is important that proper screen design considers the water quality and hydraulic
characteristics of the aquifer to maintain entrance and approach velocities within acceptable
ranges to minimize this rate of plugging and extend the intervals between well cleanings. Since
the lineal footage of well screen in a collector well is longer than for a vertical well, entrance
velocities are minimized so that the intervals between required maintenance can be extended.
This typically results in lower O&M costs over the life of the collector well.
Well maintenance can be accomplished using a variety of methods including mechanical,
chemical, or a combination of methods. The most effective method to restore well-screen
openings and well efficiency will vary from well field to well field, and is selected based upon
details of the well construction (e.g., screen design), groundwater quality, past results, the nature
of plugging (mineralogic versus biological), and other factors. The use of an ongoing monitoring
and record-keeping program should allow you to track well performance, identify operating trends,
and predict optimal times for performing maintenance.