Page 49 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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12 Chapter 1 Introduction to Water Systems

Figure 1.3 A Well Provides about 1.5 MGD (5.68 MLD) of Water to
Central Maui, Hawaii
(Courtesy of the Department of Water Supply, Maui County, Hawaii)

1.6.2 Wells
Depending on the geological formations through which they pass and on their depth,
wells are dug, driven, bored, or drilled into the ground. A well and its pumping equip-
ment are shown in Fig. 1.3. Dug and driven wells are usually confined to soft ground,
sand, and gravel at depths normally less than 100 ft (30 m). Hard ground and rock gen-
erally call for bored and drilled wells sunk to depths of hundreds and even thousands
of feet. In well-watered regions successful wells of moderate depth and diameter yield
1 to 50 gpm (4 to 190 L/min) in hard rock and 50 to 500 gpm (190 to 1,900 L/min) in
coarse sand and gravel as well as coarse sandstone. Wells in deep aquifers may yield
100 gpm (400 L/min) or more.
Except in hard rock, particularly limestone, without sand or gravel cover, wells are
generally not polluted by lateral seepage but by vertical entrance of pollution at or near
the ground surface. Pollution is excluded by watertight casings or seals extending into
the aquifer and at least 10 ft (3 m) below the ground surface, together with diversion of
surface runoff from the well area and its protection against inundation by nearby
streams.

1.6.3 Infiltration Galleries

Groundwater traveling toward streams or lakes from neighboring uplands can be inter-
cepted by infiltration galleries laid more or less at right angles to the direction of flow
and carrying entrant water to pumping stations. Water is drawn into more or less hori-
zontal conduits from both sides, or the riverside is blanked off to exclude the often less
satisfactory water seeping in from the river itself. Infiltration basins and trenches are
similar in conception. They are, in essence, large, or long, shallow, open wells. Filter
cribs built into alluvial deposits of streams intercept the underflow. Groundwater can