Civil Engineering Reference
In-Depth Information
To C it y
Recharge
Ponds
Wells
Fig. 8-22.
Groundwater recharge using spreading basins
GROUNDWATER QUALITY CONSIDERATIONS
When water seeps downward through overlying material to the water table, particles
held in suspension, including microorganisms, may be removed. The extent of removal
depends on the depth and character of the overlying material. The bacterial quality of
the water also generally improves during storage in the aquifer because time and
storage conditions are usually unfavorable for bacterial multiplication or survival. Of
course, the clarity of groundwater does not guarantee safe drinking water, and only
adequate disinfection can guarantee the absence of pathogenic organisms.
After a well has been completed and is to be tested for yield, or any time after a
permanent pump is installed or repaired, the well should be disinfected. The procedure
is outlined in detail in AWWA A100-66, American Water Works Association Standard
for Deep Wells. This standard recommends the use of hypochlorites or fresh chlori-
nated lime in a concentration of 50 mg / L of chlorine.
Four alternative procedures are given: (1) preparation of the disinfecting solution
at the ground surface to a concentration of 50 mg / L and to a quantity at least twice
the volume of the well, after which the solution is discharged rapidly into the well
interior; (2) preparation of a stock solution containing 15,000 mg / L of chlorine and
diluting it to the 50 mg / L concentration in the well by feeding it into a continuous
flow of water; (3) adding the stock solution directly to the well and agitating the well
contents with a baler or bit; and (4) placing the calculated amount of dry hypochlorite
in a perforated pipe section capped at both ends and moving the pipe up and down
within the well casing. The disinfection period should be at least 2 hours, and the
procedure should not be attempted until after the well has been thoroughly cleansed
of oil, grease, and foreign matter of any kind. After the well has been pumped until
all of the chlorine solution has been removed, a sample should be collected for bac-
teriological examination.
All groundwater withdrawal points should be located a safe distance from sources
of pollution. Sources of pollution include septic tanks and other individual or semi-
public sewage disposal facilities, sewers, and sewage treatment plants, industrial waste
discharges, land drainage, farm animals, fertilizers, and pesticides. Where water re-
sources are severely limited, groundwater aquifers subject to contamination may be
used for water supply if adequate treatment is provided.
Because many factors affect the determination of safe distances between ground-
water supplies and pollution sources, it is impractical to set fixed distances. Where
insufficient information is available to determine the safe distance, the distance should
