Page 56 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 39

(b) Number of 50-lb sand bags needed
= (47.6 ft )(112 lb/ft ) ÷ (50 lb/bag) = 107 bags
3
3
Answer: 107 bags are needed.

Discussion:
1. Packing interval should be slightly larger than the perforation
interval.
2. We should add an additional 10% to the estimate of sand usage
as a safety factor to take into consideration that borehole shape
would not be a perfect cylinder.

Example 2.22: Amount of Bentonite Seal Needed

The four monitoring wells in Example 2.21 are to be sealed with 5 ft of ben-
tonite below the top grout. Estimate the number of 50-lb bags of bentonite
needed for this application. Assume the total bulk density of bentonite is
equal to 1.8 g/cm (112 lb/ft ).
3
3
Solution:
(a) Volume of bentonite needed for each well
= {(π/4)[(10/12) − (4/12) ]}(5) = 2.29 ft 3
2
2
Volume of bentonite needed for four wells
= (2.29)(4) = 9.16 ft 3
(b) Number of 50-lb bentonite bags needed
= (9.16 ft )(112 lb/ft ) ÷ (50 lb/bag) = 20.5 bags
3
3
Answer: 21 bags are needed.

Discussion:
We should add an additional 10% to the estimate of bentonite usage as
a safety factor to take into consideration that borehole shape would
not be a perfect cylinder.

2.3.3 Well Volume for Groundwater Sampling

Purging is the process of removing stagnant water from a monitoring well
before sampling groundwater. The stagnant volume includes the water
inside the well casing and in the sand/gravel packing. A few parameters are
often monitored, such as conductivity, pH, and temperature, to ensure that
they reach a consistent endpoint before sampling. The purge volume is site
specific and depends heavily on the subsurface geology. A rule of thumb is