Page 57 - Materials Chemistry, Second Edition

40 Practical Design Calculations for Groundwater and Soil Remediation

that purging three to five well volumes before groundwater sampling can be
a starting point. The purged water is often impacted and needs to be treated,
stored, and disposed of offsite. A good estimate of the volume of purged
water is necessary for site assessment.
To estimate the amount of purged water, the following procedure can be used:

Step 1: Determine the diameter of the boring, d b.
Step 2: Determine the diameter of the well casing, d c.
Step 2: Determine the depth of the water in the well, h.
Step 3: Calculate the well volume using the following formula:

Well volume = volume of the groundwater enclosed inside the well casing
+ volume of the groundwater in the pore space of the packing


2
Well volume =  π c dh +  π ( b d 2 − d 2 c ) h   φ (2.15)



 4   4 
Information needed for this calculation:
• Diameter of the borehole, d b
• Diameter of the casing, d c
• Effective porosity of the packing, ϕ
• Depth of the well water, h

Example 2.23: Well Volume for Groundwater Sampling
The water depth inside each of the four monitoring wells in Example 2.21
was measured to be 14.5 ft. Three well volumes need to be purged out before
sampling. Calculate the amount of purge water and also the number of
55-gallon drums needed to store the water. Assume the effective porosity of
the well packing is equal to 0.40.

Solution:
(a) Well volume
= [(π/4) × (4/12) × (14.5)] + {(π/4) × [(10/12) – (4/12) ]
2
2
2
× (14.5)} × (0.4)
= 3.92 ft 3
(b) Three well volumes
= (3)(3.92) = 11.8 ft = 88 gal for each well
3