Page 86 - Materials Chemistry, Second Edition

Plume Migration in Aquifer and Soil 69

Example 3.1: Estimate the Rate of Groundwater
Entering the Existing Plume
Leachates from a landfill leaked into the underlying aquifer and created a
dissolved plume. Use the data below to estimate the amount of fresh ground-
water that enters into the impacted zone per day:

• The maximum cross-sectional area of the plume perpendicular to
the groundwater flow = 1,600 ft (20′ in thickness × 80′ in width)
2
• Groundwater gradient = 0.005
• Hydraulic conductivity = 2,500 gpd/ft 2

Solution:
Another common form of Darcy’s law (Equation 3.1) is

Q = K × (dh/dl) × A = KiA (3.2)

where i (= dh/dl) is the hydraulic gradient.
The rate of fresh groundwater entering the plume can be found by
inserting the appropriate values into Equation (3.2):
Q = (2,500 gpd/ft )(0.005)(1,600 ft ) = 20,000 gpd
2
2

Discussion:

1. The calculation itself is straightforward and simple. However,
we can get valuable and useful information from this exercise.
The rate of 20,000 gallons per day represents the rate of upstream
groundwater that will come into contact with the COCs. This
water would become impacted and move downstream or side-
stream and, consequently, enlarge the size of the plume.
2. To control the spread of the existing plume, one needs to extract
this amount of water, 20,000 gpd (or ≈14 gallons per minute
[gpm]), as the minimum. The actual extraction rate required
should be larger than this, because the groundwater drawdown
from pumping will increase the flow gradient. This increased
gradient will, in turn, increase the rate of groundwater entering
the impacted zone as indicated by Equation (3.2). In addition, not
all the extracted water will come from the impacted zone