Page 95 - Materials Chemistry, Second Edition

78 Practical Design Calculations for Groundwater and Soil Remediation

3.2.5 Determine Groundwater Flow Gradient and Flow Direction
Having a good knowledge of the gradient and direction of groundwater
flow is vital to groundwater remediation. The gradient and direction of flow
have great impacts on selection of remediation schemes to control plume
migration, such as location of the pumping wells and groundwater extrac-
tion rates, etc.
Estimates of the gradient and direction of groundwater flow can be made
from a minimum of three groundwater elevations. The general procedure is
described here, and an example follows.

Step 1: Locate the three surveyed points on a map to scale.
Step 2: Connect the three points and mark their water-table elevations
on the map.
Step 3: Subdivide each side of the triangle into a number of segments
of equal size. (Each segment represents an increment of ground-
water elevation.)
Step 4: Connect the points of equal values of elevation (equipotential
lines), which then form the groundwater contours.
Step 5: Draw a line that passes through and is perpendicular to each
equipotential line. This line marks direction of flow.
Step 6: Calculate the groundwater gradient from the formula, i = dh/dl.

Example 3.6: Estimate the Gradient and Direction of Groundwater
Flow from Three Groundwater Elevations
Three groundwater monitoring wells were installed at an impacted site.
Groundwater elevations were determined from a recent survey of these
wells, and the values were marked on a map. Estimate the flow gradient and
direction of the groundwater flow in the underlying aquifer.

Solution:
(a) Water elevations (36.2′, 35.6′, and 35.4′) were measured at three
monitoring wells and marked on the map.
(b) These three points are connected by straight lines to form a
triangle.
(c) Subdivide each side of the triangle into a number of segments of
equal intervals. For example, subdivide the line connecting point
A (36.2′) and point B (35.6′) into three intervals. Each interval rep-
resents a 0.2′-increment in elevation.