Problem 9 Solution

Question: Estimate the water level in each well and determine if there is vertical flow across the clay aquitard.

Solution:

Step 1: As an approximation, assume the water table is linear. By linear interpolation, the water level in the center is 52 m. The well on the right is half the distance from the right side to the middle, so the water level is 51 m. Similarly, the water table elevation at the location of the well on the left side is 53 m.

Step 2: The horizontal confined aquifer beneath the clay is bounded on both sides by constant head boundaries, the values of which correspond to the water level elevation in each water body (see Figures 12 and 17 for analogous scenarios). Flow is horizontal based on the geometry of the confined aquifer and the position of the boundaries (on either end).

Step 3: The medium is homogeneous, so the gradient is constant, meaning that the spacing between the equipotential lines is constant. The spacing between the lines and their values are determined by linear interpolation.

Step 4: The well on the left side is screened in the confined aquifer where the head is 53 m (as indicated by the equipotential line). At this same location, the water table elevation is also 53 m. We simplified by assuming a linear decline of the water table even though we know there will be a steeper gradient at the right side due to the decreasing flow area perpendicular to flow (as illustrated in Figure 22). Because we are assuming a linear gradient in both the confined and unconfined aquifer the head at each location will be the same above and below the clay. As a result, there is no vertical gradient and so, no vertical flow across the clay.