Lab 2 Modeling Groundwater Flow with Flownets

Purpose: Using a graphic method to model flow (1) under a dam, (2) through a deep, confined aquifer, and (3) passing from one media to another with different hydraulic conductivity.

Reading assignment: (Flownet construction procedures)
Lee and Fetter: p 53.-59.
Fetter: p. 153-161.


Groundwater flow beneath a dam

Read the problem in p. 55 of Lee and Fetter. Using the map shown in Fig. 7.1 (make several copies of them, you may need redraw your flow net), answer the following questions.

(1). Answer questions 1A, 1B, and 1C. (3 pts).

(2). Construct a flownet on Figure 7.1 showing groundwater flow (use arrows to show flow directions) under the dam. Use 3 or 4 flowtubes. (3 pts).

(3). How much water is lost (in m3/day) under the dam through discharge Q under the dam?. 3 pts.



Groundwater flow through a deep, confined aquifer

Read the problem in p. 57 of Lee and Fetter. Subsurface information and structure contour map of an impermeable bedrock is shown in Fig 7.2. and Fig. 7.3.

Note that all well data measurement in Figure 7.2 of Lee and Fetter are presented as depth relative to the ground surface. In order to correlate between wells, we must express the elevation of water table and formation boundary relative to a common datum, normally sea level. Correlation of the lithologic logs of the three wells in Fig. 7.2 are shown in the next page. The permeability of the aquifer is 100 darcys (one darcy _ 3.17_10-5 ft/s _ 2.74 ft/day). The thickness of the aquifer is about 40 feet. Base on the correlated cross-section, answer the following questions.

(1). What is the direction of "dip" (incline downward from a horizontal plane) of the aquifer? (3 pts)

(2). Does the aquifer pinch out against the ridges to the east and west as it continues to the south through the saddle? Explain. (3 pts)

(3). Identify the no-flow boundaries of the flow field, if any. (3 pts).

(4). Construct the flownet in the aquifer in Fig. 7.3 using eight flowtubes. Starting your construction in the saddle (at the constriction) and work north. Set the contour interval of the equipotential lines to three feet. Thus, the net should define three equipotential contours between wells B and C, which differ by nine feet of head. Extending the net to the north, well A should fall very close to a contour line of 265 ft. (3 pts)

(5). Determine the discharge (in ft3/day) through the aquifer (3 pts).



Groundwater refraction

The map below represents an area where geological conditions result in a series of permeability contrasts. Contaminants will migrate in the direction of flow, and although the contaminants will disperse laterally with distance, the center of the contaminant plume can be estimated if the flow paths are known accurately.

A pollutant is discharge at point X, where it enters the groundwater flow system and migrate southward. The boundaries (solid lines) between different formations (A, B, C) are oriented at 20í relative to W-E direction.

1. Draw the flow lines across Formations A, B, and C (3 pts).

2. Determine where the flow line (the center of the plume) will leave the map area (3 pts)