A Civil Action - The Woburn Toxic Trial > Learning Modules > 4 - Groundwater Hydrology > Constructing a Potentiometric Profile

Learning Module: Ground-Water Hydrology-
Constructing a Potentiometric Profile
Wells G & H Superfund Site, Woburn, MA

Student Assignment


3D illustration of the Aberjona River ValleyThree-dimensional illustration of Aberjona River valley (courtesy of Dr. Ramesh Ventakatakrishan)

Introduction

Potentiometric surface maps, like the one constructed for Problem 2 in this chapter, combined with flow line analysis are important tools for visualizing the directions of groundwater flow and changes in hydraulic gradients in an aquifer. Although these depictions can yield great insight into the conceptualization of groundwater flow, they do not show vertical aspects of groundwater flow. The two-dimensional plan view afforded by potentiometric surface maps cannot reveal information concerning vertical flow gradients and vertical flow components. As a result, in a three-dimensional flow system, flow line lengths and travel times can be underestimated if they are computed solely from a potentiometric surface map.

Construction of a potentiometric profile allows insight into regional patterns of groundwater flow, identification of recharge and discharge areas, the roles of rivers and lakes, and the effects of pumping wells. Like potentiometric surface maps, potentiometric profiles consist of a series of contour lines of equal hydraulic heads. The contours are based on water level measurements made in wells screened at different depths along a cross section through the study site. The position of the screen in each well is important and must be portrayed at the proper elevation. The contour lines of equal hydraulic head are created in the same manner as in the construction of a potentiometric surface map. However, there are a few important differences in the manner in which the contour lines in these types of maps are constructed as explained in the Reference Book.

Wells completed to different depths at the same location (well clusters, nested piezometers, or multiport wells) enable vertical hydraulic gradients to be computed and vertical flow components to be visualized. Vertical hydraulic gradients are computed by subtracting the hydraulic head value in the deeper well from the value in the shallower well and dividing the remainder by the vertical distance between the midpoints of the well screens (see digram below). A downward flow component is indicated if the gradient is negative, meaning the hydraulic head is less at depth. Conversely, an upward flow component is indicated if the gradient is positive, meaning the hydraulic head is greater at depth. The magnitude of the gradient indicates its significance.

Partially penetrating wells, rivers, lakes, and ponds are hydrologic features that can create vertical hydraulic gradients (see Reference Book). The Aberjona River and the pumping of municipal wells G and H, which partially penetrate the buried valley aquifer, at Woburn, Massachusetts create vertical hydraulic gradients that affect the configuration of equipotential lines and flow lines.

diagram illustrating how to determine the vertical hydraulic gradientVertical hydraulic head gradient is determined using water levels from two wells in close proximity to one another. In the diagram, vertical gradient is upward since the deeper well screen has a higher water level than the shallower well screen.


Instructions

The "Orientation" worksheet shows the cross section location that will be used for construction of the potentiometric profile in this exercise. It is orientated southwest to northeast across the Aberjona River valley and through well H. This exercise uses many of the same wells used in the construction of the geologic cross section in Problem 1. In this exercise, we will use the measured hydraulic head values listed on the "Cross Section" worksheet. The figure in the "Cross Section" worksheet illustrates the upper and lower boundaries of the glacial aquifer as the "ground surface" and the "top of bedrock." The top of bedrock portrayed on this figure is based on well logs and a bedrock surface map. The locations, total depths, and positions of screens for each well are shown on the figure. Thus, the hydraulic head values listed on the "Cross Section" worksheet correspond to the illustrated screened intervals. All the hydraulic head values were measured on January 3, 1986, at the completion of the 30-day constant-rate aquifer test (see Problem 2 for more information) (Myette et al., 1987).

Your objective is to construct a potentiometric profile using the provided water-level data which represent a time when the groundwater flow system was nearly steady-state in response to the pumping stresses at wells G and H.

  1. Print the "Cross Section" worksheet to use as a template for construction of the potentiometric profile.
  2. Using a pencil, label the measured hydraulic heads next to each screened interval. Continue to use a pencil until you have completed the exercise.
  3. Show the elevation of the water table at each of the wells with a small tick using the vertical scale provided. This will be the value of hydraulic head in the uppermost port (screen) in the well.
  4. Draw the water table on the profile by connecting the tick marks. Remember that this surface usually mimics the land surface elevation except where a cone of depression forms around a pumping well (e.g., Well H).
  5. Using a 5-foot contour interval, contour the hydraulic head values posted next to the well screens. Be careful to honor all the data points and have the contour lines intersect the water table at a right angle at the elevation equal to the value of hydraulic head (see Reference Book). A suggestion is to start each contour line at the water table, for example the 60-foot contour line on the east side of the cross section would start at the point where the water table is at an elevation of 60-foot. It would extend vertically downward to the base of the aquifer which is marked by the top of bedrock line. A vertical equipotential line would indicate horizontal flow. Near the pumping well (Well H), equipotential lines will form a halo around the well screen as shown in the problem setup material.
  6. Calculate the vertical exaggeration of the profile similar to Problem 1.
  7. Answer the questions about your potentiometric profile in the "Questions" worksheet.

Materials


References

Metheny, M.A., 1998, Hydrogeologic framework and numerical simulation of flow to well G & H at Woburn, Massachusetts, unpublished M.S. thesis, Department of Geological Science, The Ohio State University. 247p.

Myette, C.F., D.G. Johnson, J.C. Olimpio. 1987. Area of influence and zone of contribution to superfund site wells G and H, Woburn, Massachusetts. U.S. Geological Survey, Water-Resources Investigations 87-4100. 86p.

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