Water-Level Maps
Initial Publication Date: October 17, 2007
Summary
In this module students learn some of the fundamental concepts of groundwater flow including measurement of water levels, hydraulic heads, construction of potentiometric surfaces, construction of flowpaths, and calculation of groundwater flow velocities and traveltimes. The goals of this module are to learn how to contour irregularly spaced data (water-level measurements made in observation wells), how to draw flowpaths on a contoured water-table map, how to measure hydraulic gradients from a contoured water-table map, and how to compute flow velocities and traveltimes using Darcy's law. The goals are applied to examining flowpaths of groundwater moving from the W.R. Grace and Beatrice properties and to computing traveltimes along these flowpaths.
The EXCEL spreadsheet requires students to contour two sets of water-level data collected by the U.S. Geological Survey. One set of data was collected prior to wells G and H operating and the potentiometric surface represents the configuration of the water table during times between 1964 and 1979 when wells G and H were not operating. The other set of data was collected after wells G and H had pumped for 30 days at their average operating rates and the potnetiometric surface approximates the typical configuration of the water table when wells G and H were operating together between 1964 and 1979. These two contour maps with selected flowpaths drawn on them and and a map of the drawdown created by pumping wells G and H after 30 days of continuous pumping represent three of the major pieces of evidence to be interpreted by students and presented in the mock trial. The traveltimes of along flowpaths emanating from the Beatrice property and from the W.R.Grace property also are important to the mock trial.
Learning Goals
The overall goal of this module is for students to develop a sense of how groundwater moves through the subsurface. By constructing contours of hydraulic head (water level) and drawing flowpaths atop the contours, students develop a sense of the three-dimensionality of the water-table surface and the movement of groundwater. The calculations of hydraulic gradient and flow velocity enable students to appreciate the rate at which groundwater moves. These simple calculations expose students to Darcy's Law.
Context for Use
The contouring exercise is probably best executed in a laboratory setting. Students can prepare contours in groups of two or that they teach each other how best to fit the contour lines to the spatially irregular data. Students should use pencils and be prepared to erase and redraw the contour lines and flowpaths several times before they develop the skill (knack) required to make these maps.
Description and Teaching Materials
Examples of groundwater contour maps should be used to show students how contour lines are created and how flowpaths intersect water-level contours at right angles. These examples can be found in any groundwater textbook and in several manuals produced by the United States Geological Survey, which are cited in the references. If three-dimensional block diagrams are available, these usually help students visualization of the subsurface features.
Teaching Notes and Tips
Student's should be made familiar with triangulation as a method to determine the placement of contour lines about a cluster of three points. Extending this technique to other clusters of points produces the contour map. One-on-one interaction with the students is the best method to help them understand the contouring process.
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Assessment
Assessment of this activity should be concurrent with the completion of the contouring exercise. Student understanding of the contouring exercise is enhanced when they are assessed as they progress with completing the contour map. Following the completion of satisfactory contour map, the instructor should use a blackboard or similar 'front of room' a presentation to show groundwater flow directions normal to the contour lines and how the spacing of the contour lines indicate hydraulic conductivity.
References and Resources
In the searchable list of references, refer to: