Visiting a Local Wellfield
Taking students into the field, out of the classroom, is always beneficial. If your community or a local business uses groundwater, see if you can make arrangements to visit a wellfield so students can look at a well and begin to understand the mechanics of withdrawing water from the ground. If you can gain access to the well itself and have a water-level probe, or can borrow one, students can see how the U.S. Geological Survey personnel made the water-level measurements that they will contour in Module 4. If observation wells are also available, make water-level measurements in these wells too.
The primary goal here is to let students activity see how water-level measurements are made and to see how a well withdraws water from an aquifer. If a water treatment plant is connected to the well, take the opportunity to show students the treatment system. (Woburn's wells were not connected to a treatment system.
Context for Use
Teaching Notes and Tips
A field trip to a nearby municipal wellfield containing both pumping wells and observation wells enables students to visualize the three-dimensional nature of pumping cones of depression, to see how water-level measurements are made, and to learn about water-treatment technologies. Water-level probes and proper field techniques used to measure water levels in observation wells can be demonstrated in observations wells installed at most municipal wellfields. The field trip also enables the instructor to discuss the need for obtaining and contouring synoptic sets of water-level data (measured at the same time, as opposed to measured over a period of several weeks or months during which water-level declines or rises could occur), as was done by the U.S. Geological Survey on December 4, 1985 and January 3, 1986.
Leading such a field trip requires knowledge of your local municipal water supply; whether dams, reservoirs, or wells are used. Subsequent to 9/11, a letter of request to visit the wellfield and/or the water-treatment plant is needed as well as keys to unlock the pump houses and any observation wells. This letter is usually written to the water-plant supervisor.
It is proper procedure whenever putting a water-level probe down a well to wash/rinse the probe cable and the probe tip in distilled water prior to lowering it down the well and then to rinse the tip of the probe with distilled water after each well that the probe is lowered into. An aneroid barometer or a GPS unit commonly is be used to measure the surface elevation of the measuring point on the well collar, which serves as the measuring point for the depth to water in the well as measured by the water-level probe. Thus, students can see that the elevation of the water level in the observation well is obtained by subtracting the depth to water measured in the well from the measuring point elevation on the well collar. It is the set of these water levels measured by USGS employees that the students are contouring in their assignments.
Being at a municipal pumping well also enables the instructor to discuss local water-treatment technologies (filtration, hardness reduction with lime, chlorination, reverse osmosis, etc.) and the differences between steady-state and transient conditions in a ground-water flow system. During steady-state conditions water levels do not change over time and, as a result, ground water is not going into or out of storage and hydraulic gradients and ground-water flow velocities at any point in the flow system are constant (i.e. not changing over time). This is what is assumed whenever flowpaths are drawn on a potentiometric surface.
Under transient conditions, water levels are changing in the flow system as ground water moves into or out of storage, which causes hydraulic gradients and flow velocities at any point in the flow system to change over time. As a result, potentiometric maps and flowpaths are difficult to draw and are beyond the level of this module.
With respect to the Woburn water-level data that the students contour in this module, the December 4, 1985 data can be used to represent historic steady-state conditions in the flow system when wells G and H were not pumping and only the Riley Tannery (Beatrice) supply wells were operating. The water-level data from January 3, 1986 can be used to represent historic steady-state conditions in the flow system when wells G and H were pumping together at their average historic rates of 700 gpm and 400 gpm, respectively. Although wells G and H pumped together only about 26 percent of the time between October 1967 and May 1979, the conditions create during the 1985-86 pumping test do represent "worst-case" conditions.
Bring along a large sketch pad, fat magic markers, a water level probe (if available with a wrench and pliers), a measuring tape, etc.
In progress 09-07-08
References and Resources
- Sanders, Laura, 1998. A Manual for Field Hydrogeology. Prentice Hall, 381 pp.