Module 4: Groundwater Flow in the Area of Wells G and H
- Provide an overview of the principles of hydrogeology
- Contour water levels used in the trial
- Interpret flow directions and compute travel times from contoured potentiometric maps
When Judge Skinner divided the trial into three separate phases, it became essential in Phase I for the parties to explain how the contaminants traveled from the W.R. Grace property and Beatrice Foods property to Wells G and H. During direct examination, the expert witnesses hired by the defendants and the plaintiffs had to teach the jury many of the principles of groundwater flow and then to demonstrate how these principles could be applied to produce their expert opinions.
Basics of groundwater hydrology
The foundations of groundwater flow are based on a series of experiments performed in 1855 and 1856 by Henry Darcy, a famous engineer from Dijon, France. In these experiments, Darcy examined the flow rate of water percolating downward through a cylindrical column of sand. These experiments led him to develop Darcy's Law, which is analogous to Ohm's Law (electricity) and Fourier's Law (heat), describing the relations between flow rate, hydraulic gradient, and permeability.
One of the difficulties in understanding the flow of groundwater is the inability to directly observe it moving through subsurface materials. In lieu of visual observations, groundwater scientists and engineers use water-level measurements made in wells to determine groundwater flow rates and directions. These measurements commonly are used in association with computer models to predict the future and past behavior of a groundwater flow system. (The U.S. Geological Survey publication, Water-Supply Paper 2220, a PDF document, describes the principles of groundwater flow in a comprehensive and understandable manner.)
An important part of evaluating a groundwater flow system, like at Woburn, is determining flow directions. This entails site-specific measurement of water levels in wells, correlation of water levels to a common datum (such as sea level), and contouring the water-level elevations to make a map of the water-table surface. Once the contours, known as potentiometric levels, are drawn, the maximum slope of the contoured surface between any two points (the hydraulic gradient) on the surface reflects the direction of groundwater flow. Groundwater flows down gradient from higher elevations to lower elevations along inferred paths called flowlines, which are perpendicular to the potentiometric contours.
Activities to broaden the understanding of groundwater flow
There are likely several opportunities within your local area where groundwater operations can be observed. If nearby communities are served by municipal wells, arrange to visit the wellfield to see the pumping wells and observe the water treatment system and water distribution operations. This will provide an appreciation of groundwater as a resource. Industrial sites and gasoline service stations provide another opportunity to observe groundwater dynamics. There is a likely possibility at these locations that remediation of contaminated groundwater has been or is being implemented. Property owners may be amenable to providing a tour of the groundwater collection and treatment operations. Any of these opportunities give students first-hand experience at seeing real-world problems and opportunities related to groundwater science.