Module 6: Groundwater Hydrology

The overall goal of Module 6, Groundwater Hydrology, is to develop a basic understanding of physical processes and properties that control the occurrence and movement of groundwater in the subsurface. The module has two parts and is designed to span a two-week session. The first part (Module 6.1) focuses on aquifers and their properties, with a series of case studies illustrating examples of highly used regional aquifers. The second part (Module 6.2) focuses on the dynamics of aquifers, including the driving forces for water movement and water budgets. After completing the module, students will be able to:

• identify the properties of artesian wells and describe the conditions under which they form;
• explain the difference between porosity and permeability;
• list and describe the properties of aquifers that control the movement and storage of groundwater;
• explain the role of fractures in determining the transmission properties of aquifers;
• use Darcy's Law to explain the roles of aquifer properties and driving forces in governing the rate of groundwater flow;
• apply the concept of hydraulic head to draw flowlines on maps and cross sections;
• interpret the current and historical balance between groundwater recharge and water extraction from well hydrographs;
• propose a course of action to address overdraft in an aquifer.

Overall, this one-week module is intended for use as a stand-alone lesson or as part of an online or blended general education or introductory-level course that would satisfy a science distribution requirement. The module would be appropriate for non-majors and undeclared students looking for a major. There are two formats: (1) blended where the students meet at least once to perform the activities in teams; and (2) 100% online. As a general guideline, the delivery of content and assessment of learning goals/objectives have been designed to accommodate the logistics of large class sizes where students are expected to work approximately three hours per week covering lecture content with an additional six hours per week of additional reading and work on assessments. Note that some students will require more or less time to meet the goals and objectives of the module.

In this module, students will:

• interact with online teaching materials pertaining to groundwater hydrology;
• interpret a cross-sectional aquifer diagram;
• complete a table associating given rock properties with hydraulic conductivity;
• interpret a diagram relating fracturing to well productivity;
• use a Darcy Tube to measure flow rate as a function of aquifer characteristics, or determine the same relationships using the original data set collected by Darcy;
• interpret hydraulic gradient from a potentiometric surface map.

All materials for students are available online using the Student Materials link below. These can be implemented entirely in the context of distance learning, with students completing any discussion questions in the form of a blog or discussion group. In a traditional or blended classroom setting, students can complete the online unit as homework, using class time for discussion and the Summative Assessment.

• Student Materials - Module 6: Groundwater Hydrology

Teachers can find documentation of the activities as well as rubrics for students at this location. Rubrics for teachers are compiled under Assessment on this site. Suggestions for teaching and a list of the assessments are found below.

What works best for the module?
Module 6 provides a primer on basic aspects of groundwater flow, including an overview of aquifers and aquitards, types of aquifers, the aquifer physical properties that govern storage and transmission of water, the gradients in gravitational and pressure energy that control the direction and rate of water percolation, and the concepts of recharge and overdraft. The module material is basic, but is somewhat abstract, and a large number of individual terms, topics, and concepts are covered.

We have structured the module in two parts. Part One focuses on introductory material, various types of aquifers, and aquifer rock properties including porosity and permeability. Part Two introduces the dynamics of groundwater flow—notably the concepts of hydraulic head, recharge, and water budgets. Throughout the module, we rely on abundant schematics to illustrate aspects of subsurface geologic and hydrogeologic systems that are generally difficult to visualize, and we draw upon examples wherever possible to relate to students' common experiences. As one example, the discussion of well hydrographs incorporates data that link to recent/ongoing drought conditions in California, and compare a well response over multiple years in this area with one in an area in the eastern United States that has been unaffected by drought. Instructors might consider exploring the USGS well database in real time in class. The website is easy to navigate and interpret.

The formative assessments are designed to ensure that students have digested each individual concept (e.g., porosity, permeability, well hydrographs). The two summative assessments include: (1) a Darcy tube flow experiment (see additional instructions for construction of a simple Darcy tube apparatus for the lab activity—also available to download in the student version of the assessment); and (2) a worksheet and discussion of hydraulic head (also available in the student version of the assessment). In general, students found the first part of the module to be straightforward, but struggled with the second part—specifically the concepts of hydraulic head and its relationship to flow direction, the generation of cross-sectional diagrams illustrating the water table or potentiometric surface, and the role of pressure vs. gravitational energy in driving flow. The Darcy experiment is critical as a building block toward understanding—and developing intuition about—the physics of groundwater percolation, in particular the influence of driving gradient (controlled by the slope of the experimental tube) and aquifer conductivity (material in the tubes). The exercise works best as an interactive lab in which students are asked to make predictions and take turns running the experiment. We have also found it effective to prompt the students to take the lead by deciding what they need to measure, how to make the measurements, and where uncertainties may originate. As noted in the overall course notes, the lab itself requires at least 75 minutes to ensure ample time for setup, multiple experimental trials, and some discussion. The assigned summative assessment requires the students to synthesize concepts, and to plot their data with original data from Darcy's 1856 experiments and compare the two. The assigned exercise provides important context while also continuing to give students practice in plotting and interpreting graphical data.

What students found difficult
The formative assessments in Part 1 were relatively straightforward and followed from the module, whereas those in Part 2 required students to apply abstract concepts related to hydraulic head to answer questions about flow directions, effects of pumping wells, and links to climate conditions/trends.

Most students were able to complete the formative assessments in Part 1 correctly, though some had difficulty with Formative Assessment 3, which required them to piece together factors that control hydraulic conductivity, including grain size and sorting, fluid viscosity, and porosity. Most students were able to generate plots for the Darcy experiment and interpret them correctly in the context of hydraulic conductivity, and the formulation of Darcy's Law, by which flux is proportional to the hydraulic gradient. Some students struggled with the mechanics of generating plots, and others with the calculation of slope or the relationship between the slope of the plot and equation for a line.

The formative assessments in Part 2 posed a greater challenge to most students, and in particular assessments #2 and #3 related to well hydrographs and cones of depression. Many students did not understand the concept that groundwater would flow down gradient (perpendicular to contours of equal head); others did not understand the concept of a cross-sectional diagram or the idea that the potentiometric surface map is analogous to a topographic contour map. Confusion about the concept of hydraulic head was also apparent from some answers to formative assessment questions about confined vs. unconfined aquifers and artesian conditions. We found that by the time of the Part 2 Summative Assessment on hydraulic head, most students were able to complete the assignment and—with guidance and some individual explanation or discussion—developed a clear understanding of hydraulic head and the potentiometric surface.

Reflections
In general, we found that the volume of material in this module, combined with the incorporation of several abstract/mathematical concepts including hydraulic head and Darcy's Law, necessitated in-class discussion to review and discuss the module content and Formative Assessments.

While Module 6, Part 1 is generally straightforward, students would probably benefit from additional discussion of aquifer properties. These might include: a review of key diagrams to explain porosity, grain size and sorting, and tortuosity; and some examples of rock specimens that serve as aquifers or aquitards—for example, hand specimens of sandstones, shales, and other rocks that the students can inspect with hand lenses or pour water onto. By the same token, devoting additional class time to the Darcy lab, either via a longer class period or a second class meeting devoted to follow-up discussion, would allow students time to generate plots and interpret them with their peers and with guidance from instructors. This would also avoid overlap of assigned "homework" from the Module 6 Summative Assessment with the start of readings and formative assessments in the next module.

For Module 6, Part 2, additional time devoted to interpretation of potentiometric surface maps, further developing the analogy to topographic maps (as covered in the Module 3 Summative Assessment), tracing flowlines, and the effects of pumping wells would help to solidify key concepts. Drawing upon current events for discussion of the link between well hydrographs and climatic conditions would also be valuable in both illustrating the connection between surface processes (e.g., Module 2) and groundwater resources, and in making the relevance of the material clearer. If instructors have access to physical groundwater simulators (so called "ant farms"), these can also be a useful tool for in-class activity or demonstrations to help students visualize groundwater flow and the concept of hydraulic head. These can be constructed with simple materials as a groundwater model described by the NGWA; or are available through teaching supply companies like Ward's.

Module 6.1 Formative Assessments

• Formative Assessment 1: Aquifers
• Formative Assessment 2: Permeability and Porosity
• Formative Assessment 3: Hydraulic Conductivity

Module 6.1 Summative Assessment

• The Summative assessment for Module 6.1 is Flow in Aquifers.

Module 6.2 Formative Assessments

• Formative Assessment 1: Hydraulic Gradient
• Formative Assessment 2: Well Hydrograph Records
• Formative Assessment 3: Cone of Depression
• Formative Assessment 4: High Plains Aquifer

Module 6.2 Summative Assessment

• The Summative assessment for Module 6.2 is Hydraulic Head and Aquifers.

Student readings:

• Readings from Student Materials — Module 6: Groundwater Hydrology