Groundwater Hydrology

Prelude

1. Importance of Groundwater Hydrology. The science and engineering of subsurface hydrologic processes is still in its infancy. This chapter de
fines where, how, and why the water-resources professional is needed in today's society.

• History, from ancient to modern
• Needs, from water supply to pollution
• Careers, from entry-level to professional
• Ethics in water resources

Section I: Dynamic Earth

2. Hydraulic Perturbations and Hydrodynamics. While all hydrogeology textbooks introduce Darcy's Law for static flow, they commonly neglect the equivalent law for dynamic flow. The propagation and arrival of hydraulic perturbations is founded on hydrodynamics, and the use of Darcy's Law commonly leads to misleading or incorrect results.

• Static flows, surface and groundwater
• Hydrodynamic flows, surface and groundwater
• Periodic vs pulse stimuli

3. Responses to Natural Signals. Barometric pressure, Earth Tides, ocean tides, surface loading, and many other processes affect groundwater levels. These hydrodynamic signals can be interpreted and used to characterize aquifer hydraulic parameters and unsaturated zone properties.

• Surface loading; barometric pressure, ocean tides
• Bulk forces; Earth tides, fluid pressures
• Estimating material properties

4. Fluid Movement in the Vadose Zone. Rapid pressure responses from the ground surface to the water table are often mistaken for macropore flow. We know this is incorrect because dating of this recharge shows that the water is not \new water", in the sense that it is not rainfall that bypassed pore fluids. Instead, it is "old water", in that it is chemically consistent with ambient water in the unsaturated zone.

• Understanding responses; flux, velocity, celerity
• Wetting fronts vs diffusion waves
• Macropores vs micropores

5. Subsurface Stress and Strain. Hydraulic processes alter the in-situ stress-strain
field within aquifers and the unsaturated zone. Linking groundwater to geotechnical processes from soil and rock mechanics are needed to resolve the interaction between deformation and fluid pressure responses

• Soil compaction and loading
• Rock deformation; elastic, plastic, brittle failure
• Fluid responses

Theme 2: Groundwater and Surficial Processes

6. Critical Zone Processes. Linking the full continuum of Earth-System Sciences (from the top of canopy to the bottom of the hydrosphere) is another emerging topic. Exchanges of energy, water, nutrients, and other matter are intrinsically based on cycling and recycling through the critical zones. Mapping and understanding these processes are key for waste-assimilation, biodegradation, and contaminant transport. Fundamental concepts in fugacity and kinetics are key for evaluation and prediction of these processes.

• Extent and processes
• Physical components
• Chemical components
• Biological components

7. Soil Atmospheric Processes. The soil atmosphere, or "vadosphere", encompasses the gas-filled region of the subsurface above the water table. Having a different composition and moving differently than gasses above the ground surface, these gasses contribute to evapotranspiration losses of water as well as carbon sequestration and release.

• Gas composition
• Gas movement; advective, diffusive
• Evapotranspiration and carbon loss

8. Stream Flow Generation. While subsurface hydrologists know that most stream flow results from groundwater exfiltration, most textbooks still treat stream flow as the result of overland flow. A basic paradigm shift is occurring in the hydrologic sciences that has not been addressed in textbooks.

• Base flow
• Overland flow
• Kinematic flows
• Open channel flows

9. Surface-Water Interactions. Streams interact with groundwater through hyporheic exchanges. This dynamic zone of mixing is often called the "Liver of the River" because of the filtering and reactions that occur in geologic sediments. Commonly overlooked in both groundwater and surfacewater textbooks, studies and applications are needed to prepare hydrologists for problems in water quality.

• Hyporheic exchanges
• Recharge and discharge
• Biogeochemical processes

Theme 3: Heterogeneous Media.

10. Flow through Lineaments. Many groundwater studies neglect flow through lineaments, such as fractures, faults, and conduits. These lineaments are dynamic, in that regional stresses (as well as geochemical processes) have altered the shape, extent, and permeability of these features. More robust assessments require that thermo-mechanical, elastic deformation, chemical alteration, and even biological modi
fication of lineaments be considered and used to evaluate flow and transport through these systems.

• Types of lineaments; fractures, faults, conduits
• Lineaments as dynamic features
• Mapping and characterization

11. Saturated Fractured Media. Fracture asperities (voids) can open and close due to Earth tides, barometric pressure, rainfall, and other surface loading forces, this mechanical deformation is enhanced by thermal alteration, as well as pressure gradients. Dynamic permeabilities are likely methods for dispersion and transport in saturated fractured media.

• Fracture asperities (voids); formation, characterization
• Thermo-hydro-mechanical alteration
• Mass transport; advection, dispersion, diffusion

12. Unsaturated Fractured Media. Fracture asperities may be water or air
filled in unsaturated media. Liquid flow through regions of pendular water is likely limited, and often overwhelmed by flow through the rock matrix. On the other hand, vapor phase flow through empty fracture voids can dominate total fluid flow, especially when forced by barometric pressure and temperature changes.

• Pendular water; connected, isolated
• Vapor flux and transport
• Dual porosity flux and transport

13. Karst Terrains. Conduits in carbonate media pose special challenges in hydrogeology. These conduits are often regionally extensive, but complex in structure and shape. Regional flow can be highly concentrated in these conduits, influencing both surface and groundwater behavior. Understanding and predicting these conduits is possible given appropriate conceptualizations of geologic structure.

• Speleogenesis vs speleothemesis
• Regional vs local flow and transport
• Identification and characterization

Theme 4: Fluid Transport

14. Flownets. Classical flownets based on complex numbers are seldom used to predict pathlines. While complex numbers are difficult to teach, I use a simple expository to familiarize students with this powerful tool. I then extend the two-dimensional complex formulation to the three-dimensional, vector formation. Transient applications are also provided.

• Complex numbers (2D), vectors (3D), and quaternions (4D)
• Application to 3D-transient flow
• Defining initial and boundary conditions
• Finding flowpaths using streamlines, streaklines, and flowlines
• Incorporating decay, absorption, and dispersion

15. Tracer Tests. Predicting solute transport is facilitated by using natural and introduced tracers. Yet, little guidance is available for conducting and interpreting groundwater tracer tests. Tracer cocktails containing a mix of different tracers expands the information content of artifi
cial tracer tests, while analyzing for multiple environmental tracers provides more robust interpretation of regional groundwater flow.

• Natural vs introduced tracers
• Methods for conducting a tracer test
• Interpretation of results; advection, sorption, matrix diffusion

16. Groundwater Modeling. Here, we treat modeling as a learning tool, rather than a predictive tool. Understanding hydrogeologic processes, and the hydrologic and geochemical parameters that influence these processes, is a key goal of hydrologic modeling. The roles of analog, statistical, and process-based models are explained and examined.

• Modeling approaches; analog, statistical, process based
• Modeling goals; regulatory, engineering, scientific
• Modeling guidelines; domain, parameters, sensitivity

Wrap-up

17. New Frontiers in Groundwater Hydrology. Science is not a static construct -- our tools and approaches change with time and experience. We present a methodology for using data, information, and knowledge to build scientific constructs (paradigms) that improve our understanding and management of groundwater resources.

• Structure of science; data, information, knowledge
• Evolution of science; need for adaptation
• Goals of science; paradigms to improve understanding and management
• New tools and technologies; remote sensing, machine learning, science of big data