⛏️Intro to Geology Unit 13 – Groundwater and Hydrogeology

Key Concepts and Terminology

• Hydrogeology studies the occurrence, distribution, movement, and quality of groundwater
• Groundwater is water found beneath the Earth's surface in soil pore spaces and fractures of rock formations
• Aquifer is a geological formation that can store and yield significant quantities of water
• Aquitard is a geological formation with low permeability that restricts groundwater flow
• Porosity refers to the amount of void space in a rock or sediment that can hold water
• Permeability measures the ability of a porous material to allow fluids to pass through it
• Water table represents the upper surface of the saturated zone in an unconfined aquifer
• Confined aquifer is an aquifer bounded above and below by impermeable layers (aquitards)
• Groundwater contamination occurs when pollutants enter the groundwater system, making it unsafe for use

The Hydrologic Cycle

• The hydrologic cycle, also known as the water cycle, describes the continuous movement of water on, above, and below the Earth's surface
• Evaporation is the process by which water changes from a liquid to a gas, typically from water bodies or land surfaces
• Transpiration is the process by which water evaporates from plant leaves
• Condensation occurs when water vapor cools and changes back into liquid form, forming clouds
• Precipitation is the process by which water falls from the atmosphere to the Earth's surface in the form of rain, snow, sleet, or hail
• Infiltration is the movement of water from the surface into the soil
• Runoff is the flow of water over the land surface when precipitation exceeds infiltration capacity
• Groundwater recharge occurs when water infiltrates the soil and reaches the water table

Aquifers and Aquitards

• Aquifers are geological formations that can store and transmit significant amounts of water
  • Unconfined aquifers have no overlying impermeable layer and are directly recharged by infiltration
  • Confined aquifers are bounded above and below by impermeable layers (aquitards) and are under pressure
• Aquitards are geological formations with low permeability that restrict groundwater flow
  • Clay and shale are common examples of aquitards
• Aquicludes are geological formations that are essentially impermeable and do not transmit water
• Aquifer properties, such as porosity, permeability, and thickness, determine the amount of water an aquifer can store and transmit
• Aquifer types include sand and gravel aquifers, fractured bedrock aquifers, and karst aquifers
• Groundwater extraction from aquifers occurs through wells drilled into the water-bearing formations

Groundwater Movement

• Groundwater moves from areas of high hydraulic head to areas of low hydraulic head
• Hydraulic head is the sum of the elevation head and pressure head, representing the total energy of the groundwater at a given point
• Darcy's Law describes the flow of groundwater through a porous medium: $Q = -KA\frac{dh}{dl}$
  • $Q$ is the groundwater flow rate (volume per time)
  • $K$ is the hydraulic conductivity (a measure of permeability)
  • $A$ is the cross-sectional area of flow
  • $\frac{dh}{dl}$ is the hydraulic gradient (change in head over distance)
• Groundwater velocity is determined by the hydraulic conductivity, porosity, and hydraulic gradient
• Groundwater flow can be laminar (smooth and parallel) or turbulent (chaotic and irregular)
• Groundwater divides are boundaries between groundwater basins, similar to surface water divides

Porosity and Permeability

• Porosity is the ratio of void space to the total volume of a rock or sediment, expressed as a percentage
  • Primary porosity is the original void space in a rock formed during its deposition or formation
  • Secondary porosity develops after the rock formation due to processes like fracturing or dissolution
• Effective porosity refers to the interconnected void spaces that contribute to fluid flow
• Permeability is a measure of a material's ability to allow fluids to pass through it
  • Intrinsic permeability depends on the properties of the porous medium itself (pore size, shape, and connectivity)
  • Hydraulic conductivity is a measure of permeability that also considers the fluid properties (density and viscosity)
• Darcy's Law relates groundwater flow rate to hydraulic conductivity, cross-sectional area, and hydraulic gradient
• Porosity and permeability are influenced by factors such as grain size, sorting, and cementation
• Well-sorted, coarse-grained sediments (sand and gravel) generally have higher porosity and permeability compared to fine-grained sediments (silt and clay)

Water Table and Confined Aquifers

• The water table is the upper surface of the saturated zone in an unconfined aquifer
  • The water table fluctuates in response to recharge and discharge processes
  • The depth to the water table can be measured in wells and is an important factor in groundwater accessibility
• Confined aquifers are bounded above and below by impermeable layers (aquitards) and are under pressure
  • The potentiometric surface is the level to which water would rise in a well penetrating a confined aquifer
  • Artesian wells occur when the potentiometric surface is above the ground surface, causing the well to flow naturally
• Unconfined aquifers receive direct recharge from infiltration and are more susceptible to contamination
• Confined aquifers are recharged in areas where the aquifer is exposed at the surface (outcrop areas) and are generally better protected from contamination
• Groundwater extraction from confined aquifers can lead to land subsidence if the rate of extraction exceeds the rate of recharge

Groundwater Quality and Contamination

• Groundwater quality is determined by the presence and concentration of dissolved substances (ions, minerals, and organic compounds)
• Natural factors affecting groundwater quality include the composition of the aquifer materials, water-rock interactions, and residence time
• Anthropogenic factors, such as industrial activities, agriculture, and waste disposal, can introduce contaminants into groundwater
• Common groundwater contaminants include nitrates, pesticides, heavy metals, and organic compounds (petroleum products, solvents)
• Groundwater contamination can occur through point sources (leaking storage tanks, spills) or non-point sources (agricultural runoff, atmospheric deposition)
• Contaminant transport in groundwater is influenced by advection (movement with the flow), dispersion (spreading due to velocity variations), and retardation (interactions with aquifer materials)
• Groundwater remediation techniques include pump-and-treat systems, bioremediation, and in-situ chemical oxidation
• Groundwater protection strategies involve source control, land-use planning, and monitoring

Practical Applications in Hydrogeology

• Groundwater resource assessment involves estimating the quantity and quality of available groundwater in an aquifer
• Well hydraulics deals with the design, construction, and testing of wells for groundwater extraction
  • Pumping tests are conducted to determine aquifer properties (transmissivity, storativity) and well performance
  • Well yield is the maximum rate at which a well can be pumped without causing excessive drawdown
• Groundwater modeling uses mathematical models to simulate groundwater flow and contaminant transport
  • Models help predict the response of aquifers to stresses (pumping, recharge) and evaluate management scenarios
• Groundwater-surface water interactions are important for understanding the exchange of water and solutes between aquifers and streams, lakes, or wetlands
• Groundwater plays a crucial role in water supply for domestic, agricultural, and industrial uses
  • Aquifer management involves balancing groundwater extraction with recharge to ensure sustainable use
• Groundwater is also a significant component of the baseflow in streams and rivers, maintaining flow during dry periods
• Geothermal energy utilizes hot groundwater or steam for heating and electricity generation
• Groundwater is a key factor in geotechnical engineering, influencing the stability of foundations, slopes, and underground excavations