Geology 19 (Groundwater)

Earth and Space Sciences X

29 Oct 201544:12

EducationalLearning

32 Likes 10 Comments

TLDRThe lecture discusses groundwater, an extremely important yet hidden water resource. It comprises the majority of Earth's available freshwater and is critical for human use, like irrigation and drinking. The lecture explains concepts like aquifers, permeability, porosity and flow systems that allow groundwater storage and transmission. It also covers interactions of groundwater with surface water features. Additionally, it discusses environmental issues associated with groundwater like overuse, subsidence and contamination. Overall, the lecture provides a scientific grounding on an invaluable resource that must be properly managed to sustain civilization.

Takeaways

😀 Surface water and groundwater are linked - streams can gain or lose water to groundwater
💧 Groundwater makes up 94.5% of Earth's liquid freshwater and is stored in pores and fractures in soil and rock
🚰 Humans use large amounts of groundwater, mostly for agriculture and public supply like drinking water
🔧 Darcy's law describes the flow rate of groundwater based on hydraulic gradient, conductivity and cross-sectional area
🌡 Water table depth varies seasonally and somewhat mirrors surface topography
📉 Heavy groundwater pumping can cause land subsidence and allow seawater intrusion
🗺 Water wells create drawdown cones that pull water horizontally toward the well, potentially impacting neighbors
💦 Springs form where groundwater intersects Earth's surface, like where a confining layer creates a perched water table
☢ Groundwater can be contaminated from leaky storage tanks, septic systems, industrial waste, road salt, etc.
🤝 Managing groundwater sustainability requires understanding recharge rates and potential contamination threats

Q & A

What percentage of the world's total freshwater is found in groundwater?
-Groundwater makes up about 95% of all liquid freshwater on Earth.
What are some major uses of groundwater in the United States?
-In the United States, major uses of groundwater include irrigation (68.4%), public/domestic supply (19.3%), industrial (4.3%), livestock (3.1%), and other uses like aquaculture and mining.
What is the difference between a gaining stream and a losing stream?
-A gaining stream receives water from the inflow of groundwater through the stream bed. A losing stream loses water to the groundwater system through outflows in the stream.
How does groundwater typically become contaminated?
-Common sources of groundwater contamination include leaky underground storage tanks, industrial waste, landfills, fertilizers, pesticides, road salt, and faulty septic systems.
What causes subsidence of land and how can it impact groundwater?
-Subsidence is caused by the pumping out of groundwater from aquifers underneath the land. This can compress aquifers and reduce their water storage capacity over time.
What is an aquifer and what are some examples?
-An aquifer is a permeable rock formation that transmits water freely, such as sand, gravel, or fractured bedrock. Good aquifers have high porosity and permeability.
What causes artesian wells and springs to form?
-Artesian wells and springs form when groundwater is confined under pressure by impermeable layers above and below, forcing the water up to the surface when an outlet is available.
How can groundwater be considered a nonrenewable resource?
-While surface water is renewable with the hydrologic cycle, groundwater recharge can happen very slowly. If pumping rates exceed natural recharge rates, the groundwater can be gradually depleted.
What role do aquitards play in groundwater flow?
-Aquitards are impermeable layers that restrict water flow between aquifers. They compartmentalize groundwater reservoirs and contribute to artesian pressures.
What is Darcy's law and why is it useful?
-Darcy's law calculates groundwater flow rates based on hydraulic gradients, aquifer permeability, cross-sectional area etc. This helps determine aquifer discharge capacities and sustainable pumping rates.

Outlines

00:00

📖 Introduction to Groundwater

Introduces the lecture topic of groundwater and its importance as the largest reservoir of fresh water on Earth. Covers basic groundwater concepts like springs, porosity, permeability, aquifers and aquitards. Also discusses groundwater usage and environmental issues.

05:01

🕳️ How Groundwater Flows Underground

Explains how precipitation infiltrates into the ground and leads to the formation of a zone of saturation and groundwater below the water table. Discusses soil moisture, capillary action, gaining/losing streams and their interaction with groundwater.

10:01

⛰️ Factors Affecting Groundwater Depth

Discusses factors that impact depth to the water table, including seasonal and annual variations in precipitation. Notes that the water table tends to mirror, but is subdued compared to, the Earth's surface topography.

15:03

📏 Measuring Groundwater Porosity

Provides an example to demonstrate and calculate porosity, which determines groundwater storage capacity. Porosity depends on grain size/shape and sorting.

20:05

🚰 How Groundwater Moves Underground

Explains Darcy's law and the factors controlling groundwater flow rates, including hydraulic gradient, conductivity and viscosity. Also covers permeability, aquitards, aquifers and average flow rates.

25:05

🔭 Analyzing Regional Groundwater Flow

Notes that groundwater flow systems operate at various scales. Even large regional systems can be separated by aquitards from shallow flow interacting with surface water and topography.

30:06

⛽ Groundwater Extraction and Environmental Impacts

Discusses using wells to access groundwater, which can lead to subsidence and saltwater intrusion. Also covers contamination issues from landfills, storage tanks and septic systems.

Mindmap

Keywords

💡Groundwater

Groundwater refers to water that is underground in cracks and spaces in soil, sand, and rock. It is an important source of drinking water for humans. The video discusses groundwater depth, movement, and usage in detail.

💡Aquifer

An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials like gravel or sand. Aquifers are important sources of groundwater that can be extracted using wells.

💡Recharge

Recharge refers to the process where water moves downward from surface water to groundwater. Recharge occurs in areas where the water table is low allowing water to infiltrate into the ground.

💡Discharge

Discharge is the opposite of recharge - it is where groundwater comes back to the surface at places like springs, streams or lakes. The video discusses different types of gaining and losing streams based on discharge.

💡Porosity

Porosity refers to the open spaces in soil or rock that allow water to be stored. High porosity materials like sand can store more groundwater.

💡Permeability

Permeability refers to how easily water can flow through soil or rock. It depends on how well the pores are connected. High permeability allows aquifers to transmit water quickly.

💡Darcy's Law

Darcy's law mathematically describes the flow of groundwater based on the water table slope, aquifer permeability and cross sectional area. It allows calculating groundwater velocity and volume.

💡Wells

Wells are human-made holes drilled into aquifers to extract groundwater. Pumping from wells can cause drawdown of surrounding water levels as it gets refilled.

💡Springs

Springs form where groundwater intersects the land surface and flows out naturally. It can create waterfalls where confined perched aquifers outlet at cliffs.

💡Contamination

Various human activities like landfills, chemical spills and septic tanks can contaminate groundwater. Contaminants enter water that people drink, making aquifer protection very important.

Highlights

Groundwater makes up 95% of the world's liquid freshwater supply

Humans use 79 billion gallons of groundwater per day in the US

Most groundwater usage is for irrigation and public water supply

Groundwater moves slowly, about 4 cm per day on average

The water table depth varies seasonally and mimics surface topography

Gaining streams are fed by groundwater, losing streams feed groundwater

Porosity determines groundwater storage capacity

Permeability determines groundwater flow rates

Groundwater often moves from recharge to discharge areas

Pumping wells can locally drain aquifers

Springs form where groundwater intersects the surface

Groundwater is often nonrenewable on human timescales

Pumping can cause land subsidence

Contamination threatens groundwater quality

Groundwater sustains water supplies for the majority of the world

Transcripts

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