Revision- Hydrogeology tutorial questions Flashcards
What is the average residence time of water in oceans compared to rivers?
Oceans: ~3,000–3,200 years.
Rivers: Weeks to months.
Why do oceans have a much larger residence time than rivers?
Oceans have a massive water volume (~1.332 billion km³), while rivers have much less (~2,120 km³).
Larger reservoirs take longer to replace their water.
Why do rivers have shorter residence times compared to oceans?
Rivers have smaller storage capacities and rapid water turnover due to dynamic flows and constant replenishment by precipitation and runoff.
How does water movement differ between oceans and rivers?
Oceans: Water cycles slowly through evaporation, precipitation, and deep currents over millennia.
Rivers: Experience rapid movement and are constantly replenished by precipitation and runoff.
What is the hydrological cycle?
The hydrological cycle is the continuous movement of water within Earth’s system, involving processes like evaporation, condensation, precipitation, infiltration, runoff, and transpiration.
How does human activity interrupt the hydrological cycle through water diversion and damming?
Diverting rivers and building dams alters natural water flow, reduces downstream water availability, and affects aquatic ecosystems.
What is the effect of deforestation on the hydrological cycle?
Deforestation reduces transpiration, alters local rainfall patterns, and increases runoff and soil erosion.
How does overuse of groundwater disrupt the hydrological cycle?
Over-extraction lowers water tables, causes land subsidence, and depletes aquifers faster than they can recharge
What is interbasin water supply?
Interbasin water supply is the transfer of water from one river basin (donor basin) to another (recipient basin) to address water shortages, support development, or balance resource availability.
What are the positives of interbasin water supply?
Addresses water scarcity in arid regions.
Supports economic development, agriculture, and urban growth.
Reduces flood risks in donor basins.
Improves drought resilience.
May include hydropower generation.
What are the environmental negatives of interbasin water supply?
Alters ecosystems in donor basins.
Disrupts aquatic habitats in recipient basins.
Changes water chemistry and flow patterns.
What are the social and economic drawbacks of interbasin water supply?
High construction and maintenance costs.
Potential displacement of communities.
Overreliance on transferred water in recipient regions.
What are the political and legal challenges of interbasin water supply?
Water disputes between donor and recipient regions.
International conflicts over water rights.
Porosity (n)- equation
1-(Pb(dry bulk density)/Ps(particle density of solids))
Volumetric water content- equation
Vw(volume of water in the sample)/Vt(Votal volume)
Vadose Zone
Zone of aeration-above water table
Phreatic Zone
Zone of saturation- groundwater
The vadose zone can be absent in areas of …
High precipitation and in depressions, and more than
hundred metres thick in arid regions (commonly 5 to 25m)
Capillary Fringe
The area above the water table where water is held in the soil pores by capillary action. Water moves upward against gravity in this zone.
What happens to capillary pressure in soil as it moves from a fully saturated state to a dry state?
Capillary pressure increases as soil drains, starting low in the fully saturated state and rising as water is retained in smaller pores (field capacity) and eventually reaching its highest at the wilting point and in the dry soil.
What is Field Capacity?
The point where soil retains water against gravity, with moderate capillary pressure after drainage.
What is Hygroscopic Water?
Water tightly bound to soil particles, unavailable to plants, found at high capillary pressure.
What is the Wilting Point?
The point where plants cannot extract water due to high capillary pressure, and soil is nearly dry.
How does capillary pressure change as soil drains?
Capillary pressure rises as soil moves from saturation to dryness
What happens at the Wilting Point?
Plants can’t extract water, and only hygroscopic water remains in the soil
What’s the difference between Field Capacity and Wilting Point?
Field capacity has water available to plants, while wilting point has water unavailable to plants.
Grain size and capillary action
Smaller grains create more capillary action, allowing water to rise higher. Larger grains in soil A have larger pore spaces, which reduces capillary rise.
Surfactants
Reduce the surface tension of water, making it easier for water to spread and drain out of the soil. This is because the reduced surface tension lowers the cohesive forces holding the water in the soil.
Controls on capillary pressure in pore spaces?
Grain size and pore spaces
Interfacial tension
Fluid saturation
Pore geometry
High interfacial tension
Higher capillary pressure
How does capillary pressure influence fluid movement in soil?
Capillary pressure drives fluid movement from areas of high pressure (larger pores) to areas of lower pressure (smaller pores), controlling fluid retention and flow in soil.
Specific yield of soil- equation
Sy=Vd/Vt
Specific storage of soil- equation
Ss=pg(a+nB)
Permeability-equation
k=Ku/pg
Hydraulic conductivity equation
K=kpg/u
Relative permeability (kr)
The ratio of the effective permeability of a fluid to the absolute permeability of the porous medium.
Why is relative permeability important for groundwater recharge?
It determines how easily water moves through unsaturated zones where air occupies part of the pore space.
How does relative permeability influence soil moisture conditions?
Low Kr: Slower water infiltration and recharge.
High kr : Faster water movement.
What role do capillary forces play in groundwater recharge?
Relative permeability controls water’s ability to overcome capillary forces in unsaturated zones
How does relative permeability impact aquifer types?
In fractured rock aquifers or dual-porosity systems, relative permeability affects water movement through both matrix and fractures.
What are geofacies?
A combination of geological facies depending on the process you are looking at and sharing similar properties
What are Hydrofacies?
In fluid flow we can identify three types of facies, relative to one another: Aquifer, Aquitard, Aquiclude.
What is an aquifer?
An aquifer is a saturated, permeable geological unit that transmits significant quantities of water under normal hydraulic gradients. 𝐾>1×10^−5 m/s
What are common examples of aquifers?
Unconsolidated sands and gravels, permeable sedimentary rocks (e.g., sandstones, limestones), and heavily fractured/weathered volcanic and crystalline rocks.
What is an aquiclude?
An aquiclude is a saturated geologic unit that cannot transmit significant water quantities. 𝐾< 1x 10^-8m/s
What is an aquitard?
An aquitard is a less permeable layer that transmits water in small quantities, significant for regional groundwater flow.
K>1x 10^-8m/s 𝐾<1×10^−5 m/s
Common examples of aquitards
Clays, shales, and dense crystalline rocks.
What is an unconfined aquifer?
An aquifer where the water table is at atmospheric pressure, near the surface, and fluctuates with recharge and discharge.
What is a confined aquifer
An aquifer confined between two aquitards. Water rises above the aquifer to the potentiometric surface and may rise above the ground as an artesian well.
How can the subsurface be subdivided based on hydraulic conditions?
Unconfined aquifers: Water table at atmospheric pressure.
Confined aquifers: Confined between aquitards with a potentiometric surface.
Perched aquifers: Isolated above low-permeability layers.
How can the subsurface be subdivided based on geology/bedrock properties?
Unconsolidated: Water flows through pores between grains.
Lithified: Reduced pore space due to consolidation and cementation.
Crystalline: Water flows in fractures.
Karstified: Enlarged fissures and caves due to dissolution.
What is a fractured aquifer?
An aquifer where water flow occurs through fractures formed after consolidation or cementation.
What is groundwater recharge?
Water added to the groundwater system from infiltration, minus evaporation losses.
What is groundwater discharge?
The emergence of groundwater at the surface, such as springs, swamps, lakes, or wells.
Define local, intermediate, and regional groundwater flow- flow rate
Local: Fast, shallow flow (<10 years).
Intermediate: Slow, regional flow (10–1000 years).
Regional: Very slow, deep flow (>1000 years; fossil water >5000 years).
What influences groundwater flow patterns?
Subsurface geological structures.
Define local, intermediate, and regional groundwater flow- geologically
Local- Small discrete topographic depressions/uplifts
Intermediate- Controlled by deeper subsurface heterogeneities and intermediate geological layers
Regional- Deep basin structures
Define local, intermediate, and regional groundwater flow- geochemically
Local- Freshwater with recent recharge and limited chemical alteration
Intermediate- Moderately evolved due to longer interaction with geological materials
Regional- Highly evolved water chemistry
(saline/fossilised)
Depression springs
Formed in unconfined aquifers when the topography intersects the water table, usually due to the surface stream incision. As the Springs are formed because of earth’s gravitational pull they are named depression or gravity springs. These are usually found along the hillside and cliffs.
Fracture Springs
Fracture springs occur due to existence of permeable fracture zones in low permeability rocks. Movement of groundwater is mainly through fractures that constitute the porosity and permeability of aquifers. Springs are formed where these fractures intersect the ground surface.
Fault Springs
Faulting may also give rise to conditions in which groundwater (at depth) under hydrostatic pressure (such as in confined aquifers) can move up along such fault openings to form a spring.
Contact Springs
Contact springs emerge at contacts where relatively permeable rocks overlie rocks of low permeability. Spring water emerges at such contacts.