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.
What is the fundamental hydrological balance formula?
P=ET+Q±ΔS
Why is the hydrological balance important?
It provides the fundamental quantitative means for water budgeting in groundwater management.
What are the inflow components in a water balance?
Precipitation
Surface water inflow
Groundwater inflow
Artificial water imports (e.g., pipes, canals, sewage leaks).
What are the outflow components in a water balance?
Evapotranspiration from land
Evaporation from surface water
Surface runoff
Groundwater outflow
Artificial water exports (e.g., pipes, canals).
What storage changes need to be considered in water balance?
Surface water in streams, rivers, and lakes
Soil moisture storage
Moisture storage in the deeper aeration zone
Temporary depression storage (e.g., puddles)
Intercepted water on plants
Groundwater below the water table
What does “transient conditions” mean in groundwater management?
It means that changes in water storage are taken into account over time e.g. monsoons
When might steady-state models be used in groundwater management?
During conditions of regular precipitation where recharge and discharge are balanced.
Soil % retained- equation
% retained= (mass retained/ total mass) x 100
Hydraulic head equation
h=hp+hz
Pressure head equation
hp=z-hw or hp=P/(pwg)
Equipotential Lines
Dashed lines- lines of equal total head (h)
Elevation Head (z)- equation
=Elevation at surface - Depth of piezometer
Pressure Head
= Depth of piezometer- Depth to water
What happens to head towards the discharge zone?
It becomes lower
Where do flow lines travel from
High head to low head
What does water table follow?
The topography
What is hydraulic headd?
How high the water is above sea level: Elevation at surface- depth to water
How to work out pressure head using equipotential lines?
Point of rise to the surface
What is hydraulic conductivity measured in?
m/s
Darcy’s Law
q=-K(dh/dx)
What is q- Darcy velocity
The average speed of flow of water across the whole sample cross section
What is dh/dx
The gradient of the hydraulic head from high-low head over distance x
Flow rate equation (m^3/s)
Q=-AKi
Transmissibility equation (m2/s)
T=kb where b is the thickness of the aquifer in m
What is the equation relating permeability pressure and flow rate
q=-(k/u)* dP/dx
What happens to the flow when the pressure gradient exceeds the Reynolds number?
The relationship between the pressure gradient and flow rate becomes non-linear, requiring more pressure than expected to push fluid through a rock, often called “turbulent” flow
What is the purpose of well casing?
To provide the well’s diameter and material for structural stability and prevent collapse.
What is a well screen?
The slotted part of a well where water moves into the well, designed to retain at least 90% of the aquifer material.
What is the purpose of the filter pack and screen in a well?
To prevent aquifer material from entering the well while allowing water to flow.
What is an artificial filter pack?
A silica-based filter graded to a specific grain size, designed to match the grain size of the aquifer and prevent carbonate interference.
What is the recommended grain size distribution for an artificial filter pack?
The average grain size should be twice the grain size of the aquifer with a uniformity coefficient between 2 and 3.
What is an annular seal in a well?
A seal in the annular space of the well to prevent surface water from contaminating the aquifer.
Why is well development necessary?
To remove fine material around the well that may obstruct flow and to create a more permeable zone near the well.
What are some methods for well development?
High-rate pumping, applying hydraulic gradients, and creating pressure waves to transport fines out of the well.
Components of well design?
Well casing
Well screen
Filter pack
Annular seal
Well development
What happens to contaminate well during well development?
Contaminate surroundings with drilling mud so mud particles / mud cake must be pumped out
What to in include at top of model well design?
- Protective cover pipe
- Locking cap
- Surface seal
What to include in the middle of well design?
Annular space seal
What to include at bottom of model well?
- Filter pack seal
- Well screen
- Closed bottom
- Filter pack (bentonite above sand)
- Diameter e.g. 6.8 inches
Hydraulic testing
- Install wells or utilise existing ones
- Apply a pumping rate from 1 well
- Measure drawdown against time
How to calculate contamination advection time in porous media
Step 1 : Calculate velocity
v=K*i/n
Step 2: Calculate t
t=d/v
How to calculate retardation?
=1+(PsKd(1-n)/n)
How to calculate time with retardation?
t=d(v/R)
Lab tests for hydraulic characteristics?
- Sieve; empirical formulation (grain size distribution)
- Permeater, falling head(coarse sand- measures flow) , constant head (fine sands measures permeability)
- Gas injection, correction (simulates fluid flow)
Analogue tests for hydraulic conductivity?
- Description
- Grain size distribution analysis (permeability)
- Hydraulic conductivity/permeability charts
Field tests for hydraulic properties?
- Tracer tests (monitoring movement of tracer)
- Large scale pump tests (pumping water from well)
- Small scale borehole tests (slug tests)- displacing water in borehole
Factors influencing Hydraulic conductivity?
- Porosity
- Permeability
- Temperature
- Fractures
- Degree of Saturation
How to identify contaminated groundwater with ERT?
Contaminated groundwater has higher electrical conductivity (lower resistivity) than clean groundwater
How do monitor contaminant movement using induced polarisation (IP)?
Contaminants can affect the polarization properties of the subsurface, especially if metallic ions are present. IP measurements complement ERT by adding detail about contaminant chemistry.
What subsurface features of a Roman road can GPR detect?
Distinct material layers (e.g., stones, gravel, sand).
Boundaries between road materials and natural soil.
Drainage features.
What are key indicators in GPR data for Roman road features?
High-amplitude reflections marking construction layers.
Variations in reflection strength indicating preserved elements like foundations or drainage feature
8 main groundwater quality species
Calcium 2+, Magnesium 2+, Nitrates NO3-, Chloride Cl-, Potassium K+, Bicarbonates HCO3-, Sulfates SO4-, Sodium Na+
Species indicating mining influences
Iron Fe2+, Manganese 2+
What is Storativity ?
The amount of water a unit area of an aquifer releases per unit decline in hydraulic head.
Equation for Strorativity?
S=Ss x b
What is Specific Storage (Ss)?
The volume of water released per unit volume of aquifer per unit decline in hydraulic head.
Equation for specific storage (Ss)
Ss=pg(Bw+n*Bm)
What is Specific Yield ?
The fraction of the total aquifer volume that can drain freely under gravity.
What is the formula for Specific yield (Sy)?
Sy= Vdrainable/Vtotal
Which aquifers typically use Specific Yield?
Unconfined aquifers, where water drains due to gravity.
Which aquifers typically use Specific storage?
Confined and unconfined aquifers.
What does isotropic mean?
Properties are the same in all directions at a given point.
What is an example of isotropic behavior?
Hydraulic conductivity- K uniform in all directions
What does anisotropic mean
Properties vary depending on direction (horizontal vs vertical)
Example of anisotropic behaviour
Kx and Ky have different hydraulic conductivity
What does homogenous mean?
Properties are uniform in space
What does non-homogenous mean?
Properties vary spatially across different locations/regions
Can a region be isotropic and non-homogeneous?
Yes, it can have uniform directional properties but exhibit spatial variation.
Example of isotropic + non-homogeneous?
An aquifer with uniform directional hydraulic conductivity values at each point, but varying in value across different locations.
Example of anisotropic + homogeneous?
A sand layer that exhibits horizontal vs. vertical hydraulic conductivity but is uniform in composition.
Gravel- description
More than 50% of material is gravel size
Coarse soils
Less than 35% of material finer than 0.06mm
Fine soils- description
More than 35% of material is finer than 0.06mm
Gravelly sandy/silts and clays- description
35-65% fine
Silts and clays- description
65 to 100% fine
Fine soils- description
More than 35% of material is finer than 0.06mm
Cohesive soils
High clay/silt content- particles stick together
Uncohesive soils
Mostly sand/gravel- held together by friction
Plasticity Limit
=Liquid limit- plastic limit
Methods used for monitoring movement of contaminant in water?
- ERT profile (higher conductivity= contaminant)
- EM Survey - rapid detection of subsurface/conductivity anomalies
- Identify flow direction and create contamination maps
Importance of a base station in gravity surveys?
- Correction for instrument drift
- Correction for earth tides
- Establishing a reference point
- Tie point for regional gravity network
When are base stations made during survey?
- Start of the day: Establish the initial gravity value.
- Regular intervals: Correct for drift and tidal effects.
- End of the day: Detect cumulative changes
hat is the process of utilizing base station measurements in a survey?
Take initial base reading in the morning.
Measure field gravity values.
Return to base station periodically for drift checks.
Take final reading in the evening.
Correct field data using base station readings.
What does electrical resistivity/conductivity measure in down-borehole probing?
Variations in subsurface resistivity or conductivity.
What are the environmental applications of electrical resistivity/conductivity measurements?
Identify groundwater salinity.
Delineate contaminant plumes.
Map aquifer boundaries
What are the environmental applications of natural gamma radiation measurements?
Identify lithology (e.g., clay layers).
Correlate stratigraphic units.
Detect buried waste or radionuclide contamination.
What are the environmental applications of acoustic/seismic velocity measurements?
Determine mechanical properties of rocks and soils.
Identify voids or fractures in the subsurface.
Aid in seismic hazard analysis.
What is down-borehole probing?
A technique used to collect detailed physical, chemical, and structural information about the subsurface environment by deploying specialized instruments into drilled boreholes. This method provides high-resolution data that helps characterize geological formations, aquifers, and other subsurface features.
Key features of down-borehole probing:
- Data collection a specific depth
- Non destructive
- Versatile
What geological situation is likely to generate an Induced Polarisation (IP) anomaly?
Disseminated sulfide ore deposits (e.g., pyrite, chalcopyrite) or clay-rich aquitards.
Name an environmental situation that may generate an IP anomaly.
Contaminated soils with polarizable minerals due to industrial waste.
Describe the role of current electrodes in IP surveys.
They inject controlled electric current into the ground.
How are electrodes spaced in a typical IP survey?
Electrode spacing ranges from 5–50 meters depending on depth.
How can resistivity and IP data help in interpretation?
They identify mineralized zones or areas of contamination by detecting high chargeability anomalies.
