Confined aquifer: bounded above by aquitard β artesian pressure. Unconfined: water table is upper boundary. 'Unconfined = open to sky.'
Aquifer Types
Two fundamentally different groundwater systems β with very different behavior and vulnerability
Unconfined (water table) aquifer: upper boundary is the water table β free to rise and fall. Directly recharged by precipitation above. Vulnerable to surface contamination. Confined aquifer: bounded above AND below by aquitards (low permeability layers β clay, unfractured rock). Water under artesian pressure β if pressure head above ground surface β flowing artesian well. Recharge area: where confined aquifer is exposed at surface (often far from well). Perched aquifer: small unconfined aquifer above main water table, separated by unsaturated zone and local aquitard. Aquiclude: impermeable β no flow. Aquitard: low permeability β some flow. Aquifuge: neither stores nor transmits water.
Unconfined
Water table = upper boundary β vulnerable
Confined
Aquitard above β artesian pressure
Artesian
Pressure head above ground β flows without pumping
Perched
Small local aquifer above main water table
Darcy's Law
Darcy's Law: Q = K x i x A (Q=flow rate, K=hydraulic conductivity, i=hydraulic gradient, A=cross-sectional area). Flow = hydraulic conductivity Γ hydraulic gradient Γ area. 'Groundwater flows down the gradient.'
Darcy's Law
The fundamental equation of groundwater flow β derived experimentally by Henri Darcy in 1856
Q = K Γ i Γ A. Q: volumetric flow rate (mΒ³/s). K: hydraulic conductivity β property of material (m/s). Clay: 10β»βΉ m/s. Sand: 10β»β΅ to 10β»Β³ m/s. Gravel: 10β»Β² m/s. i: hydraulic gradient = Ξh/ΞL (head loss / distance) β dimensionless. A: cross-sectional area (mΒ²). Darcy velocity (q = Q/A = Ki): apparent velocity through entire cross-section. Seepage velocity (v = q/n): actual velocity through pore spaces β faster. n = porosity. Darcy's Law assumes: laminar flow (valid for most groundwater), saturated conditions, homogeneous material. Transmissivity (T = K Γ b): hydraulic conductivity Γ saturated thickness β aquifer productivity.
Q
Volumetric flow rate
K
Hydraulic conductivity β material property
i
Hydraulic gradient β Ξh/ΞL
A
Cross-sectional area
Seepage v
Q/(nΓA) β actual pore velocity
Hydraulic Head
Hydraulic head: h = z + Ο. Elevation head + pressure head. Groundwater flows from HIGH head to LOW head β always.
Hydraulic Head
The energy concept that drives all groundwater movement
Total hydraulic head (h): h = z + Ο. z = elevation head (potential energy from position). Ο = pressure head (pressure energy from water column height). Groundwater flows from high head to low head (energy gradient). Piezometer: tube open at bottom in aquifer β water rises to hydraulic head level. Water table: surface where pressure head = 0 (atmospheric pressure). Potentiometric surface: imaginary surface representing hydraulic head in confined aquifer β above the aquifer top. If potentiometric surface above ground β artesian well flows without pumping. Equipotential lines: connect points of equal head. Flowlines: perpendicular to equipotentials. Flow net: grid of equipotentials + flowlines.
h
Total hydraulic head
z
Elevation head β position energy
Ο
Pressure head β pressure energy
Flow direction
High head β low head always
Porosity and Permeability
Porosity: fraction of void space (n = Vv/Vt). Permeability: ease of flow. High porosity β high permeability (clay paradox).
Porosity and Permeability
Two key aquifer properties β and why they don't always go together
Porosity (n): fraction of total volume that is void space. Primary: intergranular (sand, gravel) or intragranular. Secondary: fractures, dissolution cavities (karst). Total porosity vs effective porosity: some water held by capillary forces can't drain β specific yield = drainable porosity. Values: gravel 25β40%, sand 25β50%, clay 40β70% (high!), granite 0β5%. Permeability (k): intrinsic property of material β depends on pore size and connectivity. Hydraulic conductivity (K) = k Γ Οg/ΞΌ β depends on fluid too. Clay paradox: high porosity but very low permeability (tiny, poorly connected pores) β excellent aquitard. Karst: secondary porosity from dissolution β very high permeability.
Porosity
Void fraction β how much water stored
Permeability
Ease of flow β pore size and connectivity
Clay
High porosity BUT low permeability
Karst
Dissolution cavities β very high permeability
Groundwater Flow Systems
Local: shallow, short flow paths. Intermediate: crosses topographic divides. Regional: deep, long flow paths to distant discharge.
Groundwater Flow Systems
Toth's hierarchical flow system β from local to regional scales
TΓ³th (1963): nested hierarchy of groundwater flow systems. Local systems: recharge at local highs, discharge at adjacent valleys β short flow paths, young water. Intermediate systems: flow crosses one or more topographic divides β longer paths, older water. Regional systems: recharge at continental divides, discharge at major valleys or coast β very long flow paths, ancient water (thousands of years old). Springs: where flow systems discharge at surface. Gaining streams: groundwater discharges into stream (positive baseflow). Losing streams: stream water recharges groundwater (often in arid regions). Water table configuration mirrors topography at local scale but smoothed. Residence time: local daysβyears, regional thousandsβmillions of years.
Well Hydraulics
Pumping well: creates cone of depression. Drawdown = initial head β pumped head. Theis equation gives drawdown vs time.
Well Hydraulics
What happens to groundwater when you pump a well β the cone of depression
Cone of depression: water table or potentiometric surface lowers around pumping well β cone shape. Drawdown (s): initial head minus head during pumping. Radius of influence: distance where drawdown = 0. Theis equation (1935): s = (Q/4ΟT) Γ W(u) β describes transient drawdown. Assumptions: homogeneous, isotropic, infinite, confined aquifer; fully penetrating well. Cooper-Jacob simplification: valid for large t. Pumping test: pump at constant rate, measure drawdown in observation wells β determine T and S. Storativity (S): volume of water released per unit area per unit head decline. Confined: S = 10β»β΅ to 10β»Β³ (water released by aquifer compression). Unconfined: Sy = 0.1β0.3 (gravity drainage).
Cone of depression
Water table lowers around pumping well
Drawdown
Initial head minus pumped head
T
Transmissivity β K Γ saturated thickness
S
Storativity β water released per area per head drop
Groundwater Contamination
Contaminant plume flows with groundwater. DNAPLs (Dense Non-Aqueous Phase Liquids) sink (chlorinated solvents). LNAPLs float (gasoline). Pump and treat is slow.
Groundwater Contamination
How pollutants move in groundwater β and why cleanup is so difficult
Sources: leaking underground storage tanks (USTs), landfills, septic systems, agricultural chemicals, industrial sites. Plume: contaminant spreads downgradient from source β shaped by flow field and dispersion. LNAPLs (Light Non-Aqueous Phase Liquids): gasoline, diesel β float on water table. DNAPLs (Dense Non-Aqueous Phase Liquids): chlorinated solvents (TCE (trichloroethylene), PCE (perchloroethylene)), creosote β sink through aquifer, pool at bottom β most difficult to remediate. Sorption: contaminants attach to aquifer solids β retardation factor. Biodegradation: natural attenuation. Pump and treat: extract contaminated water, treat at surface β slow, rarely achieves cleanup goals. In-situ remediation: inject oxidants, reductants, or microbes. Permeable reactive barrier: intercepts plume.
LNAPLs
Light β float (gasoline, oil)
DNAPLs
Dense β sink (TCE, PCE β solvents)
Plume
Flows downgradient from source
Pump & treat
Standard but slow β rarely fully cleans up
Groundwater Recharge
Recharge: water added to aquifer. Direct (diffuse): through unsaturated zone. Focused: through streambeds, sinkholes, fractures.
Groundwater Recharge
How aquifers are replenished β and why overdraft is a global crisis
Recharge: water that percolates through unsaturated zone to reach water table. Direct (diffuse) recharge: distributed over area β through soil and vadose zone. Focused recharge: concentrated in specific locations β stream channels, sinkholes, irrigation canals. Recharge rates: arid regions 1β5 mm/yr; humid regions 100β300 mm/yr. Vadose zone (unsaturated zone): between land surface and water table β complex flow and storage. Aquifer depletion: pumping > recharge β water table falls (Ogallala/High Plains aquifer declining 30 cm/yr in some areas). Land subsidence: over-pumping compacts clay layers β irreversible (Central Valley CA, Mexico City). Managed aquifer recharge (MAR (Managed Aquifer Recharge)): intentionally recharge aquifer with surface water, treated wastewater.
Direct recharge
Diffuse through soil β most common
Focused recharge
Streams, sinkholes, fractures
Ogallala
Great Plains β declining rapidly
Subsidence
Irreversible compaction from over-pumping
Karst Hydrogeology
Karst: dissolution of carbonate rock (limestone, dolomite) β caves, sinkholes, springs. Turbulent flow β Darcy's Law does NOT apply.
Karst Hydrogeology
Dissolution-controlled groundwater systems β fast flow, high vulnerability, spectacular landscapes
Karst: dissolution of soluble rock (limestone, dolomite, evaporites) by slightly acidic groundwater (COβ + HβO β HβCOβ). Landforms: sinkholes (dolines), caves, dry valleys, disappearing streams, large springs. Flow: through conduits β turbulent flow β Darcy's Law invalid. Rapid flow: tracer dyes move km/day (vs m/year in porous media). Florida: ~90% of drinking water from karst aquifer (Floridan). Edwards Aquifer (TX): supports endangered species, critical water supply. Vulnerability: no natural filtration β surface contamination rapidly reaches wells and springs. Sinkhole collapse: sudden subsidence over dissolving limestone β hazard in Florida, Missouri, Pennsylvania. UNESCO World Heritage karst: Carlsbad Caverns, Mammoth Cave, Guilin (China).
Isotopes in Hydrogeology
Tritium (Β³H): bomb pulse tracer β pre-1952 water is old. ΒΉβ΄C: dates groundwater 1,000β40,000 years. ΒΉβΈO/Β²H: identifies recharge source.
Isotopic Tracers in Groundwater
Environmental isotopes as natural tracers of groundwater age, origin, and flow paths
Stable isotopes: ΒΉβΈO and Β²H (deuterium) β meteoric water line (MWL (Meteoric Water Line)). Depleted values (more negative δ¹βΈO): colder/higher elevation recharge. Enriched: warmer/lower. Identifies recharge elevation and season. Radiogenic isotopes: Tritium (Β³H, half-life 12.3 yr): bomb pulse (1952β1963 nuclear testing) β presence = post-1952 recharge, absence = pre-bomb (old water). ΒΉβ΄C (half-life 5,730 yr): dates groundwater 1,000β40,000 years. Dead carbon correction needed. Β³βΆCl: million-year timescales (Great Artesian Basin β 1+ million years old). Noble gases (He, Ne, Ar): recharge temperature, excess air. CFCs and SFβ: date recent groundwater (1940sβpresent). Age dating helps: sustainable yield, contamination vulnerability, paleoclimate reconstruction.
Β³H (tritium)
Bomb pulse β present = post-1952 recharge
ΒΉβ΄C
1,000β40,000 year dating
ΒΉβΈO / Β²H
Recharge elevation and temperature
CFCs/SFβ
1940sβpresent dating
Groundwater and Society
Groundwater: 50% of global drinking water, 40% of irrigation. Ogallala depletion, arsenic in Bangladesh, saltwater intrusion in coastal cities.
Groundwater and Society
The global importance of groundwater β and the crises threatening it
Global significance: 2 billion people depend on groundwater for drinking water. 40% of global irrigation from groundwater. Ogallala (High Plains) Aquifer: one of world's largest β irrigates 30% of US groundwater-irrigated cropland, declining rapidly (recharged in Pleistocene β fossil water). Arsenic crisis: Bangladesh, India β millions drinking naturally arsenic-rich groundwater β cancer, skin lesions. Fluoride: East Africa, India β naturally elevated in some aquifers β dental/skeletal fluorosis. Saltwater intrusion: coastal cities over-pump β seawater intrudes (Miami, Jakarta, Chennai). Land subsidence: Jakarta sinking 25 cm/yr β relocating capital. Solutions: MAR, water recycling, demand reduction, conjunctive use (surface + groundwater management).
Ogallala
Fossil water β depleting, slow recharge
Arsenic
Bangladesh β natural, affects millions
Saltwater
Coastal over-pumping β seawater intrusion
Subsidence
Jakarta 25 cm/yr β Jakarta relocating
Groundwater-Surface Water Interaction
Gaining stream: groundwater feeds stream. Losing stream: stream feeds groundwater. Hyporheic zone: where they mix.
Groundwater-Surface Water Exchange
The dynamic exchange between rivers and aquifers β critical for ecology and water management
Gaining (effluent) stream: water table above stream level β groundwater discharges into stream β baseflow. Losing (influent) stream: stream stage above water table β stream recharges groundwater. Common in arid regions, losing streams in upper reaches often become gaining in lower reaches. Hyporheic zone: subsurface area where stream water and groundwater actively mix β ecologically vital (spawning habitat, temperature buffering, nutrient cycling). Bank storage: during floods, stream water infiltrates banks β released slowly after flood β extends baseflow recession. Pumping near streams: induced infiltration β well draws from stream rather than aquifer (legal issues in prior appropriation states). Baseflow separation: hydrograph technique to separate stormflow from groundwater contribution.
Gaining
GW table above stream β feeds stream
Losing
Stream above GW table β recharges aquifer
Hyporheic
Mixing zone β ecologically vital
Bank storage
Flood water stored in banks β released slowly
Mnemonic
What it means
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🎓 Common Exam Questions
Q: Explain Darcy's Law (Q = K x i x A) and what each term means.
A: Q = K x i x A (Henri Darcy, 1856). Q = volumetric flow rate (m3/day or similar). K = hydraulic conductivity β the ease with which water moves through the material. Ranges from 10-10 m/s (clay) to 10-2 m/s (gravel). i = hydraulic gradient β the slope of the potentiometric surface (change in hydraulic head / distance). A = cross-sectional area perpendicular to flow. Darcy's Law says: flow rate is proportional to hydraulic conductivity and hydraulic gradient. Groundwater flows from high head to low head at a rate controlled by permeability. A steep gradient in a permeable material = fast flow. Darcy velocity (specific discharge, q = Q/A) is the apparent velocity β actual pore velocity is higher because water only flows through pores (v = q/porosity). Darcy's Law assumes laminar flow β valid for most porous media but not karst conduits.
Q: Compare confined and unconfined aquifers β what is an artesian well?
A: Unconfined aquifer: the upper surface is the water table β groundwater surface at atmospheric pressure. Water table fluctuates seasonally and with pumping. Recharge by infiltration from above. Confined aquifer: bounded above AND below by aquitards (impermeable or low-permeability layers β clay, shale). Water is under pressure greater than atmospheric. The potentiometric surface (where water would rise in a well) can be above the top of the aquifer and even above ground level. Artesian well: in a confined aquifer where the potentiometric surface is above ground level β water flows without pumping. Examples: London Basin (18th-19th century artesian wells, now depleted), Great Artesian Basin (Australia β largest confined aquifer in world), Dakota Sandstone (US Great Plains). Confined aquifers are better protected from surface contamination but can be very difficult to recharge if aquitards are thick.
Q: What are DNAPLs and LNAPLs and why are they treated differently?
A: NAPLs (Non-Aqueous Phase Liquids): organic contaminants that do not dissolve in water β form separate phase. Two types based on density relative to water: LNAPLs (Light NAPLs, density less than water): petroleum hydrocarbons β gasoline, diesel, heating oil. Float on the water table. Remediation: pump and treat, skimming, soil vapor extraction. DNAPLs (Dense NAPLs, density greater than water): chlorinated solvents β TCE (trichloroethylene), PCE (perchloroethylene, dry cleaning fluid), carbon tetrachloride. SINK through the water table, migrate to the bottom of the aquifer, pool in depressions, and enter fractures. Extremely difficult to locate and remediate. Slowly dissolve into groundwater creating persistent plumes lasting decades to centuries. Examples: Love Canal, Cape Cod (military base solvents), Silicon Valley (semiconductor industry solvents). DNAPLs represent some of the most intractable groundwater contamination problems.
Q: What makes karst aquifers hydrogeologically unique and vulnerable?
A: Karst aquifers form in soluble rock (limestone, dolomite) dissolved by slightly acidic groundwater. Features: sinkholes (funnel-shaped depressions where the surface collapses into caves), caves, conduit networks, springs, losing streams (streams that disappear underground). Hydrogeologic uniqueness: instead of flowing slowly through pore spaces (porous media flow), water flows rapidly through conduits β cave systems and fractures. Flow velocities can be meters per second β as fast as surface streams. Travel times from sinkhole to spring: hours to days, not years to millennia. This makes karst aquifers: (1) Highly vulnerable to contamination β little filtration. (2) Difficult to characterize β standard Darcy's Law does not apply. (3) Prone to rapid changes in water level. Examples: Florida's karst is the drinking water source for millions β and extremely vulnerable to agricultural contamination and sinkholes. The Edwards Aquifer (Texas) is a critical karst aquifer for San Antonio.
Q: What isotopes are used in hydrogeology and what do they tell us?
A: Stable isotopes: Oxygen-18 and deuterium (H-2) β together form the Global Meteoric Water Line (MWL: d2H = 8 x d18O + 10). Precipitation at high altitude or latitude has more depleted (lighter) isotopic values. Comparing groundwater isotopes to modern precipitation tells us where the water recharged. Radioactive isotopes for dating: Tritium (H-3, half-life 12.3 years) β distinguishes pre-1952 (old) from post-1952 (modern) water. Carbon-14 (half-life 5,730 years) β dates groundwater up to ~40,000 years old. Chlorine-36 (half-life 301,000 years) β dates very old groundwater (Ogallala, deep fossil aquifers). Helium-4 accumulates in old groundwater from radioactive decay β another age indicator. Together these tools tell us: Is this water renewable or fossil? Where did it recharge? How long has it been underground? Is mixing occurring?