Geology tricks that make groundwater click

Unconfined (water table) aquifer: upper boundary is the water table — 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 — clay, unfractured rock). Water under artesian pressure — if pressure head above ground surface → 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 — no flow. Aquitard: low permeability — some flow. Aquifuge: neither stores nor transmits water.

Q = K × i × A. Q: volumetric flow rate (m³/s). K: hydraulic conductivity — property of material (m/s). Clay: 10⁻⁹ m/s. Sand: 10⁻⁵ to 10⁻³ m/s. Gravel: 10⁻² m/s. i: hydraulic gradient = Δh/ΔL (head loss / distance) — dimensionless. A: cross-sectional area (m²). Darcy velocity (q = Q/A = Ki): apparent velocity through entire cross-section. Seepage velocity (v = q/n): actual velocity through pore spaces — faster. n = porosity. Darcy's Law assumes: laminar flow (valid for most groundwater), saturated conditions, homogeneous material. Transmissivity (T = K × b): hydraulic conductivity × saturated thickness — aquifer productivity.

Total hydraulic head (h): h = z + ψ. z = elevation head (potential energy from position). ψ = 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 — 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 — above the aquifer top. If potentiometric surface above ground → artesian well flows without pumping. Equipotential lines: connect points of equal head. Flowlines: perpendicular to equipotentials. Flow net: grid of equipotentials + flowlines.

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 — specific yield = drainable porosity. Values: gravel 25–40%, sand 25–50%, clay 40–70% (high!), granite 0–5%. Permeability (k): intrinsic property of material — depends on pore size and connectivity. Hydraulic conductivity (K) = k × ρg/μ — depends on fluid too. Clay paradox: high porosity but very low permeability (tiny, poorly connected pores) — excellent aquitard. Karst: secondary porosity from dissolution → very high permeability.

Tóth (1963): nested hierarchy of groundwater flow systems. Local systems: recharge at local highs, discharge at adjacent valleys — short flow paths, young water. Intermediate systems: flow crosses one or more topographic divides — longer paths, older water. Regional systems: recharge at continental divides, discharge at major valleys or coast — 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 days–years, regional thousands–millions of years.

Cone of depression: water table or potentiometric surface lowers around pumping well — cone shape. Drawdown (s): initial head minus head during pumping. Radius of influence: distance where drawdown = 0. Theis equation (1935): s = (Q/4πT) × W(u) — 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 → determine T and S. Storativity (S): volume of water released per unit area per unit head decline. Confined: S = 10⁻⁵ to 10⁻³ (water released by aquifer compression). Unconfined: Sy = 0.1–0.3 (gravity drainage).

Sources: leaking underground storage tanks (USTs), landfills, septic systems, agricultural chemicals, industrial sites. Plume: contaminant spreads downgradient from source — shaped by flow field and dispersion. LNAPLs (Light Non-Aqueous Phase Liquids): gasoline, diesel — float on water table. DNAPLs (Dense Non-Aqueous Phase Liquids): chlorinated solvents (TCE (trichloroethylene), PCE (perchloroethylene)), creosote — sink through aquifer, pool at bottom → most difficult to remediate. Sorption: contaminants attach to aquifer solids → retardation factor. Biodegradation: natural attenuation. Pump and treat: extract contaminated water, treat at surface — slow, rarely achieves cleanup goals. In-situ remediation: inject oxidants, reductants, or microbes. Permeable reactive barrier: intercepts plume.

Recharge: water that percolates through unsaturated zone to reach water table. Direct (diffuse) recharge: distributed over area — through soil and vadose zone. Focused recharge: concentrated in specific locations — stream channels, sinkholes, irrigation canals. Recharge rates: arid regions 1–5 mm/yr; humid regions 100–300 mm/yr. Vadose zone (unsaturated zone): between land surface and water table — complex flow and storage. Aquifer depletion: pumping > recharge → water table falls (Ogallala/High Plains aquifer declining 30 cm/yr in some areas). Land subsidence: over-pumping compacts clay layers — irreversible (Central Valley CA, Mexico City). Managed aquifer recharge (MAR (Managed Aquifer Recharge)): intentionally recharge aquifer with surface water, treated wastewater.

Karst: dissolution of soluble rock (limestone, dolomite, evaporites) by slightly acidic groundwater (CO₂ + H₂O → H₂CO₃). Landforms: sinkholes (dolines), caves, dry valleys, disappearing streams, large springs. Flow: through conduits → turbulent flow → 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 → surface contamination rapidly reaches wells and springs. Sinkhole collapse: sudden subsidence over dissolving limestone — hazard in Florida, Missouri, Pennsylvania. UNESCO World Heritage karst: Carlsbad Caverns, Mammoth Cave, Guilin (China).

Stable isotopes: ¹⁸O and ²H (deuterium) — meteoric water line (MWL (Meteoric Water Line)). Depleted values (more negative δ¹⁸O): colder/higher elevation recharge. Enriched: warmer/lower. Identifies recharge elevation and season. Radiogenic isotopes: Tritium (³H, half-life 12.3 yr): bomb pulse (1952–1963 nuclear testing) — presence = post-1952 recharge, absence = pre-bomb (old water). ¹⁴C (half-life 5,730 yr): dates groundwater 1,000–40,000 years. Dead carbon correction needed. ³⁶Cl: million-year timescales (Great Artesian Basin — 1+ million years old). Noble gases (He, Ne, Ar): recharge temperature, excess air. CFCs and SF₆: date recent groundwater (1940s–present). Age dating helps: sustainable yield, contamination vulnerability, paleoclimate reconstruction.

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 — irrigates 30% of US groundwater-irrigated cropland, declining rapidly (recharged in Pleistocene — fossil water). Arsenic crisis: Bangladesh, India — millions drinking naturally arsenic-rich groundwater → cancer, skin lesions. Fluoride: East Africa, India — naturally elevated in some aquifers → dental/skeletal fluorosis. Saltwater intrusion: coastal cities over-pump → seawater intrudes (Miami, Jakarta, Chennai). Land subsidence: Jakarta sinking 25 cm/yr → relocating capital. Solutions: MAR, water recycling, demand reduction, conjunctive use (surface + groundwater management).

Gaining (effluent) stream: water table above stream level → groundwater discharges into stream → baseflow. Losing (influent) stream: stream stage above water table → 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 — ecologically vital (spawning habitat, temperature buffering, nutrient cycling). Bank storage: during floods, stream water infiltrates banks → released slowly after flood → extends baseflow recession. Pumping near streams: induced infiltration — well draws from stream rather than aquifer (legal issues in prior appropriation states). Baseflow separation: hydrograph technique to separate stormflow from groundwater contribution.