Hydrogeology

In short

Hydrogeology is the study of the occurrence, movement, quantity, quality and management of water below the ground surface — how rocks store water (porosity, specific yield), how they transmit it (hydraulic conductivity, transmissivity) and how wells respond to pumping. It matters because roughly 60-65% of India's irrigation and about 85% of rural drinking water come from groundwater, and the country is the world's largest groundwater user. The topic is examined in B.Sc/M.Sc applied geology papers, GATE (GG - Geology and Geophysics), UPSC Geologist/Hydrogeologist (CGWB), state PSC and junior-hydrogeologist tests, with marks concentrated in Darcy's law, aquifer parameters and pumping-test analysis. Employers are the Central Ground Water Board, state ground water departments, Jal Jeevan Mission agencies, drilling and consultancy firms (WAPCOS, NABARD-aided projects, mining and cement companies for dewatering and water audits).

Explained

Water in rocks: porosity, aquifer types and vertical zonation

• Hydrologic cycle, subsurface part: precipitation → infiltration → percolation through the unsaturated (vadose) zone → recharge to the saturated zone → lateral flow → discharge as springs, baseflow, evapotranspiration or pumping.
• Zones below surface: (i) soil-water zone, (ii) intermediate vadose zone, (iii) capillary fringe, (iv) zone of saturation below the water table (the surface where pore pressure = atmospheric). Water above the water table is held at negative (suction) pressure and is not available to wells.
• Porosity (n): volume of voids / total volume, as a fraction or %. Primary porosity forms with the rock (intergranular - sand, sandstone); secondary porosity comes later (joints, fractures, faults, solution cavities, weathering).
• Effective porosity is the interconnected pore volume; clay has n up to 50% but almost no effective transmission — the classic exam trap.
• Specific yield (Sy) = drainable water per unit volume by gravity; specific retention (Sr) = water retained by capillarity and adsorption. n = Sy + Sr.
• Rock classes by behaviour: aquifer (stores and yields usable water - sand, gravel, weathered-fractured granite, vesicular basalt, karstic limestone); aquitard (leaky, transmits slowly - silt, sandy clay); aquiclude (stores but does not yield - clay, shale); aquifuge (neither - fresh unfractured granite, massive quartzite).
• Aquifer types: unconfined (water table aquifer, upper surface free, recharged directly from above); confined (artesian - sandwiched between impermeable beds, water under pressure, piezometric surface above the aquifer top; flowing well if it is above ground); semi-confined/leaky (overlain by an aquitard that leaks); perched (local saturated body above a discontinuous impervious lens).
• Water-level surfaces: water table (unconfined) versus piezometric/potentiometric surface (confined). Contour maps of either give flow direction (normal to contours, down-gradient), hydraulic gradient, and cones of depression.
• Influent and effluent streams: an effluent (gaining) stream receives baseflow where the water table is above stream level; an influent (losing) stream recharges the aquifer - common in peninsular India in summer.

Darcy's law and the aquifer parameters

• Darcy's law (1856): discharge is proportional to the hydraulic gradient. Q = −K i A; the minus sign shows flow is from high to low head.
• Hydraulic head h = z + p/ρg (elevation + pressure head); velocity head is negligible in porous media. Flow responds to head, not to pressure alone or to depth.
• Hydraulic conductivity (K), m/day or m/s, is a property of both rock and fluid: K = kρg/μ, where k is intrinsic permeability (m² or darcy; 1 darcy ≈ 0.987 × 10⁻¹² m² ≈ 0.83 m/day for water at 20 °C).
• Typical K (m/day): gravel 100-1,000; coarse sand 10-100; fine sand 1-5; silt 10⁻³-10⁻¹; clay <10⁻⁴; fractured basalt 1-50 (highly variable); fresh granite ~10⁻⁵.
• Anisotropy and heterogeneity: layered sediments have Kh >> Kv (often 10-100×). Equivalent K for flow parallel to layers is the thickness-weighted arithmetic mean; perpendicular to layers it is the harmonic mean (dominated by the tightest layer).
• Transmissivity T = K b (m²/day), b = saturated thickness; in an unconfined aquifer b falls as the water table drops, so T is not constant.
• Storativity (storage coefficient) S = volume released per unit area per unit head decline (dimensionless). Confined: S = Ss·b, typically 10⁻⁵-10⁻³ (water expansion + aquifer compression). Unconfined: S ≈ Sy, typically 0.05-0.30 — three to four orders of magnitude larger, which is why confined aquifers show deep, fast, far-reaching drawdown.
• Darcy velocity (specific discharge) q = Q/A = Ki is a flux, not a real velocity; seepage (pore) velocity v = q/ne is the actual speed of a water particle and governs contaminant travel time.
• Validity limits of Darcy's law: laminar flow only, Reynolds number Re = ρvd/μ below about 1-10. It fails in very coarse gravel/karst near a pumping well (turbulent, hence well losses) and in very tight clays at extremely low gradients.
• Governing equation (outline): combine Darcy's law with continuity → for transient confined flow ∂²h/∂x² + ∂²h/∂y² = (S/T) ∂h/∂t (diffusion equation); steady state gives the Laplace equation. Unconfined flow uses the Dupuit-Forchheimer assumptions (small slope, horizontal flow, hydraulic gradient = water-table slope), giving the Boussinesq equation.

Well hydraulics and pumping tests

• Cone of depression: on pumping, heads fall in a radially symmetric cone; drawdown s = static level − pumping level. Radius of influence grows with time and is larger in confined and in high-T, low-S aquifers.
• Steady (equilibrium) flow — Thiem/Dupuit: confined, Q = 2πKb(h₂−h₁)/ln(r₂/r₁); unconfined, Q = πK(h₂²−h₁²)/ln(r₂/r₁). Two observation wells give K and T directly.
• Unsteady flow — Theis (1935): s = (Q/4πT) W(u), with u = r²S/4Tt and W(u) the well function. Analysed by curve matching of log s vs log t (or t/r²) on the Theis type curve; the match point gives T and S.
• Cooper-Jacob approximation (valid for small u, roughly u < 0.01, i.e. late time, small r): s = (2.303Q/4πT) log(2.25Tt/r²S). Plot s against log t — a straight line whose slope per log cycle Δs gives T, and whose zero-drawdown intercept t₀ gives S.
• Recovery test: after the pump stops, residual drawdown s′ vs log(t/t′) is a straight line; T = 2.303Q/(4πΔs′). Useful because discharge need not be held constant during recovery.
• Leaky aquifer: Hantush-Jacob (Walton) type curves with a leakage parameter r/B. Unconfined with delayed yield: Boulton/Neuman, showing an early elastic segment, a flat delayed-yield segment and a late Theis segment.
• Step-drawdown test: pumping at 3-5 increasing rates to separate aquifer loss and well loss — s = BQ + CQ², B = formation loss coefficient, C = well-loss coefficient. Well efficiency = BQ/(BQ + CQ²).
• Specific capacity = Q/s (m³/day per m), the practical yardstick for well selection and pump sizing; falls as the well clogs or incrusts.
• Partial penetration, well interference and boundaries: overlapping cones add drawdowns (superposition); image wells model a recharge boundary (river - drawdown reduced) or a barrier boundary (dyke, fault - drawdown doubled), seen as a slope change on the Jacob plot.
• Safe yield / optimum yield is the extraction a basin can sustain without undesirable effects (declining levels, quality deterioration, land subsidence, drying of springs) — a management decision, not a fixed rock property.

Indian hydrogeological framework, well construction and exploration

• Major hydrogeological units (CGWB classification):
  • Unconsolidated alluvium (Indo-Gangetic-Brahmaputra plains, coastal deltas, ~8 lakh km²) — thick multi-layer sand-clay sequences, high yields (often 100-300 m³/h), best groundwater province of India.
  • Deccan basalt (Deccan Trap, ~5 lakh km²) — water occurs in the weathered mantle, vesicular/jointed flow tops and interflow red bole/tuff zones; thin, patchy aquifers; dug wells and shallow bores; yields usually 5-30 m³/h.
  • Crystalline hard rock (granite, gneiss, charnockite, schist of the peninsular shield, ~6.5 lakh km²) — only weathered zone (10-25 m) plus fractures to about 100-200 m yield water; low storage, high failure risk, needs fracture targeting.
  • Consolidated sedimentary (Vindhyan, Cuddapah, Gondwana sandstones) — moderate, bedding and joint controlled; karstic limestone (parts of Cuddapah, Bhima, Shillong) has conduit flow, very high but erratic yields and rapid contamination.
  • Laterite (Kerala, Konkan, Odisha) — porous cap over hard rock, supports shallow dug wells and springs.
• Groundwater structures: dug well (large diameter, open well — hard rock, Maharashtra/Karnataka), dug-cum-bore well, shallow tube well / filter point (alluvium), deep tube well (multi-strainer, alluvium), bore well (hard rock, DTH drilled), infiltration gallery and collector/radial well (river bank, used for town water supply), spring tapping, qanat/surangam (traditional, north Kerala-Kasaragod).
• Drilling choices: DTH (down-the-hole) hammer for hard rock; direct/reverse circulation rotary with mud for alluvium; percussion (cable tool) for boulders. Well design: housing pipe, blank and slotted/gravel-packed strainers against the coarse zones, slot size from sieve analysis, gravel pack 4-6× the D₅₀ of the aquifer, sanitary seal at top.
• Well development (surging, jetting, overpumping, acidisation in carbonates) removes drilling mud and fines; verified by sand-free discharge and improved specific capacity.
• Exploration sequence: desk study and remote sensing → hydrogeological mapping of lithology, lineaments, springs and well inventory → water-level monitoring (pre- and post-monsoon) → geophysical survey (electrical resistivity/VES, Schlumberger, is the standard siting tool in hard rock) → exploratory drilling with lithologs and e-logs → pumping test → quality analysis. (Geophysical methods are detailed separately.)
• Monitoring: CGWB's national network of observation wells and piezometers (DWLR/telemetry) with four seasonal measurements a year; water-level and quality data published in the annual *Dynamic Ground Water Resources of India* reports and on the India-WRIS/NAQUIM outputs. NAQUIM (National Aquifer Mapping and Management Programme) maps aquifers at 1:50,000 and prepares management plans.
• Environmental tracers and isotopes: tritium and ¹⁴C for age, δ¹⁸O/δ²H for recharge source and evaporation, used with CGWB/BARC support to distinguish modern from palaeo-water.

Groundwater quality, contamination and saline intrusion

• Common dissolved constituents: major cations Ca²⁺, Mg²⁺, Na⁺, K⁺; major anions HCO₃⁻, SO₄²⁻, Cl⁻, NO₃⁻, F⁻. Reported in mg/L; EC in µS/cm; TDS ≈ 0.64 × EC(µS/cm) as a field approximation.
• Water type / graphical tools: Piper trilinear diagram (hydrochemical facies, e.g. Ca-HCO₃ fresh recharge water → Na-Cl marine/saline), Stiff, Schoeller, Wilcox and US Salinity Laboratory diagrams for irrigation suitability.
• Irrigation indices: SAR (sodium adsorption ratio), %Na (soluble sodium percentage), RSC (residual sodium carbonate); high SAR/RSC causes soil structure collapse and alkalinity.
• Drinking-water standard: IS 10500:2012 (with amendments) — Drinking Water Specification. Acceptable / permissible (in the absence of an alternative source): pH 6.5-8.5 (no relaxation); TDS 500 / 2,000 mg/L; total hardness 200 / 600 mg/L as CaCO₃; chloride 250 / 1,000; sulphate 200 / 400; fluoride 1.0 / 1.5; nitrate 45 (no relaxation); arsenic 0.01 (the 0.05 mg/L permissible relaxation was withdrawn by a 2015 amendment); iron 0.3 (relaxation withdrawn in recent amendments — confirm the current amendment before quoting); E. coli must be absent in 100 mL. Verify against the latest amendment, as individual limits have been revised.
• Geogenic problems in India: fluoride (>1.5 mg/L — Rajasthan, Gujarat, Telangana, Andhra Pradesh, Karnataka, Tamil Nadu, parts of Bihar/Jharkhand; causes dental and skeletal fluorosis); arsenic (>0.01 mg/L — Ganga-Brahmaputra-Meghna alluvium of West Bengal, Bihar, UP, Assam, Jharkhand; reductive dissolution of Fe-oxyhydroxides in young grey sands; causes melanosis, keratosis, cancer); salinity/brackishness (western Rajasthan, Gujarat, coastal belts, parts of Haryana); iron (Assam, Odisha, West Bengal); uranium (parts of Punjab, Haryana, Rajasthan); nitrate from fertiliser and sanitation (widespread; methaemoglobinaemia in infants).
• Anthropogenic sources: septic tanks and sewage, landfill leachate, industrial effluent and tanning/dyeing waste, fly-ash ponds, mine drainage (acid mine drainage - low pH, high Fe/SO₄), pesticide residues, irrigation return flow.
• Contaminant transport: advection (with the seepage velocity), hydrodynamic dispersion (mechanical dispersion + molecular diffusion), sorption/retardation, and decay/degradation. DNAPLs sink through the aquifer; LNAPLs (petrol, diesel) float on the water table.
• Vulnerability and protection: DRASTIC index method; wellhead protection zones; sanitary seal and minimum distance from latrines/septic tanks.
• Sea-water intrusion: over-pumping near the coast lets denser saline water (ρ ≈ 1.025 g/cm³) move inland and upcone. Ghyben-Herzberg relation: the fresh-water/salt-water interface lies about 40 times the fresh-water head above mean sea level, below sea level. Lowering the water table 1 m raises the interface ~40 m. Documented in Minjur (Chennai), Saurashtra and Mangaluru/Kochi coastal tracts. Controls: pumping restrictions, scattered shallow wells, recharge/injection wells, tidal regulators and bandharas, subsurface dykes (Kerala, Saurashtra).

Assessment, recharge, management, law and codes

• Groundwater balance equation: Recharge (rainfall + canal/irrigation return + tanks + inflow) − Discharge (pumping + baseflow + ET + outflow) = ΔS = Sy × area × Δh for an unconfined aquifer.
• GEC-2015 methodology (Ground Water Estimation Committee, used by CGWB and state departments): compute annual extractable ground water resource, extraction, and Stage of Ground Water Extraction = (extraction / annual extractable resource) × 100. Categories: Safe ≤70%, Semi-critical >70-90%, Critical >90-100%, Over-exploited >100% (over-exploited also where there is a significant long-term decline). Assessment units are blocks/taluks/mandals/firkas.
• Order of magnitude from the recent national assessments (2023/2024 reports): annual extractable resource ~400-410 bcm, extraction ~245 bcm, national stage ~60%, with roughly 11-15% of assessment units over-exploited — mostly Punjab, Haryana, Rajasthan, Delhi, western UP, Tamil Nadu. These numbers are revised annually; quote the latest report.
• Artificial recharge / managed aquifer recharge (MAR) structures: percolation tank, check dam/nala bund, gabion, recharge shaft and recharge well, injection well, subsurface dyke, contour bund and trench, farm pond, spreading basin, and rooftop rainwater harvesting (mandatory in many municipal bye-laws). CGWB's Master Plan for Artificial Recharge to Ground Water (2020) sets out structure numbers and costs for the whole country.
• Design norm: required recharge volume = catchment area × rainfall × runoff coefficient × fraction harvested; percolation tanks are sized to fill 2-3 times a season.
• Law and institutions: groundwater is a State subject (water, Entry 17, State List), and under Indian law has traditionally been treated as attached to land — the reason regulation is largely administrative. Central Ground Water Authority (CGWA) was constituted under the Environment (Protection) Act, 1986 following Supreme Court directions (1996) and issues NOCs for groundwater extraction under the *Guidelines to Regulate Ground Water Extraction* (notified 2020, with later revisions - check the current version), using a graded water-conservation fee and mandatory recharge/audit for industries. The Water (Prevention and Control of Pollution) Act, 1974 governs pollution; the Model Bill for Conservation, Protection and Regulation of Groundwater, 2017 has been adopted in varying form by several states; many states run their own groundwater Acts and permit systems (e.g. Maharashtra, Karnataka, Andhra Pradesh, Tamil Nadu, Kerala, West Bengal).
• Programmes: Atal Bhujal Yojana (World Bank-aided, ₹6,000 crore, community-led demand-side management in 7 water-stressed states; check current status/extension), Jal Jeevan Mission (rural tap water, heavily source-dependent on groundwater), Jal Shakti Abhiyan: Catch the Rain, Amrit Sarovar, water-conservation works under the rural employment guarantee (MGNREGA, replaced from 1 July 2026 by the VB-G RAM G Act), and NAQUIM aquifer management plans.
• Codes and standards used in practice: IS 10500 (drinking water specification), IS 3025 series (methods of sampling and test for water and wastewater), IS 11624 and related standards on irrigation water quality, IS 11189/IS 2800 series on tube-well construction and screens, IS 15792 on artificial recharge/rainwater harvesting guidance, BIS codes on pump testing. IS numbers are periodically revised or withdrawn — confirm the current number and edition before citing.
• Limitations of groundwater development: invisible resource with large uncertainty, slow response and slow recovery, over-extraction causes land subsidence and quality deterioration, free/subsidised electricity drives over-pumping, and aquifer boundaries do not follow administrative ones.

Key points

• Aquifer - a saturated formation that both stores and yields usable quantities of water; an aquiclude stores but will not yield.
• Porosity vs specific yield - clay has high porosity but very low Sy; sand and gravel have moderate porosity and high Sy.
• Water table - surface at atmospheric pressure; piezometric surface - the level to which water rises in a confined aquifer's well.
• Darcy's law - Q = KiA, laminar flow only (Re roughly < 1-10); the basis of all well hydraulics.
• Hydraulic conductivity K - m/day, fluid + medium; intrinsic permeability k - m² or darcy, medium only.
• Transmissivity T = Kb - ability of the whole thickness to transmit water, m²/day.
• Storativity - confined 10⁻⁵-10⁻³ (elastic); unconfined ≈ Sy, 0.05-0.30 (drainage) — the key numerical discriminator.
• Darcy velocity vs seepage velocity - q = Ki is a flux; real particle speed = q/ne, always larger.
• Thiem - steady-state two-well solution for K and T; Theis - transient curve matching for T and S; Cooper-Jacob - semilog straight line, valid at late time.
• Step-drawdown test - separates aquifer loss (BQ) from well loss (CQ²) and gives well efficiency.
• Specific capacity Q/s - practical index of well performance and pump sizing.
• Image wells - model recharge boundaries (less drawdown) and barrier boundaries (more drawdown).
• Ghyben-Herzberg - interface depth ≈ 40 × freshwater head above sea level; 1 m of water-table drop lifts it ~40 m.
• Hard rock aquifers - storage only in weathered mantle and fractures; shallow, patchy, high bore-well failure rate.
• Stage of extraction - Safe ≤70%, Semi-critical 70-90%, Critical 90-100%, Over-exploited >100% (GEC-2015).
• IS 10500:2012 - fluoride 1.0/1.5 mg/L, nitrate 45 mg/L, arsenic 0.01 mg/L (no relaxation since the 2015 amendment), TDS 500/2,000 mg/L (confirm latest amendment).
• CGWA - regulates extraction through NOCs under the Environment (Protection) Act, 1986; groundwater itself is a State subject.
• MAR structures - percolation tanks, check dams, recharge shafts, injection wells, subsurface dykes, rooftop harvesting.

Formulas and examples

Example 1 - Darcy flow and travel time. A sand aquifer is 20 m thick, 50 m wide across the flow; heads in two piezometers 1,500 m apart differ by 3 m; K = 25 m/day, nₑ = 0.25.

• i = 3/1,500 = 0.002; A = 20 × 50 = 1,000 m².
• Q = K i A = 25 × 0.002 × 1,000 = 50 m³/day.
• q = K i = 25 × 0.002 = 0.05 m/day; v = 0.05/0.25 = 0.2 m/day.
• Travel time over 1,500 m = 1,500/0.2 = 7,500 days ≈ 20.5 years — the point for contaminant risk.

Example 2 - Thiem test on a confined aquifer. Q = 1,500 m³/day steady; aquifer thickness b = 20 m; drawdown 3.0 m at r₁ = 10 m and 1.0 m at r₂ = 100 m.

• s₁ − s₂ = 2.0 m; ln(r₂/r₁) = ln 10 = 2.3026.
• T = Q ln(r₂/r₁) / [2π(s₁ − s₂)] = 1,500 × 2.3026 / (6.2832 × 2.0) = 3,453.9/12.566 = 274.9 m²/day.
• K = T/b = 274.9/20 = 13.7 m/day (coarse sand).
• Specific capacity of the pumped well, if its own drawdown is 3.0 m: 1,500/3.0 = 500 m³/day per m.

Example 3 - Cooper-Jacob straight line. Q = 1,000 m³/day; observation well at r = 50 m; semilog plot gives Δs = 0.8 m per log cycle and t₀ = 1.5 minutes.

• T = 2.303 Q/(4π Δs) = 2,303/(12.566 × 0.8) = 2,303/10.053 = 229.1 m²/day.
• t₀ = 1.5/1,440 = 1.0417 × 10⁻³ day.
• S = 2.25 T t₀ / r² = 2.25 × 229.1 × 1.0417 × 10⁻³ / 2,500 = 0.5370/2,500 = 2.1 × 10⁻⁴ → confined aquifer (S of order 10⁻⁴, not 0.1).

Example 4 - Resource assessment and category. A block of 100 km² gets 800 mm annual rainfall; rainfall infiltration factor 10%; other recharge (canals, tanks, return flow) 2 MCM; natural discharge allowance 5%; gross extraction 8.0 MCM.

• Rainfall recharge = 100 × 10⁶ m² × 0.8 m × 0.10 = 8 × 10⁶ m³ = 8.0 MCM.
• Total annual recharge = 8.0 + 2.0 = 10.0 MCM; natural discharge = 0.5 MCM.
• Annual extractable resource = 10.0 − 0.5 = 9.5 MCM.
• Stage = 8.0/9.5 × 100 = 84.2% → Semi-critical (70-90%).

Example 5 - Storage change and sea-water intrusion. (a) In a 10 km² unconfined aquifer with Sy = 0.18, the water table falls 1.2 m in a summer: V = 10 × 10⁶ × 1.2 × 0.18 = 2.16 × 10⁶ m³ = 2.16 MCM removed from storage. (b) In a coastal well field the fresh-water head is 1.5 m above MSL: interface depth z ≈ 40 × 1.5 = 60 m below MSL. If pumping lowers the head to 1.0 m, z becomes 40 m — the interface rises 20 m, and a 60 m deep well now draws saline water.

Where it is used

• Central Ground Water Board and state ground water departments - aquifer mapping (NAQUIM), exploratory drilling, pumping tests, water-level and quality monitoring, resource estimation, NOC processing; recruited through UPSC Geologist/Hydrogeologist and state PSC exams.
• Rural and urban water supply - Jal Jeevan Mission and municipal schemes: source finding, tube-well and infiltration-gallery design, yield testing, fluoride/arsenic/iron treatment plant siting.
• Irrigation and agriculture - bore-well siting for farmers, NABARD/ minor-irrigation appraisals, canal command water-logging and salinity studies, percolation tanks and check dams under watershed programmes.
• Mining, cement and quarrying - mine dewatering design, pit-slope seepage, hydrogeological chapters of EIA/EMP reports, impact on neighbouring wells; acid mine drainage control in coal and metal mines.
• Infrastructure and industry - dewatering for metro and deep foundations, groundwater NOC and water audit for industries, contaminated-site assessment and remediation (landfills, refineries, tanneries), geothermal and water-source heat pump projects.
• Consultancy and software roles - WAPCOS, NIH Roorkee, IITs, private hydrogeology consultancies and NGOs (ACWADAM, FES, Arghyam); tools such as MODFLOW/Visual MODFLOW, AQTESOLV, GIS and remote sensing for aquifer modelling and groundwater-potential mapping.

Interview questions

• Q: What is the difference between an aquifer and an aquiclude? A: An aquifer both stores and transmits usable quantities of water, like sand or fractured basalt; an aquiclude such as clay stores water but is too impermeable to yield it to a well.
• Q: Why is clay's high porosity useless for water supply? A: Its pores are tiny and poorly connected, so most water is held by capillary and surface forces — specific retention is high and specific yield and hydraulic conductivity are negligible.
• Q: State Darcy's law and its limitations. A: Q = −KiA, discharge is proportional to hydraulic gradient and cross-sectional area. It holds only for laminar flow (Re below about 1-10), so it fails in karst conduits, in very coarse gravel close to a pumping well, and in very tight clays at very low gradients.
• Q: Distinguish hydraulic conductivity from intrinsic permeability. A: K depends on both the medium and the fluid (K = kρg/μ) and is in m/day; k depends only on the pore geometry of the medium and is in m² or darcy.
• Q: How would you tell from a pumping test whether the aquifer is confined or unconfined? A: By the storage coefficient — confined aquifers give S of 10⁻⁵-10⁻³ with a fast, far-reaching cone of depression; unconfined aquifers give S ≈ Sy of 0.05-0.30 with a slow, local cone and a delayed-yield segment on the curve.
• Q: What does transmissivity mean physically, and why is it not constant in an unconfined aquifer? A: T = Kb is the discharge through the full saturated thickness per unit width per unit gradient; in an unconfined aquifer the saturated thickness b falls as the water table is drawn down, so T decreases during pumping.
• Q: Why is the Cooper-Jacob method only valid at late time? A: It drops all but the first two terms of the Theis well-function series, which is accurate only when u = r²S/4Tt is small (about <0.01) — that is, at large t or small r.
• Q: A step-drawdown test shows drawdown rising faster than discharge. What does that indicate? A: Significant well loss — the CQ² term from turbulent flow through the screen and gravel pack — meaning the well is clogged, badly developed or under-screened, not that the aquifer is poor.
• Q: Explain the Ghyben-Herzberg principle and its practical consequence. A: Because sea water is about 2.5% denser, the fresh-water lens floats with its interface roughly 40 times the fresh-water head below sea level; so lowering the water table by 1 m by pumping raises the saline interface by about 40 m and can salinise a coastal well.
• Q: Why do bore wells in granite terrain fail so often? A: Crystalline rock has no primary porosity — water occurs only in the weathered mantle and in discrete fractures to about 100-200 m, so unless the hole intersects a lineament or fracture zone it finds almost no storage.
• Q: How is a block declared over-exploited in India? A: Under the GEC-2015 methodology, when the stage of groundwater extraction exceeds 100% of the annual extractable resource (or where there is a significant long-term water-level decline), the assessment unit is categorised over-exploited; 90-100% is critical and 70-90% semi-critical.
• Q: Why is arsenic contamination mostly a problem in shallow Ganga plain aquifers? A: Arsenic is released by reductive dissolution of iron oxyhydroxides in young, organic-rich grey alluvial sands under reducing conditions; deeper brown oxidised sands and older aquifers are generally safer, which is why safe-aquifer targeting is used alongside treatment.
• Q: Which regulator issues permission for an industry to extract groundwater? A: The Central Ground Water Authority, acting under the Environment (Protection) Act, 1986, grants NOCs as per its extraction guidelines, except in states that operate their own groundwater legislation and authorities.

Common mistakes

• Equating Darcy velocity with the actual velocity of water — forgetting to divide by effective porosity when computing contaminant travel time.
• Using the confined (Thiem) formula with drawdowns on an unconfined aquifer instead of the Dupuit h₂² − h₁² form, especially when drawdown exceeds about 25% of thickness.
• Reporting storativity of 0.2 for a confined aquifer or 10⁻⁴ for an unconfined one — the magnitudes are diagnostic and examiners check them.
• Unit mixing: minutes with days, litres per second with m³/day (1 L/s = 86.4 m³/day), µS/cm with mg/L; and forgetting 2.303 (the ln-to-log factor) in the Jacob formula.
• Applying Cooper-Jacob to early-time data, or fitting a straight line across a slope change that is actually a boundary or leakage effect.
• Treating a low-yielding well as proof of a poor aquifer when a step-drawdown test would show it is a well-construction (well-loss) problem.
• Confusing porosity with specific yield, aquifer loss with well loss, or the piezometric surface with the water table; and quoting "safe yield" as a fixed property of the rock rather than a management limit.
• Quoting withdrawn IS numbers or old BIS limits for fluoride/arsenic without checking the current amendment of IS 10500.

Related topics

Engineering geology · Geophysical exploration · Natural hazards · Sedimentary petrology and depositional environments · Geochemistry and water-rock interaction · Remote sensing and GIS in geology