Groundwater is the primary water source for 60% of India's irrigation and 80% of rural drinking water. India is the world's largest groundwater user, extracting over 250 billion cubic metres annually. Civil engineers involved in water supply, irrigation, dewatering, and foundation design must understand groundwater behaviour.
Hydrological Cycle and Groundwater Recharge
Groundwater originates from precipitation infiltrating through the soil and percolating to the saturated zone. Factors affecting recharge:
- Rainfall intensity and duration (intense rain runs off, gentle rain infiltrates)
- Soil type: Sandy loam (high permeability) vs clay (low permeability)
- Land use: Forest (high recharge), urban impervious (low recharge)
- Topography: Flat plains (high recharge), steep hills (low recharge)
Aquifer Types and Properties
| Type | Definition | Water Level | Example in India |
|---|---|---|---|
| Unconfined Aquifer | Upper boundary is water table (free surface) | Water table at atmospheric pressure | Alluvial plains, Ganga basin |
| Confined Aquifer | Bounded above and below by impermeable layers (aquitards) | Artesian (above top of aquifer) | Deccan basalt aquifers |
| Semi-confined (Leaky) | Bounded by one or two aquitards with vertical leakage | Between confined and unconfined | Multi-layer alluvial formations |
| Perched Aquifer | Localised, above main water table, on impermeable lens | Above regional water table | Hill slopes, laterite formations |
Darcy's Law
The fundamental equation governing flow through porous media (Henry Darcy, 1856):
Q = K × i × A
or in terms of velocity:
v = K × i = K × (dh/dl)
- Q = discharge (m³/day)
- K = hydraulic conductivity (permeability) (m/day)
- i = hydraulic gradient = head loss / length
- A = cross-sectional area (m²)
- v = Darcy (seepage) velocity (not actual pore velocity)
Darcy's law is valid for laminar flow (Re < 1–10). In coarse gravel or karst, turbulent flow requires Forchheimer equation.
Hydraulic Conductivity (K) Values
| Material | K (m/day) | Classification |
|---|---|---|
| Gravel | 100–10,000 | High permeability |
| Coarse sand | 10–100 | Moderate–high |
| Medium sand | 1–10 | Moderate |
| Fine sand | 0.1–1 | Low–moderate |
| Silty sand | 0.01–0.1 | Low |
| Silt | 0.001–0.01 | Very low |
| Clay | < 0.0001 | Practically impermeable |
| Fractured rock | 0.1–100 | Highly variable |
Transmissivity and Storage Coefficient
Transmissivity (T): Rate of flow through a unit width of aquifer under unit hydraulic gradient:
T = K × b (m²/day)
where b = saturated aquifer thickness. Good production aquifers: T > 100 m²/day.
Storage Coefficient (S): Volume of water released per unit horizontal area per unit decline in head:
- Unconfined aquifer: S ≈ specific yield (Sy) = 0.05–0.35 (water drained by gravity)
- Confined aquifer: S = 10⁻⁵ to 10⁻³ (water from elastic compression and expansion of water)
Well Hydraulics — Theis Equation (Non-Steady State)
When a well pumps at rate Q from a confined aquifer, drawdown s at distance r from well at time t:
s = (Q / 4πT) × W(u)
where u = r²S / (4Tt) and W(u) is the Theis well function (exponential integral).
For small u (large T or large t): W(u) ≈ −0.5772 − ln(u) (Cooper-Jacob approximation)
s ≈ (Q / 4πT) × (−0.5772 − ln(r²S / 4Tt))
= (2.303Q / 4πT) × log(2.25Tt / r²S)
Cooper-Jacob Straight-Line Method
If u < 0.05, plot drawdown s vs log(t). The slope of the straight line:
Δs = 2.303Q / (4πT) → T = 2.303Q / (4π × Δs)
where Δs = drawdown per log cycle of time.
Storage coefficient from time-intercept (t₀ where s = 0):
S = 2.25 × T × t₀ / r²
Equilibrium (Thiem) Equation for Steady State
For unconfined aquifer, pumping with two observation wells at r1 and r2:
Q = π × K × (h₂² − h₁²) / ln(r₂/r₁)
For confined: Q = 2π × T × (h₂ − h₁) / ln(r₂/r₁)
Radius of Influence
Sichardt's empirical formula: R = C × s × √K
where C = 3000 (for ordinary wells), s = drawdown, K = hydraulic conductivity
Or from Theis: R = 1.5 × √(Tt/S)
Pumping Test Design
Step-drawdown test procedure:
- Pump at Q1 (low rate) for 30–60 min; record drawdown at pumping well and 2 observation wells
- Increase to Q2 for same duration
- Increase to Q3 (maximum rate) and continue
- Plot specific capacity (Q/s) vs Q → extrapolate to design Q
- Follow with recovery test (pump off) to confirm T and S from rising water levels
IS 2800 Part 1: Well log and construction; Part 2: Test pumping of wells
Groundwater Quality Issues in India
| Contaminant | States Affected | Health Impact | IS 10500 Limit |
|---|---|---|---|
| Fluoride | Rajasthan, AP, Telangana, Gujarat | Fluorosis (bone, dental) | 1.0 mg/L (max 1.5) |
| Arsenic | West Bengal, Bihar, UP, Assam | Skin cancer, arsenicosis | 0.01 mg/L |
| Nitrate | Punjab, Haryana (agricultural areas) | Blue baby disease (infants) | 45 mg/L |
| Iron | WB, Jharkhand, Odisha, Assam | Staining, taste, pipe corrosion | 0.3 mg/L |
| TDS | Coastal/arid zones | Taste, corrosion | 500 mg/L (max 2000) |
Frequently Asked Questions
What is the difference between an artesian well and a tube well?
An artesian well taps a confined aquifer where hydraulic pressure (artesian pressure) is high enough to make water rise above the aquifer top — if pressure is sufficient, water flows to the surface without pumping (flowing artesian well). A tube well (borewell) taps any aquifer (usually unconfined) using a submersible pump; water does not flow naturally. India has artesian zones in coastal alluvial plains and Rajasthan.
Why is specific yield much larger than storage coefficient for confined aquifers?
In unconfined aquifers, water release involves actual drainage of pores under gravity (specific yield = 5–30%). In confined aquifers, no gravity drainage occurs — water is released only by elastic compression of the aquifer skeleton and expansion of compressed water, yielding only 10⁻⁵ to 10⁻³. This is why confined aquifer yields decline rapidly with pumping — storage is much smaller.