Environmental Engineering – Sources of Water & Ground-Water Development

Population Forecasting (Recap)

• Six additional/alternative methods to the basic arithmetic & geometric increase:

  • Demographic (Component) Method

  • Uses city-specific vital statistics and migration.

  • $P<em>{n}=P</em>{0}+(B.R-D.R\pm M.R)\times n$

    • $P_{n}$ → population after $n$ years.

    • $P_{0}$ → last recorded population.

    • $B.R$, $D.R$, $M.R$ expressed as persons ∕ year.

  • Simple Graphical Method

  • Plot available census data (population vs. time), draw a smooth curve, extend by judgement.

  • Highly approximate; depends on designer’s insight.

  • Comparative (Ratio) Graphical Method

  • Select 3–5 cities that historically behaved like the subject city.

  • Plot each city’s growth; superimpose years; extend composite curve; deduce unknown future value.

  • Master-Plan Method

  • City divided into land-use zones; adopt standard permissible densities (e.g., $400\, \text{persons/ha}$ residential).

  • Ratio / Apportionment Method

  • $r<em>{t}=\frac{P</em>{\text{local},t}}{P_{\text{national},t}}$ is assumed to vary smoothly.

  • Forecast national population first, then $P<em>{\text{local},t}=r</em>{t}\,P_{\text{national},t}$.

  • GOI Recommendation

  • For Indian conditions, Geometric-Increase Method is generally preferred.

Sources of Water

• Ultimate origin: precipitation (rain/snow) ➜ runoff ➜ surface storage ➜ infiltration ➜ sea.

• Two broad classes:

  • Surface Sources

  1. Ponds & Lakes

  2. Streams & Rivers

  3. Storage Reservoirs (artificial)

  4. Oceans (desalination not yet common for municipal supply in India)

  • Sub-Surface (Underground) Sources

  1. Springs

  2. Infiltration Galleries

  3. Infiltration Wells

  4. Wells / Tube-wells

Ponds & Lakes

• Natural depressions, stagnant water body.

• Pros: usually good quality; large, old lakes exhibit self-purification.

• Cons: algal/weed growth → taste, odour, colour problems.

• Quantitatively small; best for small towns & hill areas.

Streams & Rivers

• Streams: small, non-perennial → only villages.

• Rivers: prime source for cities.

  • Perennial: fed by rain & snow; can be used directly.

  • Non-Perennial: seasonal; need storage dam.

• Water quality issues: high turbidity, silt load, sewage contamination; MUST be treated.

Storage Reservoirs (Dams)

• Man-made lake to smoothen river flow.

• Multipurpose: water supply, irrigation, hydropower, flood moderation.

• Water quality ~ natural lake.

Factors for Selecting a Source

1. Quantity – must satisfy design demand for whole design period; else import distant source.

2. Quality – wholesome, non-toxic, treatable by economical means.

3. Distance – nearer ⇒ shorter conveyance ⇒ lower capital cost.

4. Topography – flat preferred; rugged terrain ↑ cost (tunnels, siphons).

5. Elevation – source higher than city enables gravity flow (low O&M); lower elevation needs pumping.

Development of Ground Water

• Formation: rainfall infiltrates, percolates via interconnected pores ➜ accumulates on impervious strata → water table.

• Balance governed by:

  1. Geological properties (porosity $n$ & permeability $K$).

  2. Recharge rate (infiltration).

  3. Discharge rate (evaporation, transpiration, seepage, abstraction).

Fundamental Soil Properties

• Porosity $n(\%)=\frac{V_{v}}{V}\times100$

  • $V_{v}$ → volume of voids; $V$ → total soil volume.

• Permeability (Hydraulic Conductivity) $K$ from Darcy’s law:

  • $Q=K\,A\,i \quad\Rightarrow\quad K=\frac{Q}{A\,i}$ (at $20^{\circ}\text{C}$).

• Typical values

  • Granite $n\approx1.5\%$, $K\approx0.6\times10^{-5}\,\text{cm/s}$.

  • Sand $n\approx35\%$, $K\approx0.04\,\text{cm/s}$.

  • Gravel $n\approx25\%$, $K\approx0.4!\text{–}!4.0\,\text{cm/s}$.

  • Clay $n\approx45\%$, $K\approx4\times10^{-6}\,\text{cm/s}$.

Zones of Ground Water (Vertical Stratification)

1. Zone of Rock Flowage

   • Great depth; plastic deformation; contains ‘internal water’; not exploitable.

2. Zone of Rock Fracture (engineering interest)

   • Extends ground surface → up to ~$100!\text{–}!1000\,\text{m}$.

   • Sub-divided by the Water Table (W.T.) into:

     • Zone of Saturation (Saturated Zone; $S=1$): all voids full of water; source for wells.

     • Zone of Aeration (Vadose Zone; S<1): partially saturated.

     1. Soil Zone (root zone)

     2. Intermediate Zone

     3. Capillary Fringe (just above W.T.; $S=1$ but negative pressure)

Ground Water Yield Concepts

• Specific Yield $S<em>{y}=\frac{V</em>{d}}{V}\times100$ where $V_{d}$ is drainable water volume.

• Specific Retention $S<em>{r}=\frac{V</em>{r}}{V}\times100$ – water retained against gravity.

• Relationship: $S<em>{y}+S</em>{r}=n$ (porosity).

• Typical ranges (%, cm/s):

  • Clay: $n=45!–!55$, $K=10^{-5}!–!10^{-9}$, $S_{y}=1!–!10$.

  • Sand mixed: $n=20!–!35$, $K=0.005!–!0.01$, $S_{y}=15!–!30$.

Hydro-geological Units

• Aquifer: porous + permeable; yields useful water (e.g., sand & gravel).

• Aquifuge: neither porous nor permeable (e.g., massive granite).

• Aquiclude: porous but essentially impermeable (e.g., clay, shale).

• Aquitard: porous, low permeability; yields negligible water (e.g., sandy clay).

Types of Aquifers

1. Confined (Artesian) Aquifer

   • Bound above & below by aquicludes; water under pressure.

   • Piezometric surface may rise above ground → flowing artesian well.

2. Unconfined (Water-table) Aquifer

   • Upper surface is the water table; water not under pressure.

3. Perched Aquifer

   • Localised saturated body above general water table, resting on a pocket of impervious material.

Forms of Underground Sources & Their Exploitation

1. Infiltration Galleries (horizontal wells)

   • Shallow (3–5 m) tunnels parallel to river; size ~1 m × 2 m; length 10–100 m.

   • Fed by lateral perforated pipes.

2. Infiltration Wells

   • Series of shallow wells along river bank; connected via porous pipes to a jack/sump well.

3. Springs (natural exfiltration)

   • Gravity Spring – overflow of water table into valley.

   • Surface Spring – inclined impervious layer lifts water table to surface.

   • Artesian Spring – confined aquifer intersects surface; yield nearly constant.

4. Wells & Tube-wells

   • Open (Dug) Wells: dia 2–9 m, depth <20 m, discharge ≈ $18\,\text{m}^{3}! /!h$ .

     • Concepts: depression head, cone of depression, radius of influence.

   • Tube-wells

     • Cavity Type – spherical cavity at tip; suitable for fine sand.

     • Screen Type – radial entry through screens:
       • Strainer Tube-well (pre-packed screens + gravel)
       • Slotted Pipe Gravel-Packed Tube-well (gravel envelope 10–20 mm thick)

Practical / Ethical / Design Implications

• Selection of source & method must ensure sustainability, economic viability, and protection of ecosystems.

• Over-abstraction from aquifers can cause: land subsidence, reduced base-flow to rivers, water-quality deterioration (saline ingress).

• Raw river water must be analysed for industrial discharges & sewage to safeguard public health.

• Master-plan density controls integrate water-supply planning with urban zoning, promoting equitable and resilient growth.