Hydrogeology

Hydrogeological conditions mean how groundwater occurs and moves in an area: aquifer type, depth of the water table, yield and quality. The following geological factors control them.

1. Lithology: Gravel and sand have high porosity and permeability and form good aquifers; clay, shale and unfractured crystalline rocks hold water but yield little (aquicludes or aquitards).
2. Stratigraphy and sequence of beds: Alternation of permeable and impermeable layers produces confined (artesian) and unconfined aquifers and perched water tables.
3. Geological structure:
   • Folds: synclines store water and give artesian conditions; anticlines may shed water.
   • Faults: may act as conduits (broken zone) or barriers (fault gouge) and give springs.
   • Joints and fractures give secondary porosity in hard rocks.
   • Dip: decides direction of water flow and recharge area.
4. Weathering: weathered zones add porosity and permeability near the surface.
5. Solution features: limestone with caves and channels gives karst aquifers.
6. Topography and geomorphology: valleys, plains and terraces control recharge and discharge; flat plains favour deep aquifers.
7. Recharge conditions: rainfall, rivers, snow melt, infiltration through soil.
8. Igneous and metamorphic bodies: dykes and sills act as barriers, forming springs.
9. Sediment origin and depositional environment: alluvial fans and river terraces give coarse recharge zones and fine-grained basin centres.

Together these factors decide whether water is stored in unconfined, confined, perched or fractured-rock aquifers.

Darcy's law states that the rate of flow of water through a saturated porous medium is directly proportional to the hydraulic gradient and to the cross-sectional area of flow.

$$Q = K\,A\,i = K\,A\,\frac{\Delta h}{L}$$

where $Q$ = discharge ($m^3/s$), $K$ = hydraulic conductivity ($m/s$), $A$ = total cross-sectional area ($m^2$), $i = \Delta h / L$ = hydraulic gradient (dimensionless), $\Delta h$ = head loss over flow length $L$.

Discharge velocity $v = Q/A = K\,i$. The actual seepage velocity is $v_s = v/n$, where $n$ is porosity.

Relation of gradient and conductivity

For a given gradient, a higher $K$ gives larger flow. Water moves from high head to low head; the gradient is the driving force, and $K$ (depending on grain size, sorting, fractures and fluid viscosity) is the ease of flow. $K$ is large for gravel ($10^{-2}$ m/s), small for clay ($10^{-9}$ m/s).

Describing groundwater movement

Groundwater moves slowly, in laminar flow, from areas of high hydraulic head (recharge) to low head (discharge) in the direction of the gradient. Darcy's law is valid for laminar flow (Reynolds number below about 1-10).

Example (checked by calculation)

$K = 2\times10^{-4}$ m/s, head difference 5 m over 50 m, flow area 20 $m^2$:

$$\begin{aligned} i &= \frac{5}{50} = 0.1 \\ Q &= 2\times10^{-4}\times 20\times 0.1 = 4\times10^{-4}\ m^3/s \end{aligned}$$

Answer: $Q = 4\times10^{-4}\ m^3/s$ ($\approx 34.6\ m^3$/day).

Importance

• Estimating groundwater yield of wells and aquifers.
• Seepage under dams and from reservoirs and canals.
• Design of dewatering and drainage systems.
• Calculating transmissivity ($T = K b$) and aquifer testing.
• Groundwater contamination and flow modelling.

An aquifer is a saturated geological formation that stores and transmits useful quantities of water. In Nepal, aquifer systems differ by physiographic region.

1. Terai (Indo-Gangetic plain)

• Made of thick Quaternary alluvium (gravel, sand, silt, clay), hundreds of metres thick.
• Northern Terai (Bhabar zone): coarse boulder-gravel at the foot of the Churia; very high permeability; unconfined aquifer with deep water table; main recharge zone.
• Southern Terai: alternating sand and clay layers; shallow unconfined aquifer (up to about 30-50 m) and deep confined (artesian) aquifers, with flowing wells in places.
• Highest groundwater potential; shallow tube wells and deep tube wells widely used. Arsenic problem in shallow aquifers.

2. Hills (Siwalik, Mahabharat and Middle Mountains)

• Rock aquifers: fractured and weathered phyllite, schist, quartzite, limestone and Siwalik sandstone.
• Groundwater occurs in joints, fault zones, weathered layer and colluvial/river terrace deposits.
• Yield is low and variable; springs are the main source. Intermontane basins (Kathmandu, Pokhara) have thick lacustrine and fluvial sediments with good confined and unconfined aquifers.

3. High mountains

• Crystalline rocks (gneiss, granite) and glacial and moraine deposits.
• Groundwater only in fractures and glacial debris; snow melt and glaciers are the main sources; springs on slopes. Potential is small and poorly known.

 Unconfined             Confined
 ~~~~ water table      Piezometric surface ......
 ////// sand ////      -------------------------
 //////////////        ====== clay (aquiclude) =
 ==== clay =====        ///// sand aquifer /////
                       ====== clay (aquiclude) =

Groundwater moves slowly through the pores and fractures of rock and soil from areas of high hydraulic head to areas of low hydraulic head.

Factors affecting groundwater movement

1. Porosity and permeability: interconnected pores and fractures allow flow; clay has high porosity but low permeability.
2. Grain size and sorting: coarse, well-sorted material gives high flow.
3. Hydraulic gradient: greater slope of the water table or piezometric surface gives faster flow.
4. Geological structure: joints, faults, folds, dip and bedding control direction of flow.
5. Fluid properties: viscosity and temperature (warm water moves faster).
6. Recharge and discharge: rainfall, river seepage and pumping change the head.
7. Topography and vegetation.
8. Presence of impermeable layers forming barriers or confining beds.

Process of movement

1. Rain or snow melt infiltrates through the zone of aeration (vadose zone) to the saturated zone (recharge).
2. Water joins the water table and flows under gravity and pressure along the hydraulic gradient.
3. Flow is laminar and obeys Darcy's law, $v = K\,i$.
4. Flow is from recharge areas (highlands) to discharge areas (springs, rivers, lakes, wells, sea).
5. Rates are very slow, from a few cm per day in fine sand to metres per day in gravel.

An aquifer is a saturated geological formation (rock or sediment) that is sufficiently porous and permeable to store and transmit usable quantities of groundwater to wells and springs.

Examples: gravel, sand, fractured sandstone, cavernous limestone, fractured basalt.

Characteristics

• High porosity and, especially, high permeability.
• Gives economic yield to wells and springs.
• Has a recharge area and a discharge area.

Types

• Unconfined aquifer with a free water table.
• Confined (artesian) aquifer between impermeable beds, water under pressure.
• Semi-confined (leaky) aquifer and perched aquifer (small water body above an impermeable lens).

A confining bed is a geological layer of low permeability that is adjacent to an aquifer and restricts movement of water into or out of it.

Types

1. Aquiclude: saturated but practically impermeable layer that stores water but cannot transmit it (e.g. clay, unfractured shale).
2. Aquitard: layer of low permeability that transmits water slowly; it leaks into adjacent aquifers (e.g. silty clay, sandy clay). Gives a leaky aquifer.
3. Aquifuge: a rock that neither contains nor transmits water (solid massive granite or unfractured gneiss).

Characteristics

• Low hydraulic conductivity ($10^{-9}$ m/s or less for clay).
• Fine grained, often high porosity but small pores.
• Controls water pressure and creates confined (artesian) conditions.
• Protects deeper aquifers from contamination.
• Acts as a barrier in dam foundations and reservoir basins, making them favourable for water retention.

Aquifers are classified by the position of the water surface and the confining beds.

1. Unconfined (water-table) aquifer

The upper surface is the water table, free to rise and fall with recharge. Water is at atmospheric pressure.

2. Confined (artesian) aquifer

Sandwiched between two impermeable beds. Water is under pressure, so it rises above the aquifer top in wells; if the piezometric level is above ground, wells flow freely (artesian well).

3. Semi-confined (leaky) aquifer

Overlain or underlain by an aquitard through which water leaks slowly.

4. Perched aquifer

A small unconfined water body held above the main water table by a local impermeable lens.

 Recharge        Piezometric surface
  area   ..........................
 ///   _____________________  ground
 /// ==== clay (aquiclude) ========
 ///   ~~~ sand aquifer (confined) ~~~
 ///  ==== clay (aquiclude) ========
 ///  Unconfined: water table ~~~~~

Based on rock type, aquifers are also described as intergranular (alluvium), fractured-rock and karst (limestone) aquifers.

Terai (plain) aquifers

1. Thick alluvium brought by the rivers from the Himalaya: gravel and sand (coarse, permeable) alternate with silt and clay (impermeable).
2. Slope change: at the foot of the Churia range, coarse boulders form the Bhabar zone, which is the recharge area; to the south, finer sediments form confining clay beds, producing confined aquifers.
3. Recharge from heavy monsoon rainfall, rivers, canals and irrigation fields.
4. Basin subsidence gives great sediment thickness (hundreds of metres), allowing multiple aquifer layers.
5. Shallow unconfined aquifer in the upper sand, and deep confined aquifers under the clay layers.

Mountain aquifers

1. Rock type: metamorphic and igneous rocks have little primary porosity; water occurs in fractures, joints, fault zones and weathered layer.
2. Structure: folds, faults and foliation control springs and groundwater paths; permeable and impermeable layers produce contact springs.
3. Surficial deposits: colluvium, landslide debris, river terraces, moraines and glacial deposits store water.
4. Intermontane basins (Kathmandu, Pokhara) have thick lake and river sediments forming confined and unconfined aquifers.
5. Recharge from rain, snow melt and glaciers; steep slopes give rapid run-off and limited recharge.
6. Limestone areas give karst aquifers with caves and big springs.

River channel morphology is the form and shape of a river channel (plan form, cross-section and long profile) produced by the interaction of flow, sediment load, slope, bed and bank material.

Types of river channels (by plan form)

1. Straight channel: rare and short; small sinuosity; thalweg meanders inside the channel; found in steep, rocky reaches.
2. Meandering channel: single, sinuous channel with bends; erosion on the outer (concave) bank, deposition on the inner (convex) bank as point bars; gentle slope, fine-grained load; may form oxbow lakes. Common in the Terai.
3. Braided channel: many channels dividing and reuniting around bars and islands; steep gradient, coarse bed load, fluctuating discharge and erodible banks; unstable. Typical of Himalayan rivers at the foot of the hills (Koshi, Narayani).
4. Anastomosing channel: multiple stable channels separated by vegetated islands; low gradient.

Other classifications

• By stage: youthful (V-shaped, rapids, vertical erosion), mature (wider valley, meanders), old (wide flood plain).
• By stability: alluvial vs bedrock rivers.

Engineering significance

Channel shifting, bank erosion and aggradation affect bridge location, intake, embankments and settlement; stable straight reaches are preferred for bridges.

Engineering significance of groundwater

1. Water supply for drinking, irrigation and industry.
2. Foundation and excavation: high water table lowers bearing capacity and requires dewatering.
3. Slope stability: pore pressure reduces shear strength and triggers landslides.
4. Tunnels: water inflow and flooding; need drainage and grouting.
5. Dams and reservoirs: seepage, uplift pressure, leakage and piping.
6. Roads: poor drainage damages subgrade and pavement.
7. Chemical effects: sulphate and acidic water attack concrete and steel.
8. Settlement: pumping lowers water table, causing subsidence.
9. Springs are the main water source in the hills.

Water potential zones of Nepal

An artesian well is formed when a permeable aquifer is confined between two impermeable beds and tilted so that its recharge area lies at a higher level than the well.

Conditions:

1. Inclined or synclinal (basin-shaped) permeable bed such as sandstone or sand.
2. Impermeable beds (clay, shale) above and below.
3. Outcrop of the aquifer at higher elevation where rain or river water enters (recharge).
4. Water flows down-dip and gets pressure from the weight of water in the higher part.

When a well is drilled through the upper confining bed, water rises above the aquifer top because of the pressure. If the piezometric surface is above the ground level, water flows out freely without pumping.

 recharge area (outcrop)       flowing well
 high ground                       |  ^ ^
   ___.........piezometric surface.|..
  /   ___________clay_____________ | __ ground
 /   ===========sand aquifer======|====
/    ___________clay______________

Porosity

Porosity ($n$) is the ratio of the volume of voids to the total volume of rock or soil, usually in %.

$$n = \frac{V_v}{V}\times 100$$

• Primary porosity: pores between grains formed at deposition (sand, gravel).
• Secondary porosity: joints, fractures, solution cavities developed later.
• Depends on shape, packing and sorting; well-sorted rounded sand 30-40%, clay up to 50-60%, granite below 1%.
• Porosity measures storage capacity, not the ability to transmit water.

Permeability

Permeability is the ability of a rock or soil to transmit fluid through its interconnected pores. It is expressed as hydraulic conductivity $K$ (m/s) from Darcy's law, $Q = K A i$.

• Depends on pore size, connectivity, grain size, fractures and fluid viscosity.
• Gravel $10^{-2}$ m/s, sand $10^{-4}$ m/s, silt $10^{-6}$ m/s, clay $10^{-9}$ m/s.

Relation

A rock can be porous but impermeable (clay) because the pores are small and not connected. Good aquifers need both high porosity and high permeability.

Importance

Porosity and permeability control groundwater storage and yield, seepage under dams, reservoir leakage, drainage of slopes and foundation behaviour.

An aquiclude is a saturated geological formation that contains water but is so impermeable that it cannot transmit it in useful quantity to wells or springs.

• Typical examples are clay, unfractured shale, and silty clay layers. Clay may have high porosity (40-60%) but very low hydraulic conductivity ($K<10^{-9}$ m/s).
• It forms the upper and lower boundary of confined (artesian) aquifers, so water stays under pressure.
• Acts as a natural barrier against contamination and vertical movement of water.
• Differs from an aquitard (low permeability, but transmits water slowly) and an aquifuge (neither contains nor transmits water).

Engineering importance

• Good as foundation of reservoirs and ponds, since it prevents leakage.
• Clay is used as impervious core in earth dams.
• Perched water tables form over aquicludes, causing landslide and slope instability.
• Aquicludes give weak, compressible foundations and may swell.