← GW Home
Chapter 9

Groundwater Development in Nepal [3 Hours]

GW in Nepal ~8+ Past Questions

MOST IMPORTANT TOPICS & MARKS DISTRIBUTION

High-Yield Topics (by exam frequency):

  1. Kathmandu Valley aquifer system — geology, aquifer layers — Asked in 2073, 2072 Ashwin, 2071 [4–6 marks]
  2. Numericals: groundwater storage calculation, well-field planning — Asked in 2072 Ashwin, 2072 Magh [8 marks]
  3. Terai belt GW development — occurrence, shallow/deep aquifers — Asked in 2072 Magh [6 marks]
  4. Urban GW exploitation issues — over-exploitation, mitigation — Asked in 2078 [6 marks]

Marks Each Time Asked:

ExamQuestionsTotal
2078 ChaitraQ8 [6]6
2073 BhadraQ10 [4]4
2072 AshwinQ3 [8 num], Q8 [6]14
2072 MaghQ3 [8 num], Q10 [6]14
2071 BhadraQ11 [6]6

CHAPTER ROADMAP

  1. Introduction — physiographic overview
  2. Terai belt — groundwater occurrence and development
  3. Kathmandu Valley aquifer system
  4. Middle hills and high mountains
  5. Urban groundwater exploitation — issues and mitigation (Butwal)
  6. Groundwater storage and well-field planning (numerical)
  7. Past question bank for Chapter 9
  8. Chapter summary and exam key points

9.1 INTRODUCTION — PHYSIOGRAPHIC OVERVIEW

Nepal can be divided into five major physiographic zones from south to north, each having distinct groundwater characteristics:

Zone Elevation Lithology GW Potential
Terai (Southern Plain) 60–300 m Alluvial sand, gravel, clay (Quaternary) Very High — most productive aquifers
Siwalik (Churia) Hills 300–1,500 m Sandstone, mudstone, conglomerate Moderate — fractured aquifers, springs
Middle Hills (Mahabharat) 1,500–2,700 m Phyllite, schist, quartzite, gneiss Low to moderate — spring-fed
Intermontane Valleys Variable Lacustrine sediments (clay, silt, sand, gravel) Moderate to high (e.g., Kathmandu, Pokhara)
High Himalayas > 4,000 m Crystalline rocks, glacial deposits Very low — limited springs
Schematic Cross-Section: Nepal — Groundwater Zones (S to N) TERAI Alluvial aquifers GW: VERY HIGH SIWALIK Moderate MIDDLE HILLS Low–Moderate KTM Valley HIGH HIMALAYAS Very Low S (India border) N (Tibet/China)
Figure 9.1: Schematic physiographic cross-section of Nepal showing relative groundwater potential in each zone.
Key Exam Point: Nepal’s groundwater is overwhelmingly concentrated in two settings: (1) the Terai alluvial plain and (2) intermontane valleys like Kathmandu and Pokhara. Together these account for >90% of the nation’s extractable groundwater.

9.2 TERAI BELT — GROUNDWATER OCCURRENCE AND DEVELOPMENT

PAST EXAM QUESTION ADDRESSED:

2072 Magh Q10 [6]: Discuss the groundwater development possibilities of the Terai belt.

9.2.1 Geological Setting

The Terai is a narrow strip of flat alluvial plain, 20–50 km wide, along the southern border of Nepal. It is the northern extension of the Indo-Gangetic Plain. The sediments are Quaternary alluvium deposited by rivers originating in the Himalayas.

Nepal — Geological Zones (North to South) High Himalayas Gneiss, schist Limited GW Springs only Middle Hills Phyllite, quartzite Fractured aquifers Springs + dug wells Siwalik Hills Sandstone, siltstone Moderate GW Fractured + alluvial Terai Plain Alluvial sand, gravel, clay BEST aquifers in Nepal Tube wells, T = 500-5000 m²/d ↑ Elevation (N) ↓ Elevation (S) Main Boundary Thrust (MBT) separates Middle Hills from Siwalik ~80% of Nepal's GW potential lies in the Terai alluvial aquifer system Terai aquifer thickness: 100-600 m | Replenished by rivers from Himalayas GW development in hills is limited — mainly springs and shallow dug wells
Figure 9.2b: Nepal's geological zones from north to south — groundwater potential increases dramatically in the southern Terai alluvial plains.

The typical geological succession from top to bottom is:

  • Topsoil: 1–3 m of clayey/silty soil (low permeability)
  • Shallow aquifer (unconfined): 10–30 m of fine to medium sand — most widely used for hand pumps and shallow tube wells
  • Clay/silt aquitard: Acts as a confining layer
  • Deep aquifer (confined/semi-confined): Coarse sand and gravel — high-yield aquifer for deep tube wells. Total thickness increases from 50–100 m near the foothills to 500 m near the Indian border.

Hydrological Zones of the Terai (North to South)

ZoneLocationMaterialsTransmissivityKey Feature
Bhabar Zone Foothills of Siwalik range Boulders, cobbles, pebbles (coarse, porous) > 5,000 m²/day Primary natural recharge zone for the entire Terai aquifer; deep water table; high infiltration
Central Terai Middle Terai (Nawalparasi, Kapilvastu, Rupandehi, Dang, Banke) Thickest sand and gravel with silt 2,000–6,000 m²/day Most productive zone; lateral recharge from Bhabar + river seepage
Southern Terai Border area, especially eastern side Finer sediments, silt and clay interlayers 500–2,000 m²/day Shallow water table; low infiltration; recharge from river losses and flooding

9.2.2 Aquifer Characteristics

Parameter Shallow Aquifer Deep Aquifer
Type Unconfined to semi-confined Confined (artesian in places)
Depth range 5–30 m 50–500 m (increases toward Indian border)
Material Fine to medium sand Coarse sand and gravel
Transmissivity (T) 100–500 m²/day 500–6,000 m²/day (varies by zone)
Storage coefficient Sy = 0.05–0.20 S = 10−4–10−3
Well yield 5–20 m³/hr 30–100+ m³/hr
Water quality Generally good; arsenic risk in some areas Good; low iron in most areas
Primary use Domestic supply (hand pumps) Irrigation, municipal supply
Terai Aquifer System — Layered Structure Surface soil / top clay (3-10 m) Shallow Aquifer (sand/gravel) — 10-60 m Clay/silt confining layer (aquitard) Deep Aquifer (gravel/sand) — 60-300+ m Bedrock / basement STW DTW STW (Shallow Tube Well): <60 m, unconfined/semi-confined DTW (Deep Tube Well): 60-300+ m, confined, artesian in some areas Arsenic contamination common in shallow aquifers (<50 m) of inner Terai
Figure 9.2c: Terai aquifer system — shallow (unconfined) and deep (confined) aquifers separated by clay aquitards, with arsenic risk in shallow zones.

9.2.3 Groundwater Development Status

  • Over 40,000 shallow tube wells and hand pumps installed for community drinking water supply
  • Over 10,000 deep tube wells for irrigation, particularly in the Central and Eastern Terai
  • The Terai contains approximately 70–80% of Nepal’s total groundwater resources
  • Only about 20–25% of available groundwater has been developed so far — significant potential remains
GW Development in Nepal — Timeline 1960s First tube wells UNDP survey 1970s GW exploration GWRDB formed 1980-90s STW expansion Irrigation DTWs 2000s+ Arsenic testing Quality focus Current Status ~750,000+ STWs | ~1,500 DTWs | 70%+ Terai irrigation from GW GWRDB, DoI, KUKL are key GW development agencies Shift from "quantity-first" to "quantity + quality + sustainability"
Figure 9.2d: Timeline of groundwater development in Nepal — from initial UNDP surveys in the 1960s to the current focus on quality and sustainability.

9.2.4 Development Possibilities

  1. Irrigation expansion: As of 2022, only 35% of the total irrigation in Nepal is contributed by groundwater through tube wells and springs. Large-scale deep tube well irrigation can be expanded in the western Terai where surface water coverage is insufficient. Installation of eco-friendly solar irrigation systems with provisions of recharge ponds and rainwater harvesting structures can enhance crop yield and productivity.
  2. Conjunctive use: Combining surface water (monsoon) and groundwater (dry season) for year-round agriculture. Seasonal switching — using surface water in wet periods and groundwater in dry periods — is the most practical form. Surplus monsoon water can also be used for artificial recharge into the aquifer.
  3. Urban water supply: Deep wells for rapidly growing Terai cities (Birgunj, Biratnagar, Nepalgunj, Butwal, Bharatpur)
  4. Industrial use: Groundwater for agro-industries, dairy, and manufacturing in industrial corridors
  5. Recharge augmentation: Managed aquifer recharge (MAR) using monsoon surplus to sustain long-term yields
  6. Tourism and cultural value of springs: Springs are a source of recreation and natural beauty, possessing the ability to attract local and international tourists, which helps enhance the national economy.
  7. Resilient seasonal reservoirs: Aquifers, especially in areas with high recharge potential (e.g., northern Kathmandu Valley with coarse-grained sediments), can act as seasonal water-bearing reservoirs. Their major advantage over surface reservoirs is less vulnerability to evaporation and less contamination from climate extremes.
GW Development Possibilities by Region Terai Highest potential STW: irrigation DTW: municipal supply Available: ~12 BCM/yr Used: ~3-4 BCM/yr Still < 30% developed Inner Terai / Dun Moderate potential Alluvial valleys STW/DTW feasible Small-scale irrigation Arsenic in some areas ~20% developed Hills & Mountains Low potential Springs: primary source Limited bore wells Karst where limestone Seasonal variability Focus on spring mgmt Nepal's total GW potential: ~12 BCM/yr | Only 25-30% currently developed Sustainable development requires zoning, monitoring, and demand management
Figure 9.2e: Groundwater development possibilities by region — Terai has highest potential but is still less than 30% developed.

9.2.5 Key Roles of the Terai Aquifer

  • Drinking water supply: The majority of the Terai population relies on groundwater as the primary drinking water source through hand pumps, tube wells, dug wells, and boreholes. Groundwater is generally more stable and requires less treatment than surface water as soil and rock layers act as natural filters.
  • Agricultural production: The Terai is Nepal's agricultural heartland (the "breadbasket"). The aquifer system is crucial for sustaining irrigation, especially during the dry season when surface water flows are low.
  • Industrial use: Significant amounts of groundwater are used in cooling systems and for generating steam in boilers in industries.
  • Ecosystem support: Groundwater provides consistent base flow to rivers, lakes, and wetlands, especially during dry periods and droughts. This reliable water source is important for maintaining biodiversity and groundwater-dependent ecosystems.

9.2.6 Challenges

  • Arsenic contamination: Shallow aquifers in parts of the Central and Eastern Terai (Nawalparasi, Parsa, Bara, Rautahat) have arsenic > 50 μg/L. Nepal’s acceptable limit is 50 μg/L (ppb); however, WHO lowered the international guideline from 50 to 10 μg/L in 1993. According to Shrestha et al. (2004), 7.4% of samples exceeded 50 ppb and 23.7% exceeded 10 ppb. Arsenic levels generally decrease with depth; deeper tube wells were found to have levels below 10 μg/L (Smedley, 2005).
    Release mechanism (slide): Arsenic is naturally present in sediments and is released into groundwater through reductive dissolution of iron oxyhydroxides under anoxic (oxygen-deficient) conditions in the alluvial aquifer.
    Health effects (slide): Chronic exposure causes skin lesions (melanosis, keratosis), peripheral neuropathy, cardiovascular disease, and increased risk of internal cancers (bladder, lung, liver).
  • Iron and manganese: Iron and manganese in Terai groundwater come naturally from underground rocks and soil. In areas where water has little oxygen, these metals easily dissolve and mix into the water supply, causing aesthetic and health issues.
  • Nitrate contamination: A significant and growing problem in the shallow aquifers due to extensive use of fertilizers and inadequate sewage/wastewater management. Deep aquifers are generally less affected and naturally have low nitrate levels.
  • Over-extraction: Localized drawdown near urban centres due to unregulated pumping
  • Land subsidence: Potential risk if deep aquifer heads are excessively lowered
  • Lack of data: Insufficient monitoring wells and hydrogeological data for optimal management
  • Weak policies: Lack of groundwater licensing and monitoring systems, poor data availability, and insufficient research investment
Schematic Cross-Section: Terai Aquifer System GL Topsoil (clay/silt) 1–3 m SHALLOW AQUIFER (unconfined) — fine to medium sand WT Clay/Silt Aquitard (confining layer) DEEP AQUIFER (confined) — coarse sand & gravel Bedrock Hand pump Deep tube well Piez. 10–30 m 50–500 m
Figure 9.2: Schematic cross-section of the Terai aquifer system showing shallow and deep aquifers.

9.3 KATHMANDU VALLEY AQUIFER SYSTEM

PAST EXAM QUESTIONS ADDRESSED:

2073 Bhadra Q10 [4]: Discuss the aquifer system of Kathmandu Valley.

2072 Ashwin Q8 [6]: Discuss the aquifer system of Kathmandu Valley of Nepal.

2071 Bhadra Q11 [6]: Discuss, with the help of schematic zones, the availability and possibilities of groundwater resources in Kathmandu Valley.

9.3.1 Geological Background

The Kathmandu Valley is an intermontane basin (tectonic graben) approximately 25 km × 20 km in extent, filled with lacustrine and fluvial sediments of Pliocene–Quaternary age. The valley was once occupied by a lake (the ancient Paleo-Kathmandu Lake) that gradually drained, leaving behind thick sequences of clay, silt, sand, gravel, and peat.

9.3.2 Three-Layer Aquifer Model

The hydrogeology of the Kathmandu Valley is traditionally described as a three-layer system:

Kathmandu Valley — Three-Layer Aquifer System GL LAYER 1 — SHALLOW (unconfined) Sand, gravel, silt — 5–20 m depth — dug wells & shallow tube wells KALIMATI CLAY (Aquitard) ← Black clay, up to 200 m thick LAYER 2 — DEEP CONFINED AQUIFER Coarse sand & gravel (Pliocene) — 60–300 m depth — deep tube wells LAYER 3 — BASAL GRAVEL Boulder-cobble-gravel deposit at the base — highly productive where present BASEMENT ROCK (Phyllite, Quartzite, Gneiss) NORTHERN ZONE (recharge area) Coarse alluvial fan — high permeability CENTRAL/SOUTHERN ZONE Thick Kalimati clay — confined conditions Recharge from Shivapuri hills
Figure 9.3: Schematic three-layer aquifer model of the Kathmandu Valley. The Kalimati Clay (black clay) is the principal aquitard separating the shallow and deep systems.

(a) Layer 1 — Shallow Aquifer (Unconfined)

  • Comprised of recent alluvial deposits: sand, gravel, silt
  • Depth: 5–20 m below ground surface
  • Recharged by local precipitation, infiltration, and river leakage
  • Tapped by dug wells and shallow tube wells
  • Yield: 1–10 m³/hr (limited)
  • Water quality often degraded due to organic contamination, high ammonia, and bacteriological pollution in urban areas

(b) Kalimati Clay (Aquitard)

  • The Kalimati Formation is a thick layer (up to 200 m) of black, organic-rich, lacustrine clay deposited in the Paleo-Kathmandu Lake
  • Very low hydraulic conductivity — acts as the primary confining layer
  • Separates the shallow aquifer from the deep aquifer
  • Thickest in the central valley, thins towards the margins
  • Contains peat layers with high ammonia and methane — affects deep aquifer water quality through slow leakage

(c) Layer 2 — Deep Confined Aquifer

  • Comprised of Pliocene-age coarse sand, gravel, and boulder deposits
  • Depth: 60–300 m (varies across the valley)
  • Confined by the overlying Kalimati Clay — artesian conditions in places
  • Higher yield than the shallow aquifer: 10–50 m³/hr
  • Principal source for municipal water supply (KUKL/Melamchi supplement)
  • Faces declining water levels due to over-extraction

(d) Layer 3 — Basal Gravel

  • Boulder and cobble deposits at the base of the valley-fill, overlying the bedrock
  • Most productive aquifer where encountered — yields up to 100+ m³/hr
  • Not uniformly present — limited to paleochannel locations

9.3.3 Northern vs. Southern Groundwater Zones

Northern Groundwater Zone:
  • Alluvial fan deposits from rivers draining the Shivapuri range
  • Coarse material — high permeability
  • Main recharge zone for the valley’s deep aquifers
  • Water table is shallow; flows southward
  • Good quality, low contamination historically
Northern Terai (Bhabar) vs Southern Terai Bhabar Zone (North) • Coarse gravel/boulder deposits • WT very deep (15-30+ m) • High K, high recharge • Rivers disappear underground DTW needed (deep WT) Good quality water Width: 8-12 km Southern Terai • Fine sand/silt/clay layers • WT shallow (2-8 m) • Lower K, confined aquifers • Artesian conditions possible STW common (shallow WT) Arsenic risk in some areas Width: 20-30 km Bhabar → recharge zone | Southern Terai → discharge/utilization zone Bhabar protection is critical for sustaining the entire Terai aquifer system
Figure 9.3b: Comparison of Bhabar (northern) and southern Terai zones — contrasting aquifer materials, water table depths, and groundwater characteristics.
Central/Southern Zone:
  • Thick Kalimati Clay dominates the central valley floor
  • Deep aquifer is confined — artesian in Patan/Thimi
  • Limited direct recharge to the deep aquifer
  • Water quality issues: ammonia, iron, hardness
  • Over-pumping has caused 1–3 m/year water level decline

9.3.4 Current Issues

  1. Over-extraction: Abstraction exceeds recharge by 2–3 times. Deep aquifer heads have declined by 20–30 m since the 1980s.
  2. Water quality degradation: Ammonia (>1.5 mg/L in many deep wells), iron, coliform contamination, and arsenic in some deep wells (slide).
  3. Land subsidence: Reports of 2–5 cm/year subsidence in parts of inner Kathmandu, attributed to groundwater depletion compressing the Kalimati Clay.
  4. Unregulated extraction: Thousands of private tube wells without permits — no metering or monitoring.
  5. Spring discharge decline: Reduced spring flows in the surrounding hills due to lowered regional water table.
GW Issues & Challenges in Terai Arsenic WHO limit: 10 μg/L Nepal std: 50 μg/L Nawalparasi, Bara worst Iron/Manganese Fe > 0.3 mg/L common Taste, staining issues Aeration + filtration Declining WT Kathmandu: 2-3 m/yr Over-extraction Urbanization reducing R Contamination Bacterial (shallow wells) Nitrate from agriculture Industrial pollutants Institutional No regulatory framework Unregistered STWs Monitoring gaps Integrated Water Resource Management (IWRM) approach needed
Figure 9.3c: Five major groundwater challenges in the Terai — arsenic, iron/manganese, declining water tables, contamination, and institutional gaps.

9.3.5 Sustainable Development Strategies

  • Managed Aquifer Recharge (MAR): Injecting treated surface water or harvested rainwater into the deep aquifer through recharge wells, particularly in the northern recharge zone
  • Groundwater regulation: Licensing and metering of tube wells; establishment of groundwater protection zones
  • Conjunctive use: Integrating Melamchi surface water supply with strategic groundwater use for dry-season buffering
  • Monitoring network: Expansion of piezometric monitoring wells across the valley with real-time data logging
  • Rainwater harvesting: Mandatory in new buildings to reduce demand on groundwater

9.4 MIDDLE HILLS AND HIGH MOUNTAINS

9.4.1 Middle Hills (Mahabharat Zone)

The Middle Hills comprise metamorphic rocks (phyllite, schist, quartzite, gneiss) with limited primary porosity. Groundwater occurs primarily in:

Middle Hills ΓÇö Groundwater Sources Springs Contact springs Depression springs Fault-controlled springs Primary source for hill communities Q: 0.1-5 L/s Dug Wells Shallow (<15 m) Valley fill deposits Terraces/colluvium Seasonal variation in yield Contamination risk Bore Wells Fractured rock aquifers only Success rate: 40-60% Limited to favorable geology VES for siting Springs are declining across Nepal's Middle Hills due to climate change Spring-shed protection and recharge enhancement are priority interventions
Figure 9.4b: Middle Hills groundwater sources ΓÇö springs (primary), dug wells, and bore wells in fractured rock; springs are declining due to climate change.
  • Fractured rock aquifers: Water stored in joints, fractures, and faults. Limited yield (0.5–5 m³/hr).
  • Colluvial deposits: Weathered rock and debris on slopes — seasonal springs.
  • River terraces: Alluvial deposits along river valleys — moderate yields for dug wells.

The primary water source in the Middle Hills is springs. Nepal has an estimated 6,000+ springs, many of which are declining due to deforestation, climate change, and land-use changes.

9.4.2 High Himalayas

  • Crystalline rocks (granite, gneiss) with very low permeability
  • Groundwater is negligible except in glacial moraines and talus deposits
  • Springs are fed by snowmelt and glacial melt — highly seasonal
  • Not a focus for groundwater development
High Himalayas ΓÇö GW Characteristics Very Limited Groundwater Challenges ΓÇó Hard crystalline rock ΓÇó Very low permeability ΓÇó Steep terrain, thin soil ΓÇó Glacial melt dependent Available Sources ΓÇó Snow/glacier-fed springs ΓÇó Moraine deposits (limited) ΓÇó Fault-line springs ΓÇó Karst in limestone areas Population density is low ΓÇö small spring sources adequate for settlements Climate change threatens glacier-fed springs ΓÇö long-term sustainability concern
Figure 9.4c: High Himalayan groundwater ΓÇö very limited due to hard crystalline rock; population relies on glacier-fed springs and fault-controlled sources.

9.5 URBAN GROUNDWATER EXPLOITATION — ISSUES AND MITIGATION

PAST EXAM QUESTION ADDRESSED:

2078 Chaitra Q8 [6]: Discuss the overall condition of groundwater exploitation in urban areas with reference to Butwal and suggest mitigation measures.

9.5.1 General Urban Groundwater Issues in Nepal

Rapid urbanization in Nepal’s Terai cities (Butwal, Bharatpur, Birgunj, Biratnagar, Nepalgunj) and the Kathmandu Valley has led to increasing groundwater exploitation. Common issues include:

Urban GW Issues — Kathmandu Valley Crisis Indicators WT Decline 2-3 m/yr in some areas Many wells going dry Demand >> recharge Quality Degradation NH₃, NO₃, Fe, coliform Sewage + septic seepage Most wells: not potable Subsidence Risk Compaction of clay layers from WT decline Irreversible damage Needed: Managed Aquifer Recharge (MAR) Rain harvesting, recharge ponds, injection wells, reduced paving KUKL supplies only 30-40% of Kathmandu's water demand — rest from private GW
Figure 9.5b: Kathmandu Valley groundwater crisis — declining water tables, quality degradation, and subsidence risk; MAR urgently needed.
  • Declining water tables: Due to pumping exceeding natural recharge, particularly during the dry season (November–May)
  • Poor water quality: Faecal contamination from lack of sewerage; chemical contamination from industrial effluents
  • Well interference: Closely spaced wells competing for the same aquifer, reducing individual yields
  • Loss of recharge area: Paving and construction reduce infiltration; stormwater is drained away rather than recharged
  • Unregulated extraction: No licensing, no metering, no pumping limits

9.5.2 Case Study: Butwal

Butwal is a rapidly growing Terai city at the foot of the Siwaliks (Rupandehi District). Key groundwater features:

  • Aquifer type: Alluvial sand and gravel aquifers along the Tinau River fan
  • Depth range: Shallow wells at 10–30 m; deep wells at 60–120 m
  • Current status: Groundwater is the primary source of municipal water supply; Nepal Water Supply Corporation (NWSC) and private tankers rely heavily on bore wells
  • Problems:
    • Dry-season drawdown of 3–8 m in shallow wells
    • Reduction in Tinau River baseflow due to upstream and local pumping
    • Coliform contamination in shallow wells from inadequate sanitation
    • Increasing number of private tube wells without regulation

9.5.3 Mitigation Measures

  1. Groundwater regulation: Establish a licensing system for all tube wells; mandatory registration and periodic monitoring
  2. Managed Aquifer Recharge (MAR): Construct recharge pits, injection wells, and percolation ponds along the Tinau River to replenish aquifers during monsoon
  3. Rainwater harvesting: Promote household and institutional rainwater collection and recharge
  4. Conjunctive use: Develop surface water sources (Tinau diversion) for dry-season supplementation
  5. Wellhead protection zones: Establish protective zones around public supply wells to prevent contamination
  6. Monitoring network: Install piezometers across the city for continuous water level and quality monitoring
  7. Public awareness: Educate communities about water conservation and the finite nature of groundwater
  8. Sanitation improvement: Construct proper sewerage and wastewater treatment to prevent groundwater pollution
Exam tip: For a 6-mark question on urban groundwater (Butwal), organize your answer as: (a) Brief setting — 2–3 lines, (b) Current problems — 4–5 bullet points, (c) Mitigation measures — 5–6 numbered points. This structure ensures complete coverage.

9.6 GROUNDWATER STORAGE AND WELL-FIELD PLANNING (NUMERICAL)

PAST EXAM QUESTIONS ADDRESSED:

2072 Ashwin Q3 [8]: For a confined aquifer in Pokhara, estimate annual groundwater storage and determine the number of wells. (Numerical)

2072 Magh Q3 [8]: For a confined aquifer in Janakpur, estimate annual groundwater storage and determine the number of wells. (Numerical)

9.6.1 Key Formulae

A. Annual Groundwater Storage Volume:

\[ \boxed{V = S \times A \times \Delta h} \]
  • \( S \) = storage coefficient (dimensionless) — for confined aquifer, typically 10−4–10−3
  • \( A \) = aquifer plan area (m²)
  • \( \Delta h \) = annual fluctuation of piezometric surface (m)
GW Management — Key Design Principles for Nepal Safe Yield Extraction ≤ Recharge Monitor WT trends + environmental flows Well Spacing Avoid interference Min. distance = 2R R = radius of influence Quality Protection Wellhead protection Sanitary seal depth Regular testing Recharge Enhancement Rain harvesting ponds Permeable pavements Spring-shed mgmt Regulation Well registration Extraction permits Monitoring network Sustainable GW management = safe yield + quality protection + institutional governance
Figure 9.6b: Five pillars of groundwater management in Nepal — safe yield, well spacing, quality protection, recharge enhancement, and regulation.

B. Number of Wells:

\[ \boxed{n = \frac{V}{q \times t}} \]
  • \( q \) = average yield per well (m³/hr)
  • \( t \) = annual pumping hours = pumping days × 24 hr/day
Physical Meaning: The storage coefficient \( S \) for a confined aquifer represents the volume of water released per unit area per unit decline in head. It is very small because water is released only by elastic compression of the aquifer skeleton and expansion of water. For unconfined aquifers, the specific yield \( S_y \) is 100–1000× larger because water drains from pore spaces by gravity.

9.6.2 Solved Example 1: Pokhara (2072 Ashwin Q3)

PROBLEM STATEMENT

In Pokhara, a confined aquifer of 30 m average thickness extends over 800 km². The piezometric surface fluctuates annually from 19 m to 9 m above the top of the aquifer. Storage coefficient S = 0.0008. Average well yield = 30 m³/hr, pumping duration = 200 days/year. Find: (a) annual groundwater storage, (b) number of wells that can be drilled.

Given Data:
Aquifer thickness = 30 m,  A = 800 km² = 800 × 106 m²
Piezometric fluctuation: 19 m → 9 m ⇒ Δh = 10 m
S = 0.0008,  q = 30 m³/hr,  Pumping = 200 days/year

SOLUTION

Step 1 — Annual groundwater storage:

\[ V = S \times A \times \Delta h = 0.0008 \times 800 \times 10^6 \times 10 \]
\[ V = 0.0008 \times 8 \times 10^9 = 6.4 \times 10^6 \text{ m}^3 \]

\( V = 6.4 \text{ million m}^3/\text{year} \)

Step 2 — Annual yield per well:

\[ q_{annual} = 30 \;\text{m}^3/\text{hr} \times 24 \;\text{hr/day} \times 200 \;\text{days} = 144{,}000 \;\text{m}^3/\text{year} \]

Step 3 — Number of wells:

\[ n = \frac{V}{q_{annual}} = \frac{6{,}400{,}000}{144{,}000} = 44.4 \]

\( \boxed{n \approx 44 \text{ wells}} \)

9.6.3 Solved Example 2: Janakpur (2072 Magh Q3)

PROBLEM STATEMENT

In Janakpur, a confined aquifer of 20 m average thickness extends over 850 km². Groundwater table fluctuates from 15 m to 10 m above the top. S = 0.0009. Average well yield = 30 m³/hr, pumping = 210 days/year. Find: (a) annual groundwater storage, (b) number of wells.

Given Data:
Aquifer thickness = 20 m,  A = 850 km² = 850 × 106 m²
Piezometric fluctuation: 15 m → 10 m ⇒ Δh = 5 m
S = 0.0009,  q = 30 m³/hr,  Pumping = 210 days/year

SOLUTION

Step 1 — Annual groundwater storage:

\[ V = S \times A \times \Delta h = 0.0009 \times 850 \times 10^6 \times 5 \]
\[ V = 0.0009 \times 4.25 \times 10^9 = 3.825 \times 10^6 \text{ m}^3 \]

\( V = 3.825 \text{ million m}^3/\text{year} \)

Step 2 — Annual yield per well:

\[ q_{annual} = 30 \times 24 \times 210 = 151{,}200 \;\text{m}^3/\text{year} \]

Step 3 — Number of wells:

\[ n = \frac{3{,}825{,}000}{151{,}200} = 25.3 \]

\( \boxed{n \approx 25 \text{ wells}} \)

9.6.4 Comparison and Insight

Comparison: Pokhara vs Janakpur Groundwater Storage Pokhara V = 6.4 M m³ — 44 wells Janakpur V = 3.825 M m³ — 25 wells Pokhara: A=800 km², S=0.0008, Δh=10 m Janakpur: A=850 km², S=0.0009, Δh=5 m Δh is the dominant factor — double Δh ≈ double the storage
Figure 9.4: Despite similar area and S, Pokhara has 76% more wells because Δh is twice as large.
Full step-by-step solutions with additional diagrams: See Numerical Solutions — Chapter 9.

9.7 PAST QUESTION BANK FOR CHAPTER 9

2078 Chaitra Q8 6 marks

Discuss the overall condition of groundwater exploitation in urban areas with reference to Butwal and suggest mitigation measures.

ANSWER GUIDE:

(a) Setting [1 mark]: Butwal is a Terai city (Rupandehi) on the Tinau River alluvial fan. Alluvial sand/gravel aquifers at 10–120 m depth.

(b) Current problems [2 marks]: Dry-season drawdown 3–8 m, reduced river baseflow, coliform contamination in shallow wells, unregulated private tube wells, well interference.

(c) Mitigation [3 marks]: (1) Groundwater licensing & monitoring, (2) MAR along Tinau, (3) rainwater harvesting, (4) conjunctive use with surface water, (5) wellhead protection zones, (6) sewerage improvement. Section 9.5.

2073 Bhadra Q10 4 marks

Discuss the aquifer system of Kathmandu Valley.

ANSWER GUIDE:

Draw the three-layer model (Figure 9.3). Describe: (1) Shallow unconfined aquifer — sand/silt, 5–20 m; (2) Kalimati Clay — thick lacustrine aquitard up to 200 m, black organic clay; (3) Deep confined aquifer — coarse sand/gravel, 60–300 m, main supply source; (4) Basal gravel — highest yield. Mention northern recharge zone vs. central confined zone. Section 9.3.

2072 Ashwin Q3 8 marks NUMERICAL

For a confined aquifer in Pokhara, estimate annual groundwater storage from piezometric fluctuation and determine how many wells can be drilled for the given yield and pumping duration.

Aquifer thickness = 30 m, Area = 800 km², Piezometric fluctuation = 19 m to 9 m,
S = 0.0008, Well yield = 30 m³/hr, Pumping = 200 days/year

ANSWER GUIDE:

V = S×A×Δh = 0.0008 × 800×106 × 10 = 6.4×106 m³. Wells: n = V/(30×24×200) = 6.4M/144,000 = 44 wells. See Section 9.6.2 and Chapter 9 Numericals.

2072 Ashwin Q8 6 marks

Discuss the aquifer system of Kathmandu Valley of Nepal.

ANSWER GUIDE:

Same as 2073 Q10 but with more detail (6 marks vs 4). Add: geological history (Paleo-Kathmandu Lake), issues (over-extraction, 20–30 m head decline, land subsidence, ammonia contamination), and sustainable development strategies (MAR, regulation, conjunctive use). Draw schematic cross-section with zones labelled. Section 9.3.

2072 Magh Q3 8 marks NUMERICAL

For a confined aquifer in Janakpur, estimate annual groundwater storage and determine the number of wells that can be drilled under the given pumping assumptions.

Aquifer thickness = 20 m, Area = 850 km², Fluctuation = 15 m to 10 m,
S = 0.0009, Well yield = 30 m³/hr, Pumping = 210 days/year

ANSWER GUIDE:

V = 0.0009 × 850×106 × 5 = 3.825×106 m³. Wells: n = 3.825M/(30×24×210) = 3.825M/151,200 = 25 wells. See Section 9.6.3 and Chapter 9 Numericals.

2072 Magh Q10 6 marks

Discuss the groundwater development possibilities of the Terai belt.

ANSWER GUIDE:

Setting [1 mark]: 20–50 km wide alluvial plain, Quaternary sediments, two-layer aquifer system (shallow unconfined + deep confined). See Table in Section 9.2.2.

Development status [2 marks]: 40,000+ shallow wells, 10,000+ deep wells, only 20–25% of available GW developed.

Possibilities [2 marks]: Irrigation expansion (deep wells in western Terai), conjunctive use, urban supply for growing cities, MAR for sustainability.

Challenges [1 mark]: Arsenic contamination, iron/Mn, over-extraction near cities, lack of monitoring data. Section 9.2.

2071 Bhadra Q11 6 marks

Discuss, with the help of schematic zones, the availability and possibilities of groundwater resources in Kathmandu Valley.

ANSWER GUIDE:

Draw Figure 9.3 (three-layer model with northern and central/southern zones). Explain:

Northern zone: Alluvial fan, coarse sediments, main recharge area, shallow WT, good quality. Section 9.3.3.

Central/Southern zone: Thick Kalimati Clay, confined conditions, limited recharge, declining heads, quality issues. Section 9.3.3.

Possibilities: MAR in northern zone, regulation of extraction, conjunctive use with Melamchi, monitoring network. Section 9.3.5.

CHAPTER SUMMARY AND EXAM KEY POINTS

EXAM FORMULA SHEET — CHAPTER 9

Item Formula / Key Fact Context
Annual GW storage \( V = S \times A \times \Delta h \) Confined aquifer; Δh = piezometric fluctuation
Number of wells \( n = V / (q \times t) \) t = pumping days × 24 hr/day
Unit conversion 1 km² = 106 m² Always convert area before calculation
Terai — shallow aquifer Unconfined, fine sand, 5–30 m depth Hand pumps, arsenic risk in some areas
Terai — deep aquifer Confined, coarse sand/gravel, 50–500 m High yield (30–100+ m³/hr), irrigation
KTM Valley — Kalimati Black organic lacustrine clay, up to 200 m thick Principal aquitard separating shallow/deep
KTM Valley — three layers Shallow (unconfined) / Kalimati Clay / Deep (confined) Draw three-layer schematic for exam
KTM Valley — zones Northern = recharge; Central/South = confined GW flows S from Shivapuri alluvial fans
Mitigation measures MAR, regulation, conjunctive use, monitoring Common to all urban GW questions

KEY TAKEAWAYS FOR EXAM

  1. Kathmandu Valley aquifer system is the single most asked topic (appeared in 2071, 2072 Ashwin, 2072 Magh, 2073). Draw the three-layer schematic and know the northern/southern zone distinction.
  2. Terai groundwater (2072 Magh): Know two-layer system (shallow unconfined + deep confined), current development status (20–25% utilized), and development possibilities.
  3. Urban groundwater / Butwal (2078): Know common problems (drawdown, contamination, unregulated extraction) and mitigation measures (MAR, regulation, conjunctive use).
  4. Numerical — GW storage (2072 Ashwin + Magh): Simple 3-step formula: V = S×A×Δh, then n = V/(q×24×days). Easy 8 marks — do not lose these.
  5. Δh is the dominant factor in storage calculations — more impactful than area or S. Mention this insight for bonus marks.
  6. Exam allocation: Theory questions (4–6 marks, 10–15 min) + numerical (8 marks, 15 min). Total ~20 min for this chapter.
Exam Strategy: Chapter 9 Decision Flowchart CHAPTER 9 QUESTION “KTM Valley aquifer?” Three-layer schematic (Fig 9.3) Northern vs. Central/South zones Issues + strategies (Sec 9.3) “Terai GW / Urban Butwal?” Two-layer aquifer (Fig 9.2) Development status + potential Challenges & mitigation (Sec 9.2/9.5) “GW storage numerical?” V = S × A × Δh n = V / (q × 24 × days) Convert km² → m² first! Theory: 4–6 marks (diagram + structured answer). Numerical: 8 marks (3 simple steps). Always include a diagram — cross-section or schematic.
Figure 9.5: Exam strategy for answering Chapter 9 questions.