Predicted question paper — BAG701: Ground water, Well & Pumps

Time: 3 Hours — Max Marks: 70

Instructions: (i) Attempt all questions in SECTION A. (ii) In SECTION B answer one part of each question (either (a) or (b)). (iii) Use suitable assumptions wherever necessary and state them.

SECTION A — Short answer (10 × 2 = 20)

1. Define specific yield and give its units.

2. List two types of aquifers and one distinguishing feature of each.

3. Name two common types of well screens used in tube wells.

4. What is well development and why is it done?

5. Give two advantages of submersible pumps over turbine pumps.

6. State Darcy’s law in one sentence.

7. What is well interference?

8. Write the expression for specific capacity of a well.

9. Mention two common methods for artificial groundwater recharge.

10. What is the purpose of a gravel pack in a well?

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SECTION B — Long answer (5 × 10 = 50)

Q.1 a) Explain the classification of wells (open well, tube well — types) with sketches.
OR
b) Describe methods of groundwater exploration (geophysical + drilling reconnaissance) and give pros/cons.

Q.2 a) A pumping test on a confined aquifer yields the following: pumped volume = 9.5 × 10^6 m³ and drawdown = 2.4 m over area 15 km². Estimate the specific yield (assume unconfined aquifer) and compute recharge if water table rises 12.5 m in monsoon. (numerical — similar numbers appear in past papers).
OR
b) Explain Theis solution and its application for unsteady radial flow. Show how transmissivity and storativity are obtained from Theis type-curve.

Q.3 a) Describe the construction and installation steps of a gravel-packed tube well. Include selection of screen slot size and gravel gradation considerations.
OR
b) Write short notes on drilling methods: percussion, rotary, and reverse rotary. State when each is preferable.

Q.4 a) A well test gives drawdown data in observation wells — outline steps to evaluate aquifer parameters using Jacob’s straight-line method (show formula and interpretation).
OR
b) Discuss well interference and multiple well system effects. How does interference affect well spacing design?

Q.5 a) With a labelled performance curve, explain selection of a centrifugal pump for a deep-well application (include specific speed concept and net positive suction head (NPSH) idea).
OR
b) Compare deep-well turbine pump and submersible pump for an irrigation tube-well application. Discuss efficiency, installation, maintenance and suitability for large drawdown.

SECTION A

(10 × 2 = 20 Marks)

1. Specific yield of an aquifer

Specific yield is an important property of an aquifer that represents its ability to release water. It is defined as the ratio of the volume of water that an aquifer can drain under the influence of gravity to the total volume of the aquifer material. When groundwater is pumped, not all water present in the pores flows out; some water remains held by molecular attraction. The portion that flows out is known as specific yield. It is usually expressed as a dimensionless ratio or percentage. Specific yield is high for coarse materials like gravel and sand and low for clayey soils. It is an important parameter in estimating groundwater recharge and storage.

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2. Difference between confined and unconfined aquifer

A confined aquifer is a type of aquifer that is bounded above and below by impermeable or semi-permeable layers, such as clay or rock. The groundwater in a confined aquifer is under pressure, and when a well is drilled, water may rise above the aquifer level. In contrast, an unconfined aquifer has no confining layer above it and contains a free water table exposed to atmospheric pressure. The water level in wells tapping unconfined aquifers coincides with the water table. Confined aquifers generally have less recharge but provide more reliable supply, while unconfined aquifers are easily recharged by rainfall but are more vulnerable to contamination.

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3. Well development

Well development is the process carried out after the construction of a well to improve its hydraulic performance and efficiency. During drilling, fine soil particles tend to clog the pore spaces around the well screen, which reduces water flow. Well development removes these fine particles from the formation near the well by methods such as surging, pumping, air lifting, or jetting. This process increases the permeability around the well, resulting in higher discharge and reduced drawdown. Proper well development also ensures uniform flow of water through the screen, minimizes sand pumping, and increases the life of the well. It is an essential step before putting a well into regular operation.

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4. Darcy’s law for groundwater flow

Darcy’s law is a fundamental principle that governs the movement of groundwater through porous media. It states that the rate of flow of groundwater is directly proportional to the hydraulic gradient and the cross-sectional area of flow, provided the flow is laminar. Mathematically, it is expressed as
Q = K × A × i,
where Q is the discharge, K is the coefficient of permeability, A is the cross-sectional area, and i is the hydraulic gradient. Darcy’s law applies mainly to sandy and gravelly soils and is widely used in groundwater studies to estimate seepage, aquifer properties, and well discharge.

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5. Specific capacity of a well

Specific capacity of a well is a measure of its performance and efficiency. It is defined as the ratio of the discharge of a well to the drawdown produced in the well. It is usually expressed in cubic meters per minute per meter of drawdown. A higher specific capacity indicates a more efficient well with better aquifer characteristics. Specific capacity depends on factors such as aquifer permeability, well design, degree of well development, and pumping duration. It is commonly used for quick comparison of different wells and for estimating approximate aquifer transmissivity without conducting elaborate pumping tests.

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6. Types of well screens

Well screens are used in tube wells to allow water to enter while preventing sand and soil from entering the well. Common types include slotted screens and wire-wound screens. Slotted screens consist of longitudinal or horizontal slots cut into metal or PVC pipes and are simple and economical. Wire-wound screens consist of continuous wedge-shaped wires wound around supporting rods, providing uniform slot openings and high open area. This type offers better hydraulic efficiency and reduces clogging. Selection of a suitable screen depends on aquifer material, grain size distribution, and required discharge.

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7. Well interference

Well interference occurs when two or more wells are located close to each other and are pumped simultaneously. In such cases, the cone of depression formed around each well overlaps, resulting in increased drawdown in all wells. This overlap reduces the discharge and efficiency of individual wells. Well interference is significant in areas with closely spaced wells or in aquifers with low permeability. It must be considered while designing well spacing to ensure sustainable groundwater extraction. Proper planning of well location and pumping rates can minimize the adverse effects of interference.

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8. Advantages of submersible pumps

Submersible pumps offer several advantages for groundwater extraction. Since the pump and motor are submerged in water, priming is not required, making operation simpler and more reliable. These pumps are highly efficient because there is no suction loss, and water is pushed directly to the surface. Submersible pumps operate quietly and require less surface space. They are suitable for deep wells and can handle large discharges. Additionally, they have fewer mechanical problems related to cavitation and can be used in narrow bore wells.

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9. Function of gravel packing in tube wells

Gravel packing is placed between the well screen and the surrounding aquifer material in a tube well. Its primary function is to prevent fine sand and soil particles from entering the well, thereby reducing sand pumping. Gravel packing also increases the effective diameter of the well and improves water flow towards the screen. It acts as a filter, stabilizing the formation and preventing collapse. Properly designed gravel packing improves well efficiency, increases discharge, reduces wear on pumps, and extends the life of the well.

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10. Methods of artificial groundwater recharge

Artificial groundwater recharge is the process of deliberately increasing groundwater storage by allowing surface water to infiltrate into the ground. Common methods include percolation tanks and recharge wells. Percolation tanks store runoff water and allow it to slowly seep into the ground. Recharge wells directly convey surface water into deeper aquifers. Other methods include recharge pits, spreading basins, and check dams. Artificial recharge helps raise the water table, improves groundwater quality, and ensures sustainable water supply, especially in drought-prone areas.

Q1 (a) Classify wells and explain open wells and tube wells

Classification of Wells

Wells are structures constructed to extract groundwater from aquifers. Based on construction and depth, wells are classified as:

1. Open wells (Dug wells)

2. Tube wells

3. Driven wells

4. Artesian wells

5. Infiltration wells

Among these, open wells and tube wells are most commonly used for irrigation and water supply.

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Open Wells

Open wells, also known as dug wells, are large-diameter wells constructed by excavating soil up to the water table. The diameter generally ranges from 1 to 10 meters. Water enters the well from the sides and bottom. These wells are mostly suitable for shallow unconfined aquifers. They are simple in construction and economical but have limited discharge and are prone to seasonal drying.

Advantages:

• Simple and low-cost construction

• Easy maintenance

Limitations:

• Low yield

• Susceptible to contamination

(Sketch: circular masonry-lined well showing water table)

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Tube Wells

Tube wells are small-diameter, deep wells constructed using drilling methods. They consist of casing pipe, well screen, and gravel pack. Tube wells are suitable for deep confined or unconfined aquifers and can provide large and continuous discharge.

Advantages:

• High yield

• Less land requirement

Limitations:

• Higher initial cost

• Requires skilled operation

(Sketch: vertical pipe with screen and gravel packing)

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Q1 (b) Methods of groundwater exploration with merits and limitations

Groundwater exploration is the process of identifying the location, depth, and extent of groundwater resources before well construction. The main methods are:

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1. Geological Method

This method involves studying rock types, soil structure, and geological maps to identify groundwater-bearing formations.

Merits:

• Simple and inexpensive

• Useful for preliminary surveys

Limitations:

• Not accurate for depth estimation

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2. Geophysical Methods

These include electrical resistivity, seismic, and magnetic methods. Electrical resistivity is most commonly used.

Merits:

• Determines depth and thickness of aquifers

• Suitable for large areas

Limitations:

• Expensive

• Requires skilled interpretation

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3. Remote Sensing Method

Satellite images and aerial photographs are used to identify lineaments and drainage patterns.

Merits:

• Covers large areas quickly

• Useful in inaccessible regions

Limitations:

• Needs field verification

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4. Exploratory Drilling

Test wells are drilled to confirm aquifer presence.

Merits:

• Most reliable method

• Provides actual subsurface data

Limitations:

• Costly and time-consuming

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Conclusion

A combination of geological, geophysical, and drilling methods gives the best results for groundwater exploration.

Q2 (a) Pumping test on an unconfined aquifer

Given Data

Area influenced, A = 18 km² = 18 × 10⁶ m²
Total volume of water pumped, V = 11 × 10⁶ m³
Average drawdown, h = 2.2 m
Rise in water table during monsoon, Δh = 14 m

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(i) Calculation of Specific Yield

Specific yield (Sy) is given by:

$$Sy = \frac{\text{Volume of water pumped}}{\text{Area influenced × Average drawdown}}$$ $$Sy = \frac{11 × 10^6}{18 × 10^6 × 2.2}$$ $$Sy = \frac{11}{39.6}$$ $$Sy = 0.278 \approx 0.28$$ $$\boxed{Specific\ Yield = 0.28 \ (28\%)}$$

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(ii) Calculation of Recharge

Recharge (R) is given by:

$$R = Sy × A × \Delta h$$ $$R = 0.28 × 18 × 10^6 × 14$$ $$R = 70.56 × 10^6 \ m^3$$ $$\boxed{Recharge = 70.56 × 10^6 \ m^3}$$

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Final Answer

• Specific Yield of the aquifer = 0.28 (28%)

• Recharge during monsoon = 70.56 × 10⁶ m³

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Q2 (b) Theis Method for Determination of Aquifer Parameters

The Theis method is an analytical method used to determine transmissivity (T) and storativity (S) of a confined aquifer under unsteady flow conditions. It is based on the assumption that groundwater flow towards a well follows Darcy’s law and that the aquifer is homogeneous, isotropic, and of infinite extent.

The Theis equation is:

$$s = \frac{Q}{4\pi T} W(u)$$

where
s = drawdown
Q = discharge
T = transmissivity
W(u) = well function
u = \frac{r^2 S}{4 T t}

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Procedure Using Type Curves

1. A type curve of W(u) versus 1/u is prepared on log–log paper.

2. Field pumping test data (drawdown vs time) is plotted on another log–log graph.

3. The field data curve is superimposed on the type curve until best fit is obtained.

4. A match point is selected to read corresponding values of s, t, W(u), and u.

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Determination of Parameters

Transmissivity (T):

$$T = \frac{Q}{4\pi s} W(u)$$

Storativity (S):

$$S = \frac{4 T t u}{r^2}$$

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Conclusion

The Theis method is widely used for analyzing pumping test data and provides reliable values of aquifer parameters when its assumptions are satisfied. It is especially useful for confined aquifers under transient flow conditions.

Q3 (a) Construction of a Gravel-Packed Tube Well

A gravel-packed tube well is constructed in unconsolidated or loose formations where fine sand is present. The purpose of gravel packing is to improve well efficiency and prevent sand entry.

Steps in Construction

1. Site Selection
   The location of the well is selected based on groundwater exploration studies such as geological and geophysical surveys.

2. Drilling of Bore Hole
   A bore hole of diameter larger than the casing pipe is drilled using suitable drilling methods like rotary drilling.

3. Lowering of Casing and Screen
   The casing pipe and well screen are lowered into the bore hole. The screen is placed opposite the aquifer zone to allow water entry.

4. Selection of Screen
   The slot size of the screen is selected based on the grain size distribution of aquifer material. Generally, the slot opening is chosen to retain 90–95% of the formation material.

5. Gravel Packing
   Clean, rounded, and well-graded gravel is placed in the annular space between the bore hole and the screen. The gravel size is usually 4 to 6 times larger than the average grain size of aquifer material.

6. Sealing and Well Development
   The upper portion is sealed with clay or cement to prevent contamination. The well is then developed by surging or pumping.

Advantages

• High discharge

• Reduced sand pumping

• Increased well life

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Q3 (b) Drilling Methods for Tube Well Construction

Drilling methods are used to create bore holes for tube well installation. The common drilling methods are:

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1. Percussion Drilling

This method involves repeated lifting and dropping of a heavy cutting tool to break the formation. The loosened material is removed using a bailer.

Merits:

• Simple and economical

• Suitable for hard rock formations

Limitations:

• Slow process

• Not suitable for very deep wells

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2. Rotary Drilling

In rotary drilling, a rotating drill bit cuts the formation, and drilling fluid removes cuttings to the surface.

Merits:

• Fast drilling

• Suitable for deep and unconsolidated formations

Limitations:

• High initial cost

• Requires skilled operation

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Comparison between Rotary and Percussion Drilling

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Conclusion

Selection of drilling method depends on formation type, depth of aquifer, and economic considerations. Rotary drilling is preferred for deep tube wells, while percussion drilling is suitable for shallow hard rock areas. 

Q4 (a) Jacob Straight-Line Method for Pumping Test Analysis

The Jacob straight-line method is a simplified graphical technique used to determine aquifer parameters such as transmissivity (T) and storativity (S) from pumping test data under unsteady flow conditions. It is derived from the Theis solution and is applicable for late-time data, where the well function can be approximated by a straight line on semi-log paper.

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Principle

The drawdown (s) is plotted against the logarithm of time (t) on semi-log graph paper. After sufficient pumping time, the plotted points form a straight line, known as the Jacob straight line.

The equation used is:

$$s = \frac{2.3Q}{4\pi T} \log\left(\frac{2.25 T t}{r^2 S}\right)$$

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Procedure

1. Conduct a constant-rate pumping test and record drawdown at regular intervals.

2. Plot drawdown (s) versus log of time (t).

3. Identify the straight-line portion of the curve.

4. Determine the slope (Δs) for one log cycle of time.

5. Calculate transmissivity (T) using:

$$T = \frac{2.3 Q}{4\pi \Delta s}$$

6. Determine storativity (S) using:

$$S = \frac{2.25 T t_0}{r^2}$$

where $t_0$ is the time corresponding to zero drawdown extrapolated from the straight line.

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Assumptions

• Aquifer is confined, homogeneous, and isotropic

• Flow is radial and laminar

• Pumping rate is constant

• Observation well fully penetrates the aquifer

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Applications

• Estimation of aquifer transmissivity and storativity

• Design of well spacing

• Groundwater resource evaluation

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Q4 (b) Well Interference in Multiple Well Systems

Well interference occurs when two or more wells are pumped simultaneously and their cones of depression overlap. This overlap causes additional drawdown in each well, reducing discharge and efficiency.

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Mechanism of Well Interference

Each pumping well forms a cone of depression. When wells are placed close to each other, these cones intersect, resulting in:

• Increased drawdown

• Reduced yield per well

• Higher pumping costs

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Effect on Well Spacing

• Closer spacing increases interference

• Larger spacing reduces overlapping cones

• Proper spacing ensures optimum yield and sustainable pumping

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Effect on Discharge

• Individual well discharge decreases

• Combined system discharge may increase up to a limit

• Excessive interference can cause pump failure or drying of wells

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Control of Well Interference

• Maintain adequate spacing between wells

• Limit pumping rate

• Use staggered pumping schedules

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Conclusion

Well interference is a critical factor in the design of well fields. Proper analysis and planning help achieve efficient groundwater extraction while preventing excessive drawdown and aquifer depletion.

Q5 (a) Performance Characteristics of a Centrifugal Pump and Criteria for Pump Selection

A centrifugal pump is widely used for pumping groundwater from wells. Its performance is evaluated using characteristic curves obtained by testing the pump at constant speed.

Performance Characteristics

The main performance curves are plotted between:

1. Head vs Discharge (H–Q Curve)
   This curve shows that as discharge increases, the head developed by the pump decreases.

2. Efficiency vs Discharge (η–Q Curve)
   Efficiency increases with discharge, reaches a maximum at the best efficiency point (BEP), and then decreases.

3. Power vs Discharge (P–Q Curve)
   Power requirement increases with increase in discharge.

(Sketch: Combined H–Q, η–Q and P–Q curves on a single graph)

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Criteria for Pump Selection

• Required discharge and total head

• Operating near best efficiency point

• Type of water source and depth

• Pump efficiency and power consumption

• Ease of installation and maintenance

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Conclusion

Selection of a centrifugal pump near the BEP ensures efficient, economical, and reliable operation.

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Q5 (b) Comparison of Deep Well Turbine Pumps and Submersible Pumps

Deep well turbine pumps and submersible pumps are commonly used for deep tube wells.

Comparison

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Advantages

• Turbine pump: Suitable for large discharge

• Submersible pump: Compact, quiet, no suction losses

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Conclusion

Submersible pumps are preferred for deep, narrow wells, while turbine pumps are suitable for high-capacity irrigation systems.