JNTU
JNTU B.Tech III GROUND WATER DEVELOPMENT AND MANAGEMENT Previous Question Paper (February/March - 2022 - R16)
Answer any five questions. All questions carry equal marks.
Main
Discuss how the water is stored into the ground water reservoir. Briefly mention the various zones and the importance of “zone of saturation” in this connection.
Answer
Groundwater storage occurs through the process of infiltration, where precipitation or surface water percolates downward through the soil profile under the influence of gravity. As water moves through the interconnected voids of geological formations, it fills the available pore spaces or fractures within the rock and soil matrix.
The subsurface is vertically divided into two primary zones: the zone of aeration (or vadose zone) and the zone of saturation. The zone of aeration, located between the land surface and the water table, contains both water and air in its pore spaces. Below this lies the zone of saturation, where all interconnected voids are completely filled with water at a pressure equal to or greater than atmospheric pressure.
The zone of saturation is the most critical region for groundwater development because it constitutes the actual groundwater reservoir. It is from this zone that water can be extracted by wells and springs. The upper boundary of this zone is the water table, and the volume of water stored here depends on the porosity and the specific yield of the geological material.
Estimate the probable land subsidence when the artesian pressure is reduced by 7 kg/cm2 in an artesian aquifer 30 m thick having porosity of 30% and storage coefficient 2 × 10-4 (unit weight of water 0.1, bulk modulus of elasticity of water 4.7 × 10-5).
Answer
To estimate the land subsidence (ΔH) due to a reduction in artesian pressure, we use the relationship between the storage coefficient (S), the thickness of the aquifer (b), and the change in pressure (Δp). The compression of the aquifer is related to the change in effective stress, which is equivalent to the reduction in artesian pressure.
Given data:
- Thickness of aquifer (b) = 30 m
- Reduction in pressure (Δp) = 7 kg/cm2 = 70,000 kg/m2
- Storage coefficient (S) = 2 × 10-4
- Unit weight of water (γw) = 1000 kg/m3 (0.1 kg/cm3)
The storage coefficient S is defined as S = Ss × b, where Ss is the specific storage. The change in thickness (ΔH) is calculated by:
Substituting Ss = S/b:
Calculation:
The estimated land subsidence is 0.014 cm.
List the geomorphic control on groundwater and explain any two morphometric parameters which influences the groundwater potential.
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An aquifer has an average thickness of 60 m and an aerial extent of 100 ha. Estimate the available ground water storage if:
i) The aquifer is unconfined and the fluctuation in GWT is observed as 15 m.
ii) The aquifer is confined and the piezometric head is lowered by 50 m, which drains half the thickness of the aquifer. Assume a storage coefficient of 2 × 10-4 and a specific yield of 16%.
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What is permeability? Derive Darcy’s law and discuss the same.
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It was observed in a field test that 3 hr 20 min was required for a tracer to travel from one well to another 20 m apart and the difference in their water surface elevation was 0.5 m. samples of the aquifer between the wells indicated a porosity of 15%. Determine the permeability of the aquifer, seepage velocity and the Reynolds number for the flow, assuming an average grain size of 1 mm and ν water at 270 = 0.008 stoke.
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What is the nature of ground water flow? State the law governing the flow with limitations, if any.
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For conducting permeability tests in a well penetrating an unconfined aquifer, two observation wells A and B are located at distances 15 and 30 m from the centre of the well. When the well is pumped at a rate of 5 lps, it is observed that the elevations of the water table above the impervious layer, up to which the well extends are 12 and 12.5 m at A and B. Calculate the permeability of the aquifer in m/day.
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Write down assumptions in Dupit’s equation and Themi’s equation.
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Two piezometers A and B, 500 m apart are driven into the same unconfined aquifer. The ground surface elevation of piezometer A is 195 m and of piezometer B is 210 m. The depths to water in A and B are 25 m and 60 m respectively. Determine the direction of groundwater flow and use the Dupuit assumptions to calculate the water table midway between the piezometers. Also calculate the quantity of seepage through a 10 m longitudinal section of the aquifer in which K = 10-3 m/s is the horizontal base of the aquifer is at elevation 50 m.
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Derive the partial differential equation for unsteady groundwater flow with assumptions.
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A well is located in a 25m confined aquifer of permeability 30m/day and storage coefficient 0.005. If the well is being pumped at the rate of 1750 lpm, calculate the drawdown at a distance of (i) 100 m and (ii) 50 m from the well after 20 hrs of pumping.
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Write the procedure to delineate and to characterize the groundwater potential zones in the hard rock terrain using remote sensing and GIS technique with flow chart.
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What are the uses of aerial photogrammety in groundwater explorations?
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State and derive Ghyben Herzberg relation to study and measure saline water intrusion in aquifers.
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A confined fresh water aquifer outflows to a sea at the rate of 0.5 m3/d/m along the length of the seacoast. The thickness and hydraulic conductivity (freshwater) of the aquifer are 20 m and 5 m/d respectively, (i) Find the depth of the interface and pezometric head 25 m away from the toe of the interface. Assume the thickness of the top impervious layer to be 5 m below msl. (ii) Compute the length of the intrusion. Given Pf/ (Ps - Pf) = 40. If the water outflow is reduced to 0.35 m2/d, compute the factor by which the interface increases.
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