Ground Water and Aquifers Overview
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Questions and Answers

Which formations serve as good aquifers according to the listed order?

  • Rock with fissures and cracks and sandstones (correct)
  • Rock with no signs of weathering or fractures
  • Consolidated formations of clays and shales
  • Cavernous limestones and unconsolidated gravels (correct)

    • Which of the following soil properties are associated with good water yield?

  • Permeability and transmissibility (correct)
  • Specific yield and uniformity coefficient < 2
  • Specific capacity of the well > 30 lpm/m and effective size > 0.1 mm (correct)
  • All of the above characteristics

    • Which of these is incorrectly matched with its characteristic?

  • Transmissibility (T) - f(Kw, Es)
  • Storage coefficient - e - Sr (correct)
  • Aquiclude - Recuperation test (correct)
  • Specific yield (Sy) - n - Sr

    • What represents a confined aquifer?

    <p>Water-bearing rock completely surrounded by less permeable rock (C)</p> Signup and view all the answers

    Which option correctly describes ground water flow?

    <p>Can flow laterally and vertically depending on pressure differentials (B)</p> Signup and view all the answers

    What is the specific yield (Sy) of an aquifer?

    <p>Volume of water that will drain by gravity. (D)</p> Signup and view all the answers

    What does specific retention (Sr) correspond to in soil?

    <p>Field capacity of the soil. (A)</p> Signup and view all the answers

    What is the range of specific yields for alluvial aquifers?

    <p>10-20% (C)</p> Signup and view all the answers

    What characterizes an unconfined aquifer?

    <p>Saturates the formation from the ground surface. (D)</p> Signup and view all the answers

    When does a well drilled into an artesian aquifer become a flowing artesian well?

    <p>If the piezometric surface is above ground level. (B)</p> Signup and view all the answers

    What happens to a well if the piezometric surface is below the ground level?

    <p>It is classified as a non-flowing artesian well. (D)</p> Signup and view all the answers

    What is the term used for a geologic formation capable of transmitting water sufficient for economic extraction by wells?

    <p>Aquifer (A)</p> Signup and view all the answers

    How does specific yield impact the volume of water in an aquifer?

    <p>It defines the drainage potential of the saturated aquifer. (C)</p> Signup and view all the answers

    Which of the following formations is NOT considered a good aquifer?

    <p>Clays (B)</p> Signup and view all the answers

    What is indicated by porosity in an aquifer?

    <p>Total void space available for water. (D)</p> Signup and view all the answers

    What is the purpose of creating more opportunities for groundwater recharge?

    <p>To enhance natural replenishment of groundwater (A)</p> Signup and view all the answers

    What is the primary characteristic of an aquiclude?

    <p>Can absorb water but not transmit significant amounts (D)</p> Signup and view all the answers

    Which of the following is a characteristic of an aquitard?

    <p>Transmits water slowly, insufficient for pumping (B)</p> Signup and view all the answers

    Which of the following is an example of an aquifuge?

    <p>Granite (C)</p> Signup and view all the answers

    What does specific yield measure in a geologic formation?

    <p>Gravity-drainable water capacity (B)</p> Signup and view all the answers

    What modern trend aims to support groundwater sustainability?

    <p>Creation of percolation dams (C)</p> Signup and view all the answers

    What does the 'maximum safe yield' of a well refer to?

    <p>The yield under the safe working head. (C)</p> Signup and view all the answers

    In a pumping test, what is the purpose of keeping the water level constant?

    <p>To ensure consistent percolation into the well. (D)</p> Signup and view all the answers

    What variable is represented by 'D' in the equation regarding the pumping test?

    <p>The diameter of the well. (D)</p> Signup and view all the answers

    What happens during a recuperation test?

    <p>The pumping is stopped and the water level rises. (B)</p> Signup and view all the answers

    In the context of the recuperation test, what does Darcy's law help to determine?

    <p>The relationship between flow rate and hydraulic gradient. (C)</p> Signup and view all the answers

    If the water level in a well rises from s1 to s2 during recuperation, what additional measurement is needed to calculate the yield?

    <p>The time taken for recuperation. (D)</p> Signup and view all the answers

    What does 'K' represent in the formula relating to the yield of the well?

    <p>The hydraulic conductivity. (C)</p> Signup and view all the answers

    What is the significance of the negative sign in the equation Q dt = -A ds?

    <p>It indicates that the head decreases as the time increases. (A)</p> Signup and view all the answers

    What is the residual drawdown (s2) after 60 minutes of pumping?

    <p>0.760 m (C)</p> Signup and view all the answers

    What is the ratio of total drawdown to residual drawdown after 240 minutes?

    <p>1.41 (A)</p> Signup and view all the answers

    What volume of water is available per day from the system described?

    <p>27.2 m³ (D)</p> Signup and view all the answers

    How much area can be irrigated under one well in lateritic soils with the given parameters?

    <p>5,100 m² (D)</p> Signup and view all the answers

    What is the depth of recuperation per day calculated from the data?

    <p>0.76 m (B)</p> Signup and view all the answers

    What is the diameter of the well needed to yield 3000 lph based on the calculations?

    <p>7.3 m (B)</p> Signup and view all the answers

    What is the sustainable water level (SWL) referenced in the content?

    <p>5.08 m (A)</p> Signup and view all the answers

    Which outcome may occur if the well is drilled deeper in the specified area?

    <p>Saltwater intrusion (C)</p> Signup and view all the answers

    What does the variable Q represent in the context of well yield?

    <p>The safe yield of the well (D)</p> Signup and view all the answers

    How is the specific yield of the soil C commonly determined?

    <p>From a recuperation test (C)</p> Signup and view all the answers

    If the area A of the well is 1 m² and the working depression head H is 1 m, what does Q equal?

    <p>1.00 hr–1 (C)</p> Signup and view all the answers

    What is the specific yield C of fine sand?

    <p>0.50 hr–1 (C)</p> Signup and view all the answers

    In the provided example, what was the total volume of water pumped during 1 12 hours?

    <p>32.22 m³ (A)</p> Signup and view all the answers

    What change occurred to the water level after pumping for 1 12 hours?

    <p>Dropped to 5.93 m (B)</p> Signup and view all the answers

    What was the water level in the well before pumping started?

    <p>5.08 m bgl (B)</p> Signup and view all the answers

    What does the safe working depression head H refer to?

    <p>The decrease in water level during pumping (C)</p> Signup and view all the answers

    Study Notes

    Ground Water

    • Ground water is a replenishable resource unlike other Earth resources.
    • Modern trends focus on recharging ground water from natural sources (rain, percolation dams).
    • Ground water is free from pollution and atomic attacks.
    • It's economical and efficient for irrigation, especially in times of low surface water supply.

    Types of Aquifers and Formations

    • An aquifer is a water-bearing geologic formation that transmits water at a rate suitable for extraction.
    • Good aquifers include:
      • Unconsolidated gravels, sands, and alluvium
      • Lake sediments and glacial deposits
      • Sandstones
      • Limestones with cavities (caverns)
      • Granites and marble with fissures and cracks
      • Weathered gneisses and schists
      • Heavily sheeted quartzites
      • Vescicular basalts
      • Slates
    • An aquiclude absorbs water but doesn't transmit it significantly (clays, shales).
    • An aquifuge neither absorbs nor transmits water (basalts, granites).
    • An aquitard transmits water at a slow rate (clay lenses interbedded with sand).

    Confined and Unconfined Aquifers

    • Unconfined Aquifer (Water-table aquifer): A porous formation extends from the ground surface to an impermeable layer below. Groundwater table (GWT) develops. Wells drilled into these aquifers are called water table wells.
    • Confined Aquifer (Artesian aquifer): A porous formation is sandwiched between two impermeable layers (aquicludes). Water pressure within is high, and wells may be flowing artesian wells (water level above ground surface) or non-flowing artesian wells (water level below ground surface).

    Specific Yield and Specific Retention

    • Specific yield (S): The percentage of total volume of saturated aquifer that drains by gravity.
    • Specific retention (Sr): The percentage of the volume of water held in the void spaces by molecular and surface tension forces.
    • Porosity equals specific yield plus specific retention.

    Storage Coefficient

    • The volume of water discharged from a unit prism of aquifer when the water table drops by one unit. It's dimensionless.
    • For artesian aquifers, the storage coefficient accounts for the compressibility of the aquifer and the expansion of pore water.

    Darcy's Law

    • Darcy's Law (1856) states the relationship between the velocity of water flow in a porous medium and hydraulic gradient (slope).
    • Velocity of flow is directly proportional to the hydraulic gradient
    • Q = KiA where Q is the volume flow rate, K is the hydraulic conductivity, i is the hydraulic gradient, and A is the cross-sectional area.

    Transmissibility

    • Transmissibility (T) is a measure of an aquifer's capacity to transmit water.
    • It is the flow capacity per unit width under a unit hydraulic gradient.
    • For confined aquifers, T = Kb; for water table aquifers, T = KH.

    Well Hydraulics

    • Steady radial flow into a well.
    • Water table conditions (unconfined aquifer) The rate of flow is calculated using equations specific to the conditions.
    • Artesian conditions (confined aquifer).

    Well Hydraulics

    • Steady radial flow into a well.
    • Water table conditions (unconfined aquifers) and artesian conditions (confined aquifers)
    • Calculations for yield and drawdown are presented using formulas involving transmissibility (T), hydraulic conductivity (K), and geometric factors related to well and aquifer dimensions.

    Specific Capacity

    • The discharge per unit drawdown in a well (usually expressed as liters per minute per meter of drawdown).

    Cavity Wells

    • A type of well especially used in alluvial strata that has a stiff clay layer underlying it.
    • High yields are possible because flow into the hollow cavity can be relatively easy.
    • The design and characteristics of these wells are different from more conventional well designs.

    Well Spacing

    • Wells should be sufficiently far apart so that their cones of depression do not overlap.
    • Spacing is calculated according to the size of the aquifer and the anticipated pumping rate. A wider spacing is recommended in rockier areas.

    Open Well Construction

    • In alluvial areas with vertical impervious steining, the well size and depth directly influence the water yield.
    • In rocky areas, wider well diameters generally aren't required but deeper wells may be beneficial instead. Alternate bands of masonry are sometimes laid dry (without cement mortar) to improve well efficiency.

    Well Tests (Pumping and Recuperation Tests)

    • Used to estimate a well's yield.
    • Pumping tests involve pumping at a constant rate and measuring the drawdown (drop) in water levels over a period.
    • Recuperation tests involve stopping pumping and measuring the time it takes for water levels to return to their previous state.

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    Explore the essentials of ground water and its significance as a replenishable resource. This quiz covers various types of aquifers and geological formations that influence water extraction. Learn about the benefits of ground water for irrigation and its natural replenishment methods.

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