(Aktu Btech) Engineering Hydrology Important Unit-5 Groundwater Quality and Modals

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Important Questions For Engineering Hydrology:
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Q1. Write short note on well construction.

Ans. Well Construction: Drilling, installing the casing, installing a well screen and filter pack, if necessary, grouting to offer sanitary protection, and developing the well to ensure sand-free operation at maximum yield are the four or five distinct processes that comprise well construction.

• 1. Drilling Methods:
  • i. Wells can be drilled, bored, driven, jetted, or excavated. In favourable conditions, simple drilling methods such as drive point, jetting, and hand boring can be used to produce shallow wells up to 25 cm diameter and 45 m deep.
  • ii. Drilling equipment selection is influenced by the hydrogeology of the formation, the diameter and depth of the production well, the availability of funds, maintenance and parts, production capacity, volume of work, operating staff, and the ease with which the rig can be moved.
• 2. Installation of Well Screen: The common methods adopted in the case of naturally developed wells are given in the following:
  • i. Pull back method.
  • ii. Open hole method. 
  • iii. Baildown method. 
  • iv. Washdown method. 

Q2. Describe in brief various method of developing a tube well. What do you mean by recuperation test ?

Ans. A. Development of Tube Wells: It is the process of removing finer particles from surrounding the screen in order to increase the permeability of the formation, which allows water to migrate towards the well.

B. Method: Following are the methods commonly adopted for development of a tube well:

• 1. Development by Pumping:
  • i. It is the most basic and widely used method of eliminating tiny particles.
  • ii. Water is ultimately pumped from the well at a rate equal to or greater than the planned discharge in this approach. As a result, it is a case of over-pumping.
  • iii. A big capacity variable speed pump is used.
  • iv. At first, water is extracted at a relatively sluggish rate. The withdrawal rate is then gradually increased. The rate of withdrawal is kept constant between phases until no more sand particles are removed.
• 2. Development by Compressed Air: 
  • i. The basic components of this assembly are a smaller diameter air pipe (air line) and a larger dimension drop pipe.
  • ii. Because the arrangement is similar to that of an air lift pump, the drop pipe is also known as a discharge pipe.
  • iii. An air compressor is connected directly to an air tank, which is connected to the air pipe via a quick opening valve.
  • iv. An assembly of smaller diameter air pipe and drop or discharge pipe enclosing the air pipe is put into the well until it reaches approximately the bottom of the first strainer pipe.
  • v. The air pipe is positioned so that its bottom end is approximately 30 cm above the lower end of the discharge pipe. It is known as the pumping position of the air pipe.
• 3. Development by Surging: 
  • i. The reciprocating movement of a plunger in the well creates a surge.
  • ii. Water goes alternately into the soil and out of the well during the downward and backward strokes.
  • iii. The plunger’s speed is gradually increased. The plunger is operated in the casing pipe provided above the well’s screened area.
  • iv. The repeated application of surging force pushes tiny particles into the well while leaving coarser particles in the aquifer unaffected.
• 4. Development by Back-Washing:
  • i. As the name implies, it is a procedure that causes water from the well to flow into the aquifer formation via the screen.
  • ii. Back-washing agitates the formation and breaks down the bridging of sand particles.
  • iii. Back-washing aids in the effective removal of tiny particles.
• 5. Development by High Velocity Jetting:
  • i. It is one of the most successful development approaches.
  • ii. In this procedure, high velocity jets emitted by the jetting tool travel through the screen, agitating the formation behind the screen.
  • iii. It loosens fine particles in the well that can be eliminated by pumping or bailing the well water.
• 6. Development by Using Chemicals:
  • i. Dispersing agents are frequently used to backwashing or jetting water.
  • ii. Dispersing agents prevent clay’s ability to attach to sand particles.
  • iii. Common and effective dispersing agents include tetrasodium pyrophosphate, sodium tripolyphosphate, sodium hexametaphosphate (calgon), and sodium deptaphosphate.
  • iv. Once the dispersing agent has neutralised the colloidal feature of the clay, it is easily removed by surging and back-washing.

C. Recuperation Test:

• 1. Although the constant level pumping test provides an accurate number for the sate yield of an open well, it might be difficult to manage the pump in such a way that the well maintains a constant level.
• 2. In such cases, a recovery test is performed.
• 3. During the recuperation test, the water level is reduced to any level below normal and the pumping is turned off.
• 4. The time taken for the water to recuperate to the normal level is noted. From the data, the discharge from the well can be calculated as : 

Q3. Write a short note on protection of water wells.

Ans. The protection of water wells can be consists of: 

• 1. Sanitary Protection: 
  • i. Sanitary precautions must be followed wherever groundwater extracted from a well is intended for human consumption to protect water quality. Pollution sources can be found both above and below ground.
  • ii. Surface contamination can enter wells through the annular space outside the casing or through the well’s top.
  • iii. To block access points for unwanted water outside the casing. For deep wells, the annular area should be filled with cement grout.
  • iv. By providing a watertight cover to seal the top of the casing, entry from the top of the well can be avoided.
  • v. Some pumps have closed metal bases that offer the required closure.
  • vi. A seal for the annular opening between the discharge pipe and casing is necessary for pumps with an open-type base or when the pump is not installed immediately over the well.
  • vii. Seals can be formed of metal or lead packing, but asphaltic and mastic compounds are also acceptable.
  • viii. Concrete covers around the well should be elevated above the nearby land level and slope away from the well.
• 2. Frost Protection: 
  • i. It is critical to protect pumps and water lines from freezing in areas where winter frost occurs.
  • ii. Make a pitless adopter available for frostproofing a home well.
  • iii. A pitless adapter affixed to the well casing allows access to the well, while the discharge pipe runs roughly 2 metres underground in the house’s basement.

Q4. Discuss in detail about the remediation of contaminated groundwater. 

Ans. A. Contamination of Groundwater: 

• 1. Although groundwater is more protected than surface water, it can be contaminated from a variety of sources.
• 2. Because water dissolves more substances than any other substance, it is very prone to contamination or pollution.
• 3. Contamination is a change in the physical, chemical, and biological qualities of groundwater that limits or prevents its usage for a variety of applications.
• 4. It can hinder water utilization and pose risks to public health through toxicity or disease spread.
• 5. In general, the terms contamination and pollution are used interchangeably in groundwater.
• 6. There are numerous contaminants that could be present in groundwater. They could be geogenic or anthropogenic in origin.
• 7. Aquifer material dissolution contamination is geogenic. Fluoride, arsenic, iron, and other heavy metals in groundwater are typically caused by contamination from geogenic sources.
• 8. Human activities and water consumption are related with anthropogenic sources and causes of groundwater pollution.
• 9. The majority of pollution is caused by rubbish disposal on or into the ground.
• 10. The principal anthropogenic sources and causes (continuous and/or accidental types) can be categorized as follows:
  • i. Municipal: Sewer leakage, liquid waste, solid waste, and landfill, 
  • ii. Industrial: Liquid waste, leakage, mining activity, and oil field brine, 
  • iii. Agricultural: Return lows, fertilizers and pesticides, and animal waste,   
  • iv. Miscellaneous: Salt water intrusion, acid rain, and de-icing salts.

B. Remedial Measures: Measures that can be adopted to prevent, reduce and eliminate the contamination are as follows:

• 1. Industrial and sewage waste pretreatment before disposal into lagoons or other permitted receivable locations.
• 2. Disposal basin lining to prevent leaching into groundwater.
• 3. Collecting and treatment of lactate from landfills, industrial basins, and sewage lagoons via drains and wells.
• 4. Appropriate groundwater pumping management to prevent, stabilise, or delay saltwater intrusion into the coastal freshwater aquifer.
• 5. Placement of wells, injection ridges, or pumping troughs to prevent seawater intrusion.
• 6. In highly populated regions, abandonment or banning of on-site septic tank and leach field systems, and replacement of sanitary wastes with the establishment of central or municipal sewer networks.
• 7. Wastewater desalination before to discharge.

Q5. Write the different phenomenon that occurs in the movement of contaminated water.

Ans.

• 1. A transport mechanism influences the migration of dissolved contaminant in saturated flow in granular aquifers. 
• 2. The different transport mechanisms are as follows: 
  • i. Advection. 
  • ii. Diffusion. 
  • iii. Dispersion. 
  • iv. Sorption. 
  • v. Decay. 
  • vi. Physical, natural, or anthropogenic activities. 
  • vii. Hydrolysis, volatilization, and biotransformation. 
  • viii. Transport in aquifers with a pronounced bimodal permeability distribution. 
  • ix. Chemical reactions.  
• 3. The first three processes (advection, diffusion, and dispersion) are physical fundamental processes that drive contaminant migration in groundwater when no natural or anthropogenic stresses/activities exist.
• 4. The advection-dispersion relationship is used to calculate mass transport in porous media. Advection and dispersion dominate bulk fluid and solute transfer in general.
• 5. Diffusion is a mass transport phenomenon that occurs in response to a concentration gradient. It is commonly associated with fluid advective and dispersive behaviour.
• 6. The solubility of a hazardous component in a groundwater system determines its transport rate and toxicity.
• 7. The key system factors that can influence solubility are pH, sorption to solids, and temperature.
• 8. The exchange of molecules and ions between the solid and liquid phases is referred to as sorption.
• 9. Adsorption and absorption are the two types of connections between aqueous/liquid and solid phases.
• Adsorption is the attachment of molecules and ions from the solute to the rock material, resulting in a drop in solute concentration.
• 11. This is often referred to as contaminant transport retardation.
• 12. Absorption is the interaction of solute molecules with solid phase molecules, and so absorption is the dissolution of a liquid material in a solid solvent.
• 13. Desorption refers to the movement of molecules and ions from the solid phase to the solute.

Q6. What do you understand by ground water exploration and define surface evidence ?

Ans. A. Groundwater Exploration: 

• 1. The process of estimating groundwater availability is known as groundwater prospecting, groundwater investigation, groundwater exploration, or groundwater targeting.
• 2.The goal of groundwater exploration is to use various approaches to discover aquifers capable of supplying water of adequate quality in economic quantities for diverse purposes.
• 3. The techniques used to access occurrence and quality of groundwater can be classified as:
  • i. Surface investigation.
  • ii. Sub-surface investigation. 
• 4. Surface research methods include geology, remote sensing, and geophysical methods (such as electrical resistivity, seismicity, magneticity, and so on), whereas subterranean investigation methods include well drilling and logging.

B. Surface Evidence: Surface features identified on aerial photographs that aid in evaluating groundwater conditions: 

• 1. Phreatophytes and aquatic plants. 
• 2. Geologic land forms likely to contain relatively permeable strata:
  • i. Modern alluvial terraces and floodplains.
  • ii. Stratified valley-fill deposits in abandoned meltwater and spillway channels. 
  • iii. Glacial outwash and glacial deltas. 
  • iv. Kames and kame-moraine complexes. 
  • v. Eskerine-kame complexes. 
  • vi. Alluvial fans. 
  • vii. Beach ridges. 
  • viii. Sand dunes assumed to overlie sandy glaciofluvial sediments.
• 3. Lakes and streams:
  • i. Drainage density of stream network.
  • ii. Localized gain or loss of streamflow. 
  • iii. Nearby small perennial and intermittent lakes (e.g., lakes in outwash, elongate saline lakes in inactive drainage systems). 
  • iv. Perennial rivers and larger creeks in valleys having inactive floodplains. 
  • v. Small intermittent drainages (including misfit creeks in abandoned glacial spillways and meltwater channels). 
  • vi. No defined drainage channel in former glacial spillways and meltwater channels. 
• 4. Moist depressions and seepages: 
  • i. Moist depressions, marshy environments, and seepages (significance depends on interpretation of associated phenomena). 
  • ii. String of alkali flats or lakes (playas, salinas) along inactive drainage systems. 
• 5. Springs:
  • i. Depression springs (where land surface locally cuts the water table or the upper surface of the zone of saturation). 
  • ii. Contact springs (permeable water-bearing strata overlying relatively impermeable strata).

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