Groundwater Development For Portable Water Supply
Groundwater hydrology may be defined as the science of the occurrence distribution, and movement of water below the surface of the earth. Geochydrology has an identical connotation, and hydrogeology differs only by its greater emphasis on geology. Utilization of groundwater dates from ancient times, although an understanding of the occurrence and movement of subsurface water as part of the hydrologic cycle has come only relatively recently.
SCOPE: Groundwater referred to without specification is commonly understood to mean water occupying all the voids within geologic stratum.
This saturated zone should be distinguished from an unsaturated, or earation zone where voids are filled with water and air. Water contained in saturated zone is important for engineering work, geologic studies and water supply development consequently, the occurrence of water in this zones will be emphasized here. Unsaturated zone a re usually found above saturated zones and extend uquad to the ground surface. Because this water includes soil masture within the root zone, it is a major concern for agriculture, binary, and soil science. No rigid demarcation of water between the two zones is possible, for they possess an interdependent boundary and water can move from owe zone to the other in either direction. The interrelationships are described more in some higher hydrogeology texts.
Table of Contents
- Title page
- Letter of transmittal
- Table of contents
- 1.0 Historical background and groundwater theories
- 1.1 Water theories
- 1.2 Recent centuries
- 2.0 Importance of groundwater
- 2.1 Groundwater in the hydrologic cycle
- 2.2 Occurrence of groundwater
- 2.3 Rock properties affecting groundwater
- 2.4 Vertical distribution of groundwater
- 2.5 Types of aquifers
- 3.0 Searching / exploration of groundwater
- 3.1 Methods of groundwater exploration
- 3.2 Groundwater basin investigation
- 3.3 Data collection and fieldwork
- 4.0 Drilling for groundwater (wells)
- 4.1 Test whole and well logs
- 4.2 Methods for drilling shallow wells
- 4.3 Methods for drilling deep wells
- 4.4 Quality of groundwater
- 4.5 Measures of water quality
- 4.6 Water quality coterie
- 5.0 Completion of wells
- 5.1 Well development
- 5.2 Protection of wells
- 5.3 Well rehabilitation
- 5.4 References
1.1 Historical Back Ground and Ground Water Theories
Groundwater development dates from ancient times the Old Testament contains numerous references to groundwater, springs, and wells, other that dug wells, groundwater in ancient times we supplied from horizontal wells known as QAUNATS. These persist to the present day and can be found in a band across the regions of the South Western Asia and North Africa extending from Aghanistan to Morocco. A cross section a long a qanat ie shown in fig 1.1 typically, a gently sloping tunnel dug through alluvial material leads water by gravity flow beneath the water table at its upper end to a ground.
A vertical cross section along a qanat surface outlet and irrigation canal at its lower end. Vertical shafts dug at closely s paced intervals provide access to the tunnel. Qanats are laboriously hand constructed by skilld workers employing techniques that date back 3000 years.
Iran possesses the greatest concentration of qanats; here some 22,000 qanats supply 75 percent of all water used in the country. Lengths of qanats extend up to 30km, but most are less than 5km. The depth of the qanat mother well (see fig 1.1) ie normally less than 50m, but instances of depth exceeding 250m have been reported. Discharge of Qantas varies. Seasonally with water table fluctuations and seldom exceed 100m3/hr.
1.2 Groundwater Theories
Utilization of groundwater greatly preceded understanding of its origin, occurrence, and movement. The writing of Greek philosophers to explain origins of springs and groundwater contain theories ranging from fantasy to nearly correct accounts. As late as the seventeenth century it was generally assumed that water emerging from springs could not be derived from rainfall, for it was believed that the quantity was in adequate and the earth too impervious to permit penetration of rain water for below the surface. Thus, early Greek philosophers such as Homer, Thates and Plato hypothesized that springs were formed by seawater. Conducted through subterranean channels below the mountains, then Aristotle suggested that air enters cold dark caverns under the mountains where it condenses into water and contributes to springs.
The Roman philosophers, including Seneca Pliny, followed the Greek ideas and contributed little to the subject. An important step forward, however was made by the Roman architect Vitnvius he explained the now accepted infiltration theory that the mountains receive large amounts of rain that percolate through the rock strata and emerge at their base to form streams.
The Greek theories persisted through the Middle Ages with no advances until the end of the Renaissance. The French Poffer and Philosopher Bernard Palissy (1510 – 1589) reiterated the infiltration theory in 1580, but his teachings were generally ignored. The German astronomer Johannes Kepler (1571 – 1630) was a man of strong imagination, who likened the earth to a huge animal that takes in water of the ocean, digests and assimilates it, and discharges the end products of these physiological processes as groundwater and springs. The seawater theory of the Greeks, supplemented by ideas of vapourizaton and condensation processes within the earth, was restated by the French Philosopher Rene’ Descarfes (1596 – 1650).
A clear understanding of the hydrologic cycle was achieved by the latter part of the seventeenth century. For the first time theories were based on observations and quantitative data. Three European countries made notable contributions, although others contributed to and supported these advances. Pierre Perrault (1611 – 1680) and estimated runoff of the upper sein drainage basin. He reported in 1674 that precipitation on the basin was about six times the river discharge, thereby demonstrating false the early assumption of inadequate rainfall.
The French Physicist Edme Mariotte (1620 – 1684) made measurements of the same of paris and confirmed paraults work. His publication appeared in 1686, after his death, and contained factual data strongly supported the infiltration theory. Meinzer once stated. Mariotte probably deserves more than any other man the distinction of being regarded as the founder of groundwater hydrology, perhaps I should say the entire science of hydrology”. The third contribution came from the English astronomer Edmund Halley (1656 – 1742), who reported in 1693 on measurements of evaporation demonstrating that sea evaporation was sufficient to account for all springs and stream flow.
1.3 Recent Centuries
During the eighteenth century fundamentals in geologic were established that provided a basis for understanding the occurrence and movement of groundwater. During the first half of the nineteenth century many artesian wells were drilled in France stimulation interest in groundwater. The French hydraulic engineer Henry Darcy (1803 – 1858). Studied the movement of water through sand. His treatise of 1856 defined the relation, now known as Darcy’s law, governing groundwater flow in most alluvial and sedimentary formations. Later European contributions of the nineteenth century emphasized the hydraulics of groundwater development. Significantly contributions were made by J. Boussineq, G.A. Daubree, J. Dupuru and A. Thiem. In the twentieth century, increased activity in all phases of groundwater hydrology has occurred. Many Europeans have participated with publications of either specialized or comprehensive works. There are too many people to mention them all, but R. Dachler, E. Imbeaux, K. Keihack are best known in the United States.
American contributions to groundwater hydrology date from near the end of the nineteenth century. In the past 90 years, tremendous advances have been made. Important early theoretical contributions were made by A. Hazem, F.H. King while detailed field investigations were begun by men such as T.C Chamberline, N.H Darton through his consuming interest in groundwater and his dynamic leadership of groundwater activities of the U.S geological survey, stimulated many individuals in the quest for groundwater knowledge.
In recent decadeds the publications of M.S. Hantush, C.E. Jacob. Within the lat 20 years the surge in university research on groundwater problems, the establishment of professional consulting firms specializing in water resources, and the advent of the digital computer have jointly produced a competence for development and management of groundwater resources that was nonexistent hereto fore.
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Completion of Wells
After a well has been drilled, it must be completed. This can involve placement of casing, cementing of casing, placement of well screens, and gravel packing, however, wells in hard rock formations can be left as open holes so that these components may not be required.
Well casing serves as a lining to maintain an open hole from ground surface to the aquifer. It seals out surface water and any undesirable groundwater and also provides structural support against caving materials outside the well. Materials commonly employed for well casing are wrought iron, alloyed or unalloyed steel, and ingot iron. Joints normally consist of threaded couplings or are welted, the object being t o secure water tightness.
Wells are cemented in the annualar space surrounding the casing to prevent entrance of water of unsatisfactory quality to protect the casing against exterior corrosion and/or to stabilize caving rock formations. Cement grout, consisting of a mixture of cement and water and sometimes various additives, can be placed by a pump bailer, by a tremie pipe, or by pumping. It is important that the grout be introduced at the bottom of the space to be grouted to ensure that the zone is properly sealed.
In consolidated formations, where the material surrounding the well is stable, groundwater can enter directly into an uncased well. In unconsolidated formations, however, wells are equipped with screens. These stabilize the sides of the hole, prevent sand movement into the well and allow a maximum amount of water to enter the well with a minimum of hydraulic resistance.
Screens are made of a variety of metals and metal alloys, plastics concrete, asbestos cement, fibre glass reinforced epoxy, coated base metals and wood. Because a well screen is particularly susceptible to corrosions and incrustation, nonferrous metals alloys and plastics are often selected to prolong well life and efficient operation.
Following completion: a new well is developed to increase its specific capacity, prevent sanding and obtain maximum economic w ell life. These results are accomplished by removing the finer material from the natural formations surrounding the perforated sections of the casing. Where a well has been gravel packed, much of the same purpose has been accomplished, although development is still beneficial. The importance of developing wells cannot be under estimated all too often development is not carried out adequately to produce full potential yields.
Development procedures are varied and include pumping, surging, use of compressed air, hydraulic getting, and addition of chemicals, hydraulic fracturing and use of explosives. These are briefly described in subsequent paragraphs.
This procedure involves pumping a well in a series of steps from a low discharge to one exceeding the design capacity. To be more effective the intake area of the pump should extend to near the centre of the screened section. At each step the well is pumped until it clears after which the power is shut off and water in the pump column surges back in to the well. The step is repeated until only clear water appears. The discharge rate is than increased and the procedure repeated until the final rate is the maximum capacity of the pump or well. The coarser fraction entering the well is removed by a boiler or sand pump from the bottom. This development method of by pumping is recommended as a finishing procedure after any of the development techniques described subsequently.
Another method for developing a well is by the up and down motion of a surge block attached to the bottom of a drill stem. Such blocks are particularly applicable with a cable tool rig. Solid, vented and spring loaded surge blocks, often constructed by well drilling contractors, are employed. The cylindrical block is 2 to 5cm smaller than the well screen and fitted with belting rubber leather that will not damage the screen; as the block is moved up and down in the screen a surging action is imparted to them. The down stroke causes back wash to break up any bridging that may occur, while the upstroke pulls dislodged sand gram in the well.
Initially, surging should begins with a slow stroke at the bottom of the screen and progress to the top of the screen. This then be repeated with increasingly faster strokes. The procedure is completed when material accumulating in the bottom of them becomes negligible for wells in rock aquifers, surging can be accomplished in the casing above open holes.
- SURGING WITH AIR
- BACKWASTING WITH AIR
- HYDRAULIC JETTING
Hydraulic fracturing, a technique borrowed from the petroleum industry, is occasionally employed to enhance the yield of open hole rock wells. Inflatable packers on pipe extending to ground surface isolate a section of aquifer. After filling the pipe with isolated section with water, pump pressure is applied to fracture the rock. Sand is sometimes pumped into the section to force the grains into the rock fractures so as to maintain the openings.
Donation of explosives in rock wells often increases yields by enlarging the hole, increasing rock fractures, and removing fine grained deposits on the face of the well bore.
Protection of Wells
Sanitary Protection: Wherever groundwater pumped from a well is in tended for human consumption proper sanitary precaution must be taken to protect the water quality, pollution sources may exist either above or below ground surface.
Surface pollution can either enter the well through the annular space outside of the casing or through the top of the well its self to prevent foreign bodies from entering the well or undesirable water outside the casing, the annular space should be filled with cement grout for deep well water light should be provided to avoid the entrance of undesirable material from entering the well.
Whenever a new well is completed or an old well repaired, contamination from equipment, well materials, or surface water may be introduced into the well. Addition and agnation of a chlonne compound will disinfect the well. The following disinfection, the well should be pumped to waste until all traces of chlomie are removed. As a final check on the portability of the water, a sample should be collected and sent to a laboratory for bacteriological examination.
FOST PROTECTION: In regions where winfer frost occurs, it is important to protect pumps and water lines from freezing. The pitless Adapter, attached to the well casing, provides access to the well, while the discharge pipe runs about 2m underground to the basement of the house.
Abandonment of Wells: Whenever a well is abandoned, for whatever reason, it should be sealed by filling it with clay, concrete or earth. Not only is surface contamination them unable to enter the well, but sealing serves other useful purposes, prevents accidents, avoids possible movement of inferior water from one aquifer to another and conserves water in flowing wells.
A well, properly drilled, cased, and developed, will little attention. Many wells fail, however, that is the yield decreasing quantities of water with time, well rehabilitation refers to the treatment of a production well by mechanical, chemical or other means to recover as much as possible of the cost production capacity.
One cause of failure is depletion of ground supply, not a fault of the well, this trouble can sometimes be remedied by decreasing pumping drafts, resetting the pump, or deepening the well.
A second cause of well trouble results from faulty well construction, such items as poor casing connections, improper perforations or screens, incomplete placement of gravel packs, and poorly seated wells are typical of difficulties encountered.
Depending on the particular situation as determined from a television or photographic survey of the well it may be possible to repair the well, but sudden failures involving entrance of sand or collapse of a casing often requires replacement of the entire well.
The third and the most prevalent cause of well failure results from corrosion or incrustation of well screen. Corrosion may result from direct chemical action of the groundwater or from electrolytic action caused by the presence of two different metals in the well. The effects of corrosion can be minimized by selecting nonmetallic well screens or one of corrosion resistant metal (such as nickel, copper, or stainless steel) and by providing cathodic protection.