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Ground Water, continued
Recovery
Principles of Recovery
When a well is at rest, the pressure of the water outside the well is equal to the pressure of the water inside. When water is removed from the well by pumping, the resulting drawdown or lowering of the water level produces a differential in head or pressure and water flows into the well. When water is being discharged from a well, the water table is lowered in an area around the well to form a depression in the water table that somewhat resembles an inverted cone. This depressed area is known as the cone of depression or the cone of influence. As the pumping rate of the well is increased, the drawdown becomes greater. When a well is first pumped the water level will fall very rapidly, but as pumping is continued at a steady rate the drawdown increases at a diminishing rate. When pumping is stopped, the recovery is rapid at first but gradually tapers off and may continue for many hours or for several days.
The yield or capacity of a well is the continuous rate at which it will yield water after the water stored in the well has been removed. The capacity depends upon the amount by which the water level can be lowered, the thickness and permeability of the water-bearing bed, and the construction and condition of the well. The capacity of a well generally is expressed in gallons a minute.
The specific capacity of a well is the rate of yield per unit of drawdown and is commonly expressed in gallons a minute per foot of drawdown. If a well yields 50 gallons per minute and has a drawdown of 10 feet when pumped at that rate, the specific capacity of the well is 5 gallons a minute per foot of drawdown. In testing the specific capacity of a well, pumping is continued until the water level remains approximately stationary.
Dug Wells
Dug wells are constructed by excavating with picks and shovels or by power machinery. Those in Marshall County are generally more than 2 feet in diameter and are relatively shallow. Dug wells may be constructed in nearly any type of geologic formation, but in Marshall County they are most common in areas where the aquifers are consolidated bedrock or sand and gravel deposits lying near the surface. This type of well is simply a pit penetrating the water-bearing rocks and walled with rock, brick, or concrete. The advantage of a dug well is the large infiltration area and storage reservoir provided by the large diameter. Dug wells are, in general; more subject to contamination and failure during dry weather than deeper drilled wells.
Bored Wells
Bored wells are made by augers or post-hole diggers. Such wells are limited to unconsolidated sediments. Bored wells are generally 12 inches or more in diameter and are generally cased with bell-joint tile. Contamination by surface water seeping through the tile joints is likely to occur in bored wells.
Driven Wells
Some of the wells in the major valleys of Marshall County are driven wells, generally made by driving a 1 1/4- or 1 1/2-inch pipe, equipped at the bottom with a screened drive point, below the water table. Such wells generally are constructed only where the water-bearing material is sufficiently permeable to permit water to flow freely into the pipe, where the material is unconsolidated enough to permit a pipe to be driven, and where the depth to water level is not more than about 20 feet below the land surface. Where the surface material will not allow a pipe to be driven, the upper part of the well is dug or augered with a post-hole digger, and the pipe is then driven into the bottom of this hole. Driven wells are sometimes equipped with a pitcher pump; if the water level is too deep for a pitcher pump, a cylinder is placed at the bottom of a dug or augered part of the well and the driven pipe placed below this part.
Drilled Wells
Most of the wells in the areas of thick unconsolidated sediments, as well as many of the deeper wells in other parts of the county, are of the drilled type. Wells may be drilled either by the percussion method or by the hydraulic-rotary method. The drilled wells for stock and domestic use in Marshall County are generally 3 1/2 to 8 inches in diameter and are cased with galvanized-iron or wrought iron casing.
Drilled wells obtaining water from consolidated sediments that will not cave are generally cased only in the upper part. Water may enter the well along the entire uncased part of the hole, only the surface seepage water being cased off.
Drilled wells obtaining water from unconsolidated sediments are generally cased to the bottom of the hole. Water enters the well through the bottom of the casing, and, if they are provided, through perforations in the casing. The size of the perforations is a very important factor in the construction of a well. They should be small enough that only the finest material can enter the well, but they should be large enough to allow free flow of water into the well and not become clogged by sand particles. Care should be taken to locate the perforated sections of casing within the water-bearing beds. Some wells in unconsolidated sediments are equipped with special well screens or strainers. By selecting the proper slot size in a well screen, a certain amount of the finer water-bearing material is passed through the screen and the coarser material is retained to form a natural gravel packing around the screen.
Municipal and industrial wells in unconsolidated materials may be artificially gravel-packed. In this type of construction a large-diameter hole is first made and cased. A smaller casing with sections of well screen spaced to correspond to the water-bearing beds is then centered in the hole and the annular space between the large casing and the smaller casing is filled with carefully selected gravel. The larger casing then is withdrawn to permit the water to flow into the well. This type of construction increases the effective diameter of the well and helps to prevent fine sand from entering the well.
Methods of Lift and Types of Pumps
Most of the domestic and stock wells in Marshall County are equipped with lift or force pumps. The cylinders in lift pumps and in force pumps are similar and are below the land surface. A lift pump generally discharges water only at the pump head, whereas, a force pump can force water above this point. Stock wells in Marshall County are operated generally by windmills or electric motors, but many of the domestic wells are hand operated. Public-supply wells are equipped almost entirely with turbine pumps powered by electric motors. Electrically powered jet pumps with automatic controls are used on many of the modern domestic installations in the county.
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Kansas Geological Survey, Geologic History of Kansas
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Web version March 2004. Original publication date March 1954.
URL=http://www.kgs.ku.edu/General/Geology/Marshall/05_gw3.html