lecture 7 water wells

8/2/2019 Lecture 7 Water Wells

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Chapter 7: Groundwater and Well Design

Chapter 7 Key Concepts:

Groundwater principles and components

Well description and componentsWell design and calculation

The use of groundwater and the construction of wells are other aspects of a water system

that require design knowledge. Groundwater is generally less contaminated than surface water

and has less stringent contaminant regulations.

About Groundwater

Groundwater is one part of the hydrological cycle (see Figure 1), which includes the

oceans, surface water, the atmosphere, ice and groundwater. In the ground, large quantities of

water are stored in aquifers where most groundwater is drawn from. Groundwater is the

principal source of water for small water systems in California and across the country.

Figure 1: Diagram of the hydrological cycle47

(Water Science [website])


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Springs and wells are the main types of groundwater sources. Springs are the water of

aquifers that reach the surface and wells retrieve water from underground aquifers. Well

construction requires multiple aspects of design and engineering. Wells may be either vertical

which is most common or horizontal into the side of a hill. Springs are easily assessable

however further design and engineering is likely since this kind of source may be under the

influence of a surface water and likely requires treatment. The rest of the chapter is focused on

wells and the design and construction of wells. Figure 2 shows a typical well profile.

Figure 2: Typical well profile with various levels of ground and aquifer48

(Santa Clara p. 4)

There are two types of aquifers: confined and unconfined. Unconfined aquifers are

restricted below a certain depth by an impervious stratum. Confined aquifers are not restricted

by a barrier.

Well System Description

Types of Well Systems

A well system is usually placed in a pump house in order to protect the equipment. There

are two versions of a well system: above-ground discharge (figure 2) and below-ground


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discharge (Figure 3). With above-ground, the well system equipment is more accessible. With

below-ground, the well equipment is less accessible.

Figure 2: Example of above-ground discharge with submersible pump in a pump house


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Figure 3: Example of below-ground discharge with submersible pump

Types of Well Pumping Systems

Table 1: Types of Well Pumping Systems49

(Water Well Standards 74-81, p. 37)

Type of WellPumping System

Description Diagram of Installation49

(WaterWell Standards 74-81, p. 37)

Turbine Pump The turbine pump, which isdescribed in detail in Chapter 6, iseasier to maintain than thesubmersible pump for wellsystems.

Submersible Pump For deep water wells, thesubmersible electric pump is themost common choice. The pumphas several stages to generateenough pressure to lift the waterout of the well and to pressurize itsufficiently for use. The electricmotor is long and narrow so it canfit down into the well casing forwells as small as 4 inches in

diameter. Overall, submersiblepumps are good for deep wells andare highly efficient.


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Type of WellPumping System

Description Diagram of Installation49

(WaterWell Standards 74-81, p. 37)

Jet Pump Jet pumps recirculate partof the water delivery back to thesuction line to raise the pressure inthe suction line sufficiently to

prevent pump damage bycavitation (low-pressure boiling) inthe pump impeller. The deeper thewell, the greater the fraction ofdelivered water must berecirculated. The maximumpractical lift is limited toapproximately 200 feet byeconomics.

Pressure Tank

A pressure tank is usually included in a well system to delay pump turn-on and

eliminate frequent, short on/off cycles. When water is used, the pressure tank drains

water into the plumbing system to delay pump turn-on. If the well system pressure

lowers to the turn-on pressure typically 20 psi then the pump turns on and it does not

turn off until the air in the pressure tank is compressed to the turn-off pressure, which is

typically 40 psi. With a pressure tank, the well pump does not burn up as fast and pump

run time is extended. The pressure tank is not meant to provide water storage.

Pressure tanks can contain a diaphragm or bladder that is charged by an air

compressor to roughly 75% of the turn-on pressure. These pre-charged tanks allow a

smaller tank volume to produce a larger useable storage volume.


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Figure 4: Typical pressure tank set up (left) and pre-charged pressure tanks (right)

Well Construction and Abandonment

Wells are typically constructed using a drilling method with steel casing and

bentonite or soil concrete grout sealer. Older methods include well construction by

digging and pile driving (see Figure 5). However, digging wells can be extremely

dangerous because of possible cave-ins and are more susceptible to surface water

contamination.

Figure 5: Well construction methods


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In present day, well construction is fairly standardized with an established collection of

steps. The major construction steps include:

Well drilling by cable, air hammer and drill bit, or reverse rotary

Welding the steel casing or sealing PVC

Placing the casing in the well

Mixing bentonite or soil concrete grout

Pumping in the bentonite soil concrete grout

Most wells are constructed by drilling about 10 to 20 feet at a time and placing the casing

section by section instead of drilling the entire well at one time and then placing all the casing.

Wells that are abandoned or not used should be decommissioned to prevent surface

contamination from entering the well and affecting the underground aquifers. To decommission,

a well casing is pulled then grout is pumped to fill and seal the well.

Figure 6: An abandoned well (left) and a properly decommissioned well (right)

Well Design

California Well Standards

Standards for well design, construction, and operation in California are set by the

Department of Water Resources (DWR) under bulletins 74-81 and 74-90 (see Figure 7). The

standards in both bulletins must be met for water wells, monitoring wells, and cathodic

protection of wells.


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Figure 7: Timeline of California DWR well standards50

(California Well Standards 74-90 p. 5)

Well Offset

Well offset is the distance from a well to certain activities or objects to help prevent well

contamination. Requirements for well offsets are set by California and often depend on the

potential contamination impact of the activity or object (see Figure 8).


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Figure 8: Example of well offset requirements for various activities 48(Santa Clara p. 7)

Well Seal and other Contamination Prevention

Well seal is used to prevent surface water from running along the well line and is

typically applied along sections of the well line that may contact poor quality water (see Figure

9).


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Figure 9: Typical well seal profiles49

(Water Well Standards 74-81 p. 47)

To prevent well contamination from surface water, sealing a well to a certain depth is required

(see Table 2). This depth varies based on well location and type.

Table 2: California rules for well seal requirements50

(California Well Standards 74-90 p. 14)

Well Type Minimum Depth Seal Must Extend BelowGround Surface

Community Water Supply 50 feetIndustrial 50 feetIndividual Domestic 20 feetAgricultural 20 feetAir-Conditioning 20 feetAll Other Types 20 feet

Sealing the submersible pump and a water tight well cap are other ways of preventing well

contamination (see Figure 10).


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Figure 11: Well screen (left)

51

(How Wells are Designed) , submersible pump seal (center)

52

(Ground WaterManual) and water tight cap (right)53

(Home*A*Syst: Protecting Well Water Supply)

Pressure Tank Design

The amount of water that is drained from a pressure tank between the turn-on and turn-

off points of the pump is called useable storage capacity and depends on the tank volume and the

set points. Normally the turn on point is at 20 psi, but may be as high as 30 psi. Turn off usually

occurs at 40-psi but may be 50 psi. The spread between the on and off points is normally 20 psi.

Only a small fraction of the tanks volume is useable storage, 6.5 gallons for a 42 gallon tank

between 20 and 40 psi (see Figure 11).

Figure 11: Design and useable storage capacity of uncharged pressure tanks


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The recommended size of a pressure tank is 10 times the pump flow rate for 1 minute for

non-charged tanks, and 6 times the pump flow rate for 1 minute for pre-charged tanks. So a 10

gpm pump should have a 100 gallon non-charged tank or a 60 gallon pre-charged tank. This

insures the pump will operate for nearly 2 minutes before shut-off every time it start and frequent

starting and stopping is avoided.

Specific Capacity Calculation

To design a well, the specific capacity is a key factor that must be determined. Well

drawdown is one concept used in specific capacity calculations (see Figure 12) and it is the

change in water level from static water level to the level when the well pump is running for at

least and 8 hour period.

Figure 12: Well drawdown diagram and calculation54 (Understanding Your Water Well [website])

A pump capacity test is used to determine specific capacity. In Figure 13, the pumping test was

run for 24 hours at 2,500 gpm with a 21 feet drawdown. Specific capacity is calculated as:

Specific Capacity = 2,500 gpm/21 feet = 119 gpm/ft


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Figure 13: Example of pump capacity test results 55(Specific Capacity [website])

Transmissivity

Transmissivity is a factor determined for wells that is simple to calculate once specific

capacity is known. It measures how large of an area the well pumping affects the aquifer.

Below are the equations used to calculate transmissivity:

T = 1500 * Q/s (for an unconfined aquifer)

T = 2000 * Q/s (for a confined aquifer)T = Transmissivity, in gallons per day per foot, gpd/ft

Q/s = Specific Capacity, in gallons per minute per foot, gpm/ft

With greater transmissivity, the wells radius of influence is greater (see Figure 14). Also, the

effect of drawdown is less when transmissivity is greater.

Figure 14: Diagram of well drawdown and radius of influence for two transmissivities


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Safe Yield

Safe yield is a measure of the highest flow rate possible without any risk of drawdown to

the well screen. Below is the equation for safe yield:

SY = AW * SC

SY = Safe Yield (gpm)

AW = Available Water (ft.)

SC = Specific Capacity (gpm/ft.)

Available water is the distance between the static water level and the top of the well screen.


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Well Inspection

Figure 15 is a good general guideline to follow when inspecting wells for potential

failures or hazards.

Figure 15: Guidelines for well inspection48

(Santa Clara p. 5)


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References

47. Water Science for Schools: The Water Cycle. 7 Nov. 2008. United States Geological

Survey. 26 Feb. 2008. .

48. A Guide for the Private Well Owner. May 2001. Santa Clara Valley Water District. 20April 2009. .

49. Water Well Standards: State of California (Bulletin 74-81). December 1981. California

Department of Water Resources. 20 April 2009. .

50. California Well Standards: Bulletin 74-90. June 1991. California Department of WaterResources. 20 April 2009. .

51. How Wells Are Designed. American Ground Water Trust. 16 March 2009.

.

52. Ground Water Manual for Small Water Systems. 1999. Montana State University:

The Montana Water Center. 16 March 2009. < http://watercenter.montana.edu/training/

gw/default.htm>.

53. Home*A*Syst: Protecting Well Water Supply. 2002. North Carolina State University

Cooperative Extension Services. 16 March 2009. .

54. Understanding Your Water Well. July 2001. Ohio Department of Natural Resources. 16March 2009. < http://dnr.state.oh.us/Water/pubs/fs_div/fctsht62/tabid/4149/

Default.aspx>.

55. Specific Capacity A Measure of Well Performace, Well Problems, and Aquifer

Transmissivity: Part 1 of 2 (WRD Technical Bulletin Volume 2, Winter 2005). 2005.

Water Replenishment District of Southern California. 16 March 2009..

lecture 7 water wells

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