Good Practices In Maintaining Hand Pumps & Production Wells

By Upul Wickramaratne

Hydrogeologist / Manager (Groundwater)Groundwater Section

National Water Supply & Drainage BoardChilaw RoadWariyapola

Groundwater ??

Animals and Villagers are using same Water in Drought Affected Tanks in Pothanegama

Dug well Constructed on Dry Tank-Warawewa

Dry Dug Well at Weewalayaya Village

Distributed among common place in village by Social Service

Department and Water Supply by Bowser.

Dug Well Constructed on Dry Stream - Polpithigama

Construction of Dug Well, blasting rocks – Andarayaya, Polpitigama

Relevance to an Emergency

• Groundwater can be developed quickly in some situations – shallow tube wells/ Deep tube wells available drilling equipment (eg. Manika Farm – 2009)

• If deep wells may be available – Could be included in emergency water supply solution– Need to understand capacity, condition, constraints

on use • Potential for future development later in

emergency

What is groundwater? Groundwater is water that is

found underground in the cracks and spaces in soil, sand, and rocks.

Groundwater is stored in—and moves slowly through—geologic formations called AQIFERS which capable of yielding enough water to supply peoples' uses.

How does groundwater move?

Underground, water slowly moves from an aquifer’s recharge areas (areas where water seeps into the aquifer from rain fall to it’s discharge area (like streams, springs and lakes).

Groundwater is always moving (this is called groundwater flow) and moves very slowly--only inches per year.

groundwater flow discharge area

evaporationrecharge

area

precipitation

condensation

runoff

transpiration

aquifer

water tableinfiltration

Hydrologic Cycle

Hydrogeology: Science which treats of occurrence distribution and movement of water under the earth surface

Major Features of the GW System to Remember

Porosity: is defined as the ratio of the volume of voids to the volume of aquifer material. It refers to the degree to which the aquifer material possesses pores or cavities which contain air or water.

Permeability: The capacity of a porous rock, sediment, or soil to transmit ground water. It is a measure of the inter-connectedness of a material's pore spaces and the relative ease of fluid flow under unequal pressure.

Pores, Porosity and PermeabilityPores: The spaces between particles within geological material (rock or sediment) occupied by water and/or air.

Primary Porosity

Secondary Porosity (Hard Rock Aquifers)

Hard Rock

Lime Stone

Cavities

Fracture

• Aquifers are replenished by the seepage of Precipitation that falls on the land, but there are many geologic, meteorologic, topographic, and human factors that determine the extent and rate to which aquifers are refilled with water.

• Rocks have different porosity and permeability characteristics, which means that water does not move around the same way in all rocks. Thus, the characteristics of ground-water recharge vary all over the world

How can we trap the groundwater

• By dug wells• Shallow tube wells• Deep Tube well• Natural springs

Well installation methods

• Hand excavation / boring• Hand drilling

– incl auguring and jetting• Machine drilling

– Rotary, percussion, down hole hammer

• Drilled wells– Reach greater depths– Penetrate wider range of rocks

Drilling…

Summary of Types of Wells

Cross section of fractured aquifer

High yield TW

Maintaining Of Tube wells• Should be a proper design and construction

Fractured Aquifer• Fractures form conduits through which

groundwater and pollution can move• Rock matrix stores water• Boreholes intercepting fractures

can have high yields

FractureRock Matrix

Water LevelSand & Clay

Sand & Clay

Hard Clay (Brown)

Gravel Sand Clay & Calcareous Rock

Fragments

Gravel & Clay

Blank PVC Casing 225mm (1000T)

Gravel Pack (Filter Pack)

Clay filled

Screen PVC Casing 225mm (1000T)

Sandy clay

Gravel & Clay

Hard Clay (White)

Main Aquifer

TW design..

Hard rock Open hall

22.4m

27.8m

31.0m

14.2m

CVWD Well 5681Aquifer and Filter Pack

0

10

20

30

40

50

60

70

80

90

100

0.001 0.010 0.100 1.000 10.000 100.000

Sieve Opening, mm ------>

Cum

ulat

ive

% P

assi

ng

Design Criteria (D = Pack d = Aquifer) Well 5681

Pack/Aquifer Ratio (50% Passing): 4:1 to 6:1 5.6

Terzhagi Migration Factor: D15 / d85 < 4 1.4

Terzhagi Permeability Factor: D15 / d15 > 4 16

% Filter Pack Passing Screen Slot: 10% to 20% 6%

Uniformity Coefficient (Aquifer) ; cu = d60 / d10 4

Uniformity Coefficient (Pack): Cu = D60 / D10 1.8

Pack/Aquifer Sorting Factor: Sf = Cu / cu 0.44

1080-1090 ft

1220-1230 ft

840-850 ft

940-950940-950 ft

1130-1140 ft

GranuleSiltClay Fine Sand Med Sand Coarse Sand Pebble Cobble

100200 60 40 30 20 16 10 6 1/ 4 in. 3/ 8 in.US Std. Sieve

Recommended Design

"Rancho Mix" with3/32 in. Screen Slot

Size, in. % Passing0.375 1000.187 68.0937 7.0787 3.0661 2.0469 1

Test pumping of wells

• Completed after well development• Tests the potential pumping rate and

resulting drawdown• Assists in determining :

– Long term pumping rate and water level– Suitable pump– Impact on surrounding wells or environment

Pumping tests

• Step drawdown test – short periods of pumping at progressively higher rates

and measuring drawdown in the well– helps work out achievable pumping rate

• Constant rate test– Pumping at constant rate (could be for days or weeks)– Measure drawdown in well and nearby “observation”

wells– Helps assess pumping impacts on the aquifer under

different pumping scenarios

Pumping Test……

Step Drawdown Test

Q = 3,000 gpmDrawdown = 17 ft

Specific Capacity = 176 gpm/ft

SWL 280 ft

TD = 830 ft

Time Drawdown Test

Failures • Being operating HPTW or PW

performance of wells such as WQ variation and quantity could be declinedDue to

Construction failure, Formation failure, Aquifer failure, Clogging or Scaling of Ca, iron & iron +

magnesiumOVER PUMPING

OVER PUMPING• What is Over pumping

When exceed the given recommendation of pumping rate and duration

• Is that Pumping Test recommendation everlasting No, because more than 80% of groundwater

recharge from directly precipitation. If any change of rain fall pattern then recommendation could be changed.

.

Specific Capacity of a Well

• The rate of discharge of water well per unit of drawdown, usually expressed as m3/d/m.

• Allows a check of well efficiency• Helps indicate bore deterioration eg

blocked screens• Reduced capacity (greater drawdown to

pumping rate) indicates reduced well efficiency

Decline In Specific Capacity

STATIC WATER LEVEL

130 GPM130 GPM

Original Drawdown, 40 ft SC = 50 gpm/ft

Spec

ific

Cap

acity

, gpm

/ft

TIME ---->

50

40

30

20

10

60 FT SC = 33 gpm/ft

80 FT SC = 25 gpm/ft

AQUIFER

Pumping Level ---->

Clogging Of Screen SlotsDue To Scale

Pumping Level ---->

Pumping Level ---->

• Loss In Specific Capacity

• Degradation Of Water Quality

• Pumping Excess Sand

• Pumping Any Gravel

• Pumping Air

Time to Rehabilitate Well

Discharge Rate, gpmD

raw

dow

n, ft

Des

ign

Des

ign

Dis

char

geD

isch

arge

Design DrawdownDesign Drawdown

Analysis Of Well Efficiency - Specific Capacity Diagram

Wel

l Effi

cien

cy, %

100

0

Well EfficiencyWell Efficiency

Cones of Depression

• Cone of depression – forms in the water table when water is pumped from a well

• The pumping of wells can have a great deal of influence on water levels below ground, especially in the vicinity of the well, as this diagram shows. If water is withdrawn from the ground at a faster rate that it is replenished by precipitation infiltration and seepage from streams, then the water table can become lower, resulting in a "cone of depression" around the well.

• Depending on geologic and hydrologic conditions of the aquifer, the impact on the level of the water table can be short-lived or last for decades, and the water level can fall a small amount or many hundreds of feet. Excessive pumping can lower the water table so much that the wells no longer supply water—they can "go dry."

How intensive ground-water pumping can cause salt-water intrusion in coastal aquifers.

Theoretically 1m drawdown creates 40m uplift

What Causes The Well To Need Rehabilitation?

• Clogging of Perforations

• Corrosion

– Enlargement of Perforations

– Holes in Casing or Screen

• Cementation in the Near-well Zone (Eg. Filter Pack)

• Sand Sealing

When the Well Screen Slot Opening Is Too Small, Fine-grained Material Which Normally Migrates Through the Filter Pack on Start Up, Consolidate Near the Well Screen Creating a Low Permeability Zone Which Greatly Accelerates Well Losses.

Well Screen Filter Pack Aquifer

“Sand Sealing”

Low Permeability ZoneAdjacent to Well ScreenDue to too small a slot size

How Do I Know If My Loss In Production Is Due To My Well Or Is It

A Well-field Problem?• Well-field Interference• Regional lowering of ground water levels

leaving the most productive aquifers “High And Dry”

• Seasonal Fluctuations in Recharge

Do All Wells Need Rehabilitation ?

• Experience in the Industry has shown that approximately 10% of all Large-capacity Water Supply Wells will be relined at some time during their life

• Rehabilitation Percentage is much higher -- 70%• It is rare that a well will operate without some

rehabilitation during its life

Clogging

• Iron• Iron Magnesium • Clogging process could increased due

depletion of water level by creating aerobic environment

HCL 10% HCL 5%Well Klean 20%

HCl 5%Citric 1% Citric 10% Phosphoric 8.5%

Laboratory ResultsAfter Dissolving

Scale In Acid

Decision Tree For Rehabilitation

ARE POTENTIAL BENEFITS INSUFFICIENTARE POTENTIAL BENEFITS INSUFFICIENTOROR

REHABILITATION COSTS > REDRILLING COSTSREHABILITATION COSTS > REDRILLING COSTS

REDRILLREDRILL REHABILITATIONREHABILITATION

POTENTIAL BENEFITS SUFFICIENTPOTENTIAL BENEFITS SUFFICIENTANDAND

REHABILITATION COSTS < REDRILLING COSTSREHABILITATION COSTS < REDRILLING COSTS

EVALUATE POTENTIAL BENEFITS AND COST OF REHABILITATIONEVALUATE POTENTIAL BENEFITS AND COST OF REHABILITATION

IS WELL APPROPRIATE FOR REHABILITATION?IS WELL APPROPRIATE FOR REHABILITATION?(design, age, operational history, location)(design, age, operational history, location)

IF SOIF SO

IDENTIFY PROBLEM AND CAUSEIDENTIFY PROBLEM AND CAUSE

• The development of tube well is essential process to obtain  an efficient and long –lasting well. The main objectives of well development  are as under:  

• Repair  to damage done to the formation by drilling operation and restore the original  hydraulic conductivity of the aquifer. 

• To  increase the porosity and permeability of the water bearing formation in the  vicinity of the well by removing finer material of aquifer  

Tube well development

Tube well development cont..

• To  stabilize the formation around well screen to yield sand free discharge.

• All the new wells should be developed before being put  into production well. It well achieve highest specific capacity and ensure  energy efficient well.

•  We know that Tube well is an important and costlier  structure in water supply schemes.

Well development Methods    • Surging and pumping with air-

compressor• Re-drilling with air-compressor• Over pumping    • Back washing   • High velocity water jetting • Use of explosive • Use of acid• Hydro fracturing

Life time of TW

• Developed countries (Sweden, Denmark, Norway, UK, USA) – 30 to 60 years

• Sri Lanka - 4 to 10 years

If proper maintenance system implemented could be improved

- 20 to 30 years

• GROUNDWATER MONITORING is very

IMPORTANT to avoid this problem

Thank you