ngwa 2004 microbial gwq interest group

04/19/23 EGIS, Well Video & Geophysical Services: 919-929-8363

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NGWA 2004Microbial GWQ Interest Group

Introduced Coliform Bacteria during Pump

Replacement and Well Service: Diagnosis

Prevention and Decontamination


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ByMichael L. Vaught

EGIS441 Northside Dr

Chapel Hill, NC 27516

9199298363 or [email protected]


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Two Wells that Became Coliform Positive after Pump Service

Calvary Baptist Daycare – Piedmont granitic rock aquifer– Salisbury, NC

Orchard at Altapass– Blue ridge gneiss rock aquifer– Altapass, NC (Blue Ridge Parkway)


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History of Clean Tests

Calvary Baptist Daycare– Owners memory

Drilled in 1987 Daycare started in 1996 Tested clean until pump pulled

– Public Water records NCDENR Public Water Supply since 1996 Coliform positive = pump service in August, 20003


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History of Clean Tests

Orchard at Altapass– Owner/ operators memory

Drilled in 1976 Tested clean until pump pulled Coliform positive = pump service in October, 20003

– Public Water records Available since Feb 2004 Owners records


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Fractured Rock Aquifers

Partially Cased Open Rock Wells Unconfined Water Table Topography and elevation head Tectonic (Dikes, Faults, Emplacements) Borehole Stability Shallow Fractures = Less Filtration Time? Grout Seal Casing/Conduit


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Downhole Video & Pumping

Observe All Three Stages– Static rest.– Pumping stress.– Recharge.

Characterize well.– Construction (leaky casing?)– Borehole stability (geology)– Biofouling– Water zones


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Salisbury Field Project– Well Data

Calvary Baptist Daycare– 21’ of 6.25” galvanized casing- tight– 16’ to 170’ Slight biofouling– No shallow exposed water zones– 87’ slowly recharging water level– 170’ downward walls clean– 340’four inch standard submersible pump– 415’ significant water producing fracture– 455’ drillers well tag “Bottom of the Well”.

(well yield 17 gpm)


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Altapass Field Project– Well Data

Orchard at Altapass– 30’ Water level. Walls are clean.– 55.5’ Bottom of the 6.25’galv csg. – tight.– 74’ Weathered fracture yielding 2 gpm– 155’ Major water bearing rock fracture zone– 273’ four inch standard submersible pump– 304’ “Bottom of the Well” from video.

(well yield about 10 gpm)


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1st Failed attempt to disinfect

Salisbury– First ever Pump replacement in August– 8/14/03 coliform present in regular test– Well Super chlorinated and recirculated


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2nd Failed attempt to disinfect

Salisbury– 8/27/04 coliform present in repeat tests– Well Super chlorinated and recirculated


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3rd Failed attempt to disinfect

Salisbury– 10/06/04 coliform present in repeat tests– Well Super chlorinated and recirculated– 11/13/04 coliform present in repeat tests


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Source of Introduced Coliform

Diagnosis by Downhole Video Survey – Construction (Ridgewood – Leaky Csg?).– Shallow water.– Space below the pump.– Grass and debris in the well

Pump service work – Salisbury is playground with wood chips– Altapass surrounded by apple orchard (Oct.)


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Eliminating Introduced Coliform

Salisbury well– Extend pump to within four feet of the bottom– Draw the well down and back flush; cycles– Raise pH to 11 and recirculate overnight– Draw the well down and back flush– Create a 200 ppm chlorine solution in the well– Maintain pH of 6 with the chlorine at 200 ppm– Recirculate solution at least 4 hours– Apply positive head by back flushing– End of second day restore service, chlorinated water


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Eliminating Introduced Coliform

Altapass well– Extend pump to within four feet of the bottom– Draw the well down and recirculate– Pulled the pump assembly onto wooden deck– Add chlorine tablets and gravel– Reinstall pump with sleeve to the bottom– Create a 200 ppm chlorine solution in the well– Maintain pH of 6 with the chlorine at 200 ppm– Recirculate solution overnight– Cyclic Draw down and recirculation– End of second day restore service, chlorinated water


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Prevention

Pump service work – Ground cover or reel system– Clean & disinfect drop-pipe and wire– Replace twisted wire– Remove and replace tape– Rubber gloves– Work clean– “… Prevention is Worth a Pound of Cure”


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Summary

Avoid repeat chlorinations Verify Well construction and condition Flow sleeve for pumps. Disinfection depends on distribution. Efficient chlorination (pH & ppm) Wells remain clean


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References

Mansuy, N. (1999) Water Well Rehabilitation: a practical guide to understanding well problems and solutions, Lewis Publishers

Smith, S. A., Borch, M. A., and Noble, L. N. (1993) Evaluation and Restoration of Water Supply Wells, AWWA Research Foundation

Schnieders, J.H. (1999) Chemicals Used In Water Well Rehabilitation, U.S. Filter, Johnson Screens

ngwa 2004 microbial gwq interest group

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