Microbiological Well Contamination: One Water Supplier’s Resolution to a Persistent Problem
The Greenlawn Water District, located within the Town of Huntington, Long Island, NY, provides water to a population of approximately 42,000, pumping an average of 5 mgd.
The Greenlawn Water District, located within the Town of Huntington, Long Island, NY, provides water to a population of approximately 42,000, pumping an average of 5 mgd. During the summer months, the community produces water demands with peaks of 17 mgd. The District’s Well No. 9 at the Buttercup Lane plant experienced persistent microbiological contamination over a three year period, which kept the well out of service for extended periods of time.
Originally constructed in 1965, the well is 600 foot deep and is fitted with a deep well turbine pump. The well screen is set at 520 feet to 600 feet below grade. This screen setting extracts water from the Magothy formation in a strata that is primarily sand and mica. Above the Magothy aquifer is a clay layer (about 200 feet thick) and a shallower glacial aquifer.
The well was removed from service in late 2000 in order to conduct routine rehabilitation.
As part of the work, the contractor removed the pump motor, discharge head, column pipe and well pump. The well casing was also wire brushed and cleaned. In January 2001, the contractor installed a new 10-stage vertical turbine pump. The existing discharge head was reconditioned and new column pipe was also installed. The original oil-lubricated well pump was replaced with a water-lubricated pump.
The contractor disinfected the well upon completion of the work and testing indicated no bacteria present at that time. However, over the next 17 months intermittent positive total coliform bacteria results were obtained. The well was again removed from service.
Various corrective methods were attempted to eliminate the bacteria. It was felt that residual oil from the oil-lubricated pump might still be resident inside the well thus prohibiting the induced chlorine from killing the bacteria. The district tried to eliminate the bacteria by adjusting the pH of the water by introducing an acid to increase the effectiveness of the chlorine used for disinfection. Staining and streaking inside the well was observed via an internal television inspection. The district pumped chlorine gas into the well, hoping that the gas would find its way into any small crevice, crack or opening within the casing and kill off the bacteria. Both projects proved unsuccessful.
With the well being 600 feet deep, it was not believed that the bacteria source was coming from the screened area of the aquifer (520 feet to 600 feet). The next possible source was either through a leak in the casing or by short-circuiting of the aquifer system. Contaminated surface or sub-surface water was thought to be running down along the exterior wall of the well casing. The casing, in turn, may not have been properly sealed with cement grout when originally installed.
Many wells are constructed with a packer that prevents any material from accumulating above the gravel pack. Unfortunately, this well was not constructed with a packer but with an annular space between the well casing and the well riser pipe, on top of the gravel pack. It was believed that this space may have been harboring bacteria from the recent casing cleaning or scale that normally accumulates. In May/June 2003, the district arranged for a contractor to remove a majority of this accumulation and disinfect the well. The annular space below the top of the riser pipe was then filled with additional gravel and capped with concrete grout at the top of the riser. This effort also proved unsuccessful.
The district’s next attempt to resolve the problem was by depositing approximately 50 gallons of food-grade oil into the well. It was expected that the oil would float on the top of the static water and act as a seal to prevent any contaminated water leaking down inside the well casing. When bacteria still continued to be detected intermittently, H2M was retained to investigate the problem. H2M recommended that a mechanical fix be made, and a casing liner be installed in the well.
The proposed installation called for a 16-inch diameter steel casing liner to be installed inside the existing 20-inch diameter steel well outer casing. The liner was to be fitted with a mechanical seal that was to fit over the screen riser pipe, on top of the gravel pack, and intended to secure the well from any contamination from above. However, an alignment and plumbness test, performed prior to installation, indicated that the well was 2.5 feet out of plumb and approximately one inch out of alignment. The video also revealed that, from an earlier attempt to seal off the gravel pack, a large mass of concrete formed at the top of the riser reducing and distorting its shape. Therefore, before the liner could be installed, measures needed to be taken to remove the concrete buildup and correct the distortion.
The contractor removed about 20 fist-size chunks of concrete from the well. The video, however, indicated that the removal process had unavoidably distorted the top of the riser, yielding a non-uniform surface. It was certain that these conditions would not allow the seal to fit properly.
H2M, working with the contractor, proceeded with an alternative method of sealing the casing at the riser. Four conical shaped heavy gauge rubber pieces (shale traps) were strapped to the bottom section of the liner, with the wider open section facing towards the top of the well.
The interstitial space between the new liner and the existing casing was filled with a mixture of bentonite and sand between elevation 272 feet and 438 feet belowgrade. The shale traps were filled with bentonite as they were lowered into the well.
The new casing liner, as installed, extends down to the top of the gravel pack to about elevation 445 feet. In order to get these shale traps inside the existing casing and so as to have enough space into which the bentonite/sand mixture could be placed, the liner had to be modified. The liner was reduced from a 16-inch diameter to 14-inch below the bottom of the well pump setting (elevation 272 feet) to just above the 12 foot diameter riser at 438 feet. Below 438 feet down to the top of the gravel pack the liner is 16-inch in diameter. The liner was fitted over the riser pipe, with its bottom edge supported by the gravel pack and concrete collar. The interstitial space between the new liner and the existing liner above elevation 272 feet was filled with the sand/bentonite mixture to complete the installation.
With much determination, the Greenlawn Water District, with H2M’s assistance, finally succeeded in reviving this long standing problematic well. After four successful rounds of extensive bacteriological testing, and with Suffolk County Department of Health Services approval, the well was placed back into service in May 2004. It has not experienced any further bacteriological contamination since then, and continues to provide high quality water to the Greenlawn community. WW
About the Author:
Benjamin Bletsch, P.E., is Senior Water Manager at Holzmacher, McLendon & Murrell, P.C. (H2M). Robert Santoriello is Superintendent of the Greenlawn Water District.