Articles & Case Studies

Operation Fair Isle

Posted: Friday 2nd March 2012

Drilcorp was commissioned by Scottish Water to clean the production borehole and test the monitoring well on Fair Isle

Owned by the National Trust for Scotland and situated approximately 40km south-west of Sumburgh Head between Shetland and Orkney, Fair Isle is beautiful and remote. It has a population of approximately 70 inhabitants who mostly reside on the southern third of the lowlands of the island. The island is famous for birds, knitwear and historic shipwrecks and is possibly better known for its shipping weather forecasts.

The public water supply borehole for Fair Isle is 73m deep and 200mm in internal diameter. As well as providing the public water supply, water is pumped from the production well to a small reservoir which provides storage for approximately three days’ supply of water.

Previous survey work had identified that the well was fouled with manganese and/or iron deposits, which had choked the pump, screen and possibly rock fissures. This is a common problem and can also choke the pipework away from the well and cause discolouration of the water.

Scottish Water commissioned Drilcorp to clean the production borehole and test the monitoring well. The chemical selected for the operation was “Boresaver C”. This is an oxalic acid-based material and is UK government-approved for use in potable water supplies.

Works undertaken

The purpose of the works was to:

· run a short pump test to establish the existing yield and characteristics of the well;

· clean the well and pump using chemical methods

· pump test the well again; and

· carry out a CCTV survey on completion of the cleaning; and a yield test was carried out on the monitoring borehole.

On arrival at the site all personnel were given an induction by Scottish Water to ensure they were aware of any hazards. Work began once Scottish Water issued Access Certificates and the power and rising main were isolated.

A number of chemicals were being stored on site by Scottish Water and, because some of the chemicals were strong oxidising agents, the Boresaver C was kept totally separate. It was mixed in an open atmosphere and only the amount required was mixed at any one time. Any spills were to be cleaned up and stored in strong plastic bags.

Production well

Using data loggers, the first test on borehole one was carried out over a 6-hour period, with the pump running at its maximum. The rest water level was recorded as 11.97mbgl.

After 6 hours, the water level had drawn down to 43.77mbgl. Once the pump was switched off, the well recovered back to 25.07mbgl in 1 hour, 17.7mbgl in 2 hours and 13.87mbgl in 3 hours. As the water level drew down, the efficiency of the pump reduced because of the increase in head of water it had to pump.

It initially pumped 36.71l/min, with only 11.97m head to pump. By the time it had been pumping for 3 hours the head was 43.77m and the flow rate was reduced to 14.57l/min. Based on the meter readings, the test averaged 16.66l/min, indicating that the well was not running to its full potential.

Using the existing borehole pump, 90kg of ‘Boresaver C’ was injected into the well and circulated for a period of 24 hours. The pump was then removed from the well and the well was then pumped clean with an air lift eductor. To allow the installation of a dip tube, the wellhead was taken away to be modified.

A CCTV survey was then carried out to ensure that the well was clean and all contamination removed. The pump was inspected and further cleaned at the surface prior to installing back into the well.

On completion of the air lifting and cleaning operation, the pump was re-installed and another pump yield test was carried out over a 6 hour period. The data logger results were recorded. The rest water level started at 11.78mbgl. After 6 hours of running, the water level drew down to 40.77mbgl and produced a flow rate of 18.95l/min. The water level recovery after 1 hour was 21.73mbgl; after 2 hours, it was 15.67mbgl; and after 3 hours it was 13.54mbgl. Based on the meter readings, the test averaged 22.22l/min, which was a significant increase in performance for the well.

A monitoring well was also established and pumping tests were carried out using the same procedures as in the production well before and after its rehabilitation. The results were recorded to use as a comparison.

Good recovery

During the initial pump test, a lot of black material was pumped from the well. This is typical of manganese contamination. The drawdown of the well after cleaning was 3m less, indicating that water was flowing into the well more freely after cleaning.

The results showed that the amount of water pumped in following the final test after 6 hours was 2m3 greater. Because the well recovered more quickly after cleaning, it can be assumed that contaminant material was also removed from the rocks, allowing water to flow more freely into the well. Because the Scottish Water pump was also used for the cleaning procedure, it was likely that any build-up of manganese in parts not easily seen was also removed.

No appreciable effect was noted on the adjacent monitoring well during the 6-hour pumping tests. The pump test on the monitoring borehole showed similar characteristics to the production borehole, although the results were not quite as good, since the monitoring well was generally slower to recover.

There is a range of Boresaver products for various applications. All are UK government-approved for potable water supplies.

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