Maintenance of Well Yield in the Long Term

Posted on December 26, 2010 by Jes

Extended abstract presented at the New Zealand Hydrological Society 2010 Annual Conference in Dunedin

Introduction
Good well maintenance practise can be cost effective and assist in providing the optimal yield from groundwater wells in the long term.
Over time, well yield can reduce, water quality alter and operational costs increase, however with appropriate well maintenance these problems can be remediated or avoided altogether.  Well maintenance comprises activities undertaken to allow a well to continue to operate at maximum efficiency, thereby prolonging well life.  If such measures are not undertaken, well rehabilitation such as pump replacement or well screen replacement may be necessary.
This paper considers some simple low cost methods for checking and maintaining your well to maximise performance in the long term.

Monitoring and Diagnosis
In order to avoid loss of a strategic groundwater supply, regular performance evaluation is necessary. This allows a baseline to be established against which future performance can be compared.  Therefore well maintenance begins with regular inspections and monitoring of well yield, water quality and pumped water level. Should well performance reduce by 10 % to 15 %, maintenance is required; if performance is reduced by 25 %, rehabilitation may be necessary. Diagnosis of the problem requires review of well construction details and ongoing monitoring data in combination with more specific water quality analyses and CCTV inspection to examine the well casing and screen.  Maintenance may require chemical treatment of the well, physical cleaning or development or re-evaluation of the pumping regime.
The results of insufficient well development or lack of well maintenance over time have proved to have a significant effect on the performance of pumping wells. Comparison of stepped rate well tests carried out before and after a mechanical development programme demonstrates that well performance can be significantly improved by relatively simple methods.

Mechanical Well Development
Mechanical development is essential when a well is constructed but it can also play an important role when treating a clogged or partially clogged screen. High velocity jetting combined with air lift pumping and perhaps mechanical surging has proven to be an effective way to flush the screen and the surrounding formation and remove particles from the well.
Common development methods involve air lifting at a low rate and slowly working up to the intended long term yield.  However a range of examples from new and existing wells have shown that the most effective (and most timely) results are obtained if the well is developed at pumping rates exceeding the anticipated yield (over pumping) when the well is put back in operation.

Chemical Treatments
Chemical and biological encrustation of well screens is one of the major causes of well failure, and where identified a chemical treatment (usually in conjunction with physical development) may be required.  The type of encrustation, and hence the treatment, is largely dependent on the water quality of the aquifer and therefore a good understanding of the nature of the aquifer and natural water quality is required for diagnosis.
The most common causes of encrustation are the precipitation of iron, manganese, magnesium and calcium, or biofouling caused by slime producing organisms such as iron bacteria.  Treatment generally comprises repeated dosing of the well with an acid or oxidising agent such as chlorine.
Chemical treatments can also be used to address the physical plugging of the screen or surrounding formation (where fine particles have migrated over time into the open area surrounding the screen).
Chemical treatments have been found to be more effective when combined with physical development.  Generally such treatments will be required on a regular basis and will comprise part of the long term maintenance plan.

CCTV
When the well is cleaned up by physical and chemical treatment it is good practice to check if there is any physical damage to the inside of the screen, the sump or the casing. This can easily be done by undertaking a CCTV (Closed Circuit Television) inspection of the inside of the well. A CCTV inspection can normally be completed in a couple of hours and it provides information about the welding, the connection between casing and screen, any damage to the screen and if there is an excess of, or unexpected material present in the well sump.

Case Studies
Water quality samples (including microbiological analyses) collected from existing wells in the Central North Island have shown elevated iron values and the presence of iron bacteria. Iron bacteria often occur because of an excessive drawdown in the pumping well. Development, such as air lift pumping followed by over-pumping with a submersible pump, has been shown to improve the specific capacity of the wells, resulting in a smaller drawdown (and thereby limiting aeration of the pumped aquifer).  This development in combination with a shock chlorination to kill the remaining iron bacteria has proven effective, extending the life of relatively old production wells and production of new ones.
A new well constructed in Canterbury was initially developed by the contractor using mechanical surging. Following the initial development the well operated under sand free conditions, but the specific capacity was lower than expected. Instead of constructing another well to provide the required yield, further well development was recommended. A development programme was proposed consisting of high velocity jetting, air-lift pumping and surging and finally over-pumping with a submersible pump. This procedure resulted in removal of a significant amount of fine material from the formation and the subsequent stepped rate pumping test resulted in a well equation indicating an improvement of the well by more than a factor of 2.  Well yield improved from 15 l/s to 33 l/s.

Conclusion
With a small effort, well maintenance measures offer increases in well productivity and longevity, avoiding the need for expensive repairs and re-drilling, whilst improving the quality and reliability of the drinking water supply.
Regular maintenance of a well can increase the yield and reduce electricity usage and running costs. It can eliminate or delay the need for drilling a new well, resulting in cost savings with minimal downtime.
Case studies demonstrate the effectiveness of appropriate maintenance measures, A CCTV inspection in conjunction with water quality sampling is a relatively quick and cost effective way of diagnosing well problems, allowing the specification of targeted chemical and physical treatment to improve well yield.