• Borehole Monitoring and Deterioration

Borehole Monitoring

Continuous monitoring of borehole performance can be cost-effective, helping to detect any problems before they become serious. Maintenance programs should consist of regular field visits, water sampling (for chemical/microbial analyses), water level measurements, and routine monitoring by simple step-drawdown tests. The data collected can be compared with those obtained when the well was new or last monitored. A regular testing schedule consisting of a basic step-drawdown test every year is sufficient, with maintenance carried out if there is any sign of deterioration. Low-risk areas (in terms of borehole incrustation or corrosion) may require maintenance work only every few years. It is prudent to erect a lockable fence around the borehole to prevent tampering and accidental or malicious damage. The Table below sets out the symptoms to be noted in a monitoring program, along with causes and suggested remedial actions.

Monitored symptomCausesRemedial action
Regional fall of groundwater levelRegional factors, e.g. drought, large-scale abstraction, extensive deforestationLower pump inlet Deepen borehole Drill new (deeper) borehole
Localized fall of groundwater levelOver-pumping
Blocked screens or gravel pack		Check/compare earlier test pumping data Reduce pumping rate Rehabilitate: Inspect screens, surge-develop to clean screens and gravel pack
Change in water quality (chemical)Chemical pollution
Saline influx
Aquifer mixing		Analyze water; if hazardous, shut down borehole production and reassess situation
Change in water quality (biological)Pollution
Change in water chemistry		Analyze water. If hazardous, shut down borehole production.
If temporary, pump out water and disinfect borehole
Unusual corrosion/incrustation of borehole head works equipmentWater quality, e.g. carbonate (hard water), acidic water, iron, bacteriaRemove pump, inspect borehole. Rehabilitate
Reduction of yield (pumping level unchanged)Pump faulty
Piping blocked (incrustation)		Remove and inspect pump Inspect piping; replace if necessary
Unusual noise or vibration (submersible pump)Damaged/faulty pumpRemove and inspect pump
Inspect borehole
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Local staff should be recruited and trained in the monitoring of boreholes, and in the repair of pumps (especially hand-pumps), particularly in those areas where the failure of the water supply would have the most serious consequences. This would apply most to boreholes supplying settlements or institutional facilities in remote arid or semi-arid regions of the world and in places where rapid borehole deterioration is a possibility.

Water quality monitoring
Chemical analysis of the bore water should indicate the potential for damage to borehole structures. The physical condition of the abstraction system at a borehole may give an indication of developing conditions within the borehole itself. If unusual and significant corrosion or incrustation is taking place among borehole headwork structures, the same is likely to be happening inside the borehole. Water quality monitoring is particularly important if boreholes are close to densely populated areas or in coastal zones.

Pollution (chemical or biological) may be caused by the former; in coastal areas there is the possibility of intrusion by salt water, from a fluctuating fresh water/sea water transition zone. In the latter case, of course, simple tasting will confirm the problem, but regular conductivity or total dissolved solids (TDS) analysis will provide predictive data.

Distilled water has a conductivity of 1 μS,

good fresh water <2,000 to 3,000 μS, and saline water >6,000 μS
(S = Siemen, 1 Ω-1cm-1).

The equivalent TDS classification is:

fresh water 0 to 1,000 mg/l;
brackish water 1,000 to 10,000 mg/l;
and saline water 10,000 to >100,000 mg/l.

Borehole monitoring should include regular step-drawdown tests, which can be further analyzed to determine the basic hydraulic parameters of aquifers. Drawdown in a borehole is essentially the sum of losses due to movement of water from the aquifer into the borehole space. Mathematical analysis of step-test data allows these losses to be determined, along with the relationship between drawdown and discharge for the borehole under test. From these data, an indication of the efficiency of the borehole (and hence of any reduction of efficiency over time) can be obtained.

Causes of Borehole Deterioration