These should be high-priority areas for rapid screening. Deep wells are used in the south of Bangladesh to avoid this problem and these have a very high probability of being safe. However, all new wells should be tested before commissioning. Changes with time Limited water quality monitoring over the last few years shows that the arsenic concentration of some wells has increased slightly. However the results do not show a universal tendency to increase.
Survey data show that higher proportions of older wells are contaminated by arsenic.
This trend is shown for wells ranging in age from their first year of operation to about twenty years of age. This trend suggests, but does not prove, that arsenic concentrations at wells may increase with time. Certainly, the wide range of concentrations observed strongly suggests that concentrations do not increase at all wells, or if so, at such a slow rate as to be irrelevant in human terms. However in the context of an arsenic mitigation strategy, it would be advisable to adopt a precautionary approach and to assume that the arsenic concentration could increase slowly with time. The implication is that a well tested as safe, but in an affected area, cannot be presumed permanently safe.