Four purpose-constructed observation boreholes in chalk were found to contain water with pH of 10-11.5 and abnormal chemistry within fourteen months of commissioning. Experiments show that this was due to contamination by alkaline porewater solutions from the concrete grout used in construction. Contamination was still detectable nearly three years after construction but was cleared by pumping. A six-hour pumping test is recommended as standard procedure during commissioning of observation boreholes.

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