Reverse QMRA to design monitoring after contamination event

Bank filtration provides an excellent barrier against pathogens. However in this case contamination of the bank filtration well field occurred due to construction failure of some wells. The water supply from this well field was stopped immediately and safe water was supplied from another plant. After repairing the site, the site needed to be remediated by ‘flushing’ out any contaminants. The question was when the water could be considered safe again, since monitoring indicators would provide insufficient basis for this.

A reverse QMRA approach was developed to design a monitoring plan that would sufficiently verify compliance to a health based target of 1 infection per 10,000 persons per year. For the Netherlands, this corresponds to approximately one pathogen in one million litres of water. It is not feasible to monitor water quality for compliance to this target directly. The QMRA Treatment calculator was used to determine the effect of each treatment barrier on each pathogen. Thus a maximum allowable pathogen concentration in the raw water could be calculated, which can be monitored directly through large volume samples. Using this approach the water company could guarantee safe water for its clients and satisfy the health inspector.