Risk modeling of pathogens in captured rainwater: Application of QMRA to human exposure from on-site, non-potable, fit-for-purpose reuse

Rainwater collected on suburban and municipal properties from rooftops and stored in barrels and cisterns is a potential, alternative source of water for non-potable uses such as vehicle and equipment washing, in addition to potable use with an appropriate level of treatment. As good stewards of public resources, homeowner and municipal use of rainwater benefits cities by decreasing stormwater flow from the property to sewers and also decreases reliance on city drinking water. But is roof-collected rainwater safe to use, and what standards exist to evaluate human health risk? Because the water isn’t normally being ingested, protozoa and bacterial pathogens from roof animals are the most probable concern for human health (as opposed to metals or chemical contaminants), and skin contact and respiration are the potential routes of exposure. Unfortunately, risk-based standards for determining safe use of harvested rainwater are lacking, leaving cities to devise their own approach. A risk-based, holistic, analysis is recommended using Quantitative Microbial Risk Assessment (QMRA), consistent with the World Health Organization guidelines and supported by the USEPA. QMRA provides a systems view of pathogen sources, fate and transport pathways in the environment and receptor populations. We demonstrate the use of this evidence-based, data and risk assessment methodology to inform decision making on the potential human health risks of on-site, fit-for-purpose use of roof-collected rainwater, from source and pathway modeling to receptors, dose response modeling and sensitivity analysis.