Citation:

Johnston, J., D. Demaree, S. Ghimire, AND M. Jahne. Risk modeling of pathogens in captured rainwater: Application of QMRA to human exposure from on-site, non-potable, fit-for-purpose reuse (ISEM 2023). International Society for Ecological Modelling Global Conference 2023, Toronto, CANADA, May 02 - 06, 2023.

Impact/Purpose:

The EPA Office of Water protects human health by implementing the Clean Water Act (CWA) and Safe Drinking Water Act (SDWA, among others). The OW works with EPA's 10 Regional offices and other federal agencies to inform and empower state and local governments, with the goal of building capacity and delegating responsibility to them for CWA and SDWA implementation. This includes American Indian tribes, the regulated community, and landowners and land managers. OW provides guidance, specifies scientific methods and data collection requirements, performs oversight, and facilitates communication among those involved in protecting public health. Much of the science needed to inform better decisions exists. Risk assessment methods such as QMRA are peer reviewed and grounded in empirical data. For stormwater use, exposure scenarios from pathogen sources to receptors needs to be better described. Use of stormwater requires unique exposure scenarios to be described and quantified. Challenges of stormwater use will benefit from a holistic approach that includes improved problem formulation and quantitative risk assessment.

Description:

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.

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