De facto reuse refers to the use of surface water that contains a considerable portion of wastewater effluent from upstream communities as a source water for drinking water supplies. In contrast to the highly regulated domestic wastewater reuse practices in the U.S., not much is known regarding the human health risk associated with de facto reuse. Trinity River, Texas, a wastewater dominated river that is used as the main source of drinking water for Houston, is used as an exemplar to quantify the health risk associated with exposure to Cryptosporidium and norovirus through drinking water contaminated with wastewater effluents. The results show that the annual infection risks exceed the U.S. EPA recommended annual safe drinking water health risk of 10-4 per person per year (pppy) for all scenarios considered. However, a comparison of the corresponding disease burdens with the World Health Organization's annual disease burden for safe drinking water of 10-6 DALYs pppy indicates the disease burdens are mostly within the acceptable range of the guideline. This difference in risk interpretations reflects the difference in the fundamental definition of the two risk benchmarks as well as the gaps in data and risk models (including the conversion from infection to illness risk and expected DALY per infection). The results also imply that the current safe drinking water regulatory framework should be reexamined to best achieve the goal of human health protection. The outcomes of this study provide an overall picture of the potential risk associated with de facto reuse, which contributes to a better understanding of the practice and decisions in wastewater reuse in the U.S. and the rest of the world.

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