Quantitative Microbial Risk Assessment for Private Wells in Flood-Impacted Areas

Abstract

Microbial contamination of private well systems continues to be a prominent drinking water concern, especially for areas impacted by floodwaters. Hurricane Harvey deposited nearly 60 inches of rain, resulting in extensive flooding throughout Houston, Texas, and neighboring counties. A sampling campaign to test private wells for fecal indicator bacteria was initiated in the weeks following flooding. Escherichia coli concentrations measured in wells were utilized in a quantitative microbial risk assessment to estimate the risk of infection for both drinking water and indirect ingestion exposure scenarios. Derived reference pathogen doses indicated that norovirus (1.60 × 10−4 to 8.32 × 10−5) and Cryptosporidium (2.37–7.80 × 10−6) posed the greatest health risk via drinking, with median health risk estimates exceeding the U.S. Environmental Protection Agency’s modified daily risk threshold of 1 × 10−6 for a gastrointestinal infection. Bathing (1.78 × 10−6), showering (4.32 × 10−7), and food/dish washing (1.79 × 10−6) were also identified to be exposure pathways of health concern. A post-flood microbial risk assessment of private wells in the Gulf Coast has not previously been conducted. Estimating these health risks can provide scientifically supported guidance regarding which well water practices are safest, especially when well water quality is unknown. Developing this guidance is critical as coastal communities experience increased vulnerability to flooding.