Can We Swim Yet? Systematic Review, Meta-Analysis, and Risk Assessment of Aging Sewage in Surface Waters

This study investigated the risk
of gastrointestinal illness associated with swimming in surface waters
with aged sewage contamination. First, a systematic review compiled
333 first order decay rate constants (k) for human
norovirus and its surrogates feline calicivirus and murine norovirus, Salmonella, Campylobacter, Escherichia
coli O157:H7, Giardia, and Cryptosporidium, and human-associated indicators in surface water. A meta-analysis
investigated effects of sunlight, temperature, and water matrix on k. There was a relatively large number of k for bacterial pathogens and some human-associated indicators (n > 40), fewer for protozoans (n =
14–22), and few for human norovirus and its Caliciviridae surrogates (n = 2–4). Average k ranked: Campylobacter > human-associated markers
> Salmonella> E. coli O157:H7
> norovirus and its surrogates > Giardia > Cryptosporidium. Compiled k values were
used in a quantitative microbial risk assessment (QMRA) to simulate
gastrointestinal illness risk associated with swimming in water with
aged sewage contamination. The QMRA used human-associated fecal indicator
HF183 as an index for the amount of sewage present and thereby provided
insight into how risk relates to HF183 concentrations in surface water.
Because exposure to norovirus contributed the majority of risk, and
HF183 k is greater than norovirus k, the risk associated with exposure to a fixed HF183 concentration
increases with the age of contamination. Swimmer exposure to sewage
after it has aged ∼3 days results in median risks less than
30/1000. A risk-based water quality threshold for HF183 in surface
waters that takes into account uncertainty in contamination age is
derived to be 4100 copies/100 mL.