Abstract
Given the 24–48 h turn-around time of conventional surveillance approaches, methods are needed that improve the timeliness and accuracy of recreational water quality risk assessments. Although one useful approach is to combine existing monitoring programmes with predictive faecal indicator bacteria (FIB) models, these models are largely ‘top-down’ in their approach to safeguarding public health. Beyond being simply ‘advised when to avoid swimming’, there is an increasing awareness amongst the general public regarding the role they can play in water quality monitoring. Using quantile, maximum value and optimized incremental modelling approaches, this study reports on the possibility of developing intuitive, public-friendly models that are based on the physical appearance of water (clarity), to estimate 8103 nation-wide E. coli concentrations in rivers, and to assess whether water is safe to swim in. If swimmers were to avoid river waters with <1.1 m black disc visibility during autumn and summer, and river waters with values <0.5 m black disc visibility during spring and winter, they would also avoid microbial hazards that are associated with exceedances of the 540 CFU/100 mL single sample bathing water standard. Regardless of the climatic season, stream order classification, catchment land cover or geology of streams considered, the clarity-based E. coli models performed well as they presented with sensitivity, specificity and accuracy values of at least 72%. The developed models offer the benefit of providing a faster method for estimating E. coli concentration, potentially engaging the public in water monitoring, and allowing them to make informed decisions on whether it is safe to swim.
Highlights
Surface waters are prone to contamination from various point and nonpoint sources and can serve as a vehicle for transmission of potentially pathogenic bacteria (Dada & Hamilton, 2016; Devane, Gilpin, & Moriarty, 2015; Praveena et al, 2018)
The proportion of samples with E. coli concentrations that exceed bathing water standards was highest for Waitara, Waipa, Waihou, and Waikohu as more than 30% of exceedances were observed during the nine-year period included in this study (Figure 3)
Median E. coli concentrations were closer to the bathing water standard for water samples having water clarity values that fell within the first quartile (Figure 4)
Summary
Surface waters are prone to contamination from various point and nonpoint sources and can serve as a vehicle for transmission of potentially pathogenic bacteria (Dada & Hamilton, 2016; Devane, Gilpin, & Moriarty, 2015; Praveena et al, 2018). Beach monitoring programmes have been adopted and implemented in many nations to protect beachgoers from health risks caused by potentially harmful bacteria. In New Zealand, policies such as the National Policy Statement on Freshwater (NPS, 2014) provide guidelines for regulators to ensure that monitoring programs are in place to warn the public about the risk of exposure to these pathogens during recreational contact. Current risk assessment is based on microbiological culturing of Escherichia coli, and results are used to inform the issuance of swimming advisories to reduce the risks of exposure to potentially pathogenic bacteria at recreational sites. While dissemination of accurate and timely information is critical to preventing illness, water quality advisories often do not present accurate assessments of such risk in a timely manner
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